7??????Tunable infrared laser spectroscopy

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7 Tunable infrared laser spectroscopy
R. F. Curl and F. K. Tittel
Departments of Chemistry and Electrical and Computer Engineering,
Rice Quantum Institute, Rice University, Houston, TX 77005, USA
1 Introduction and scope
The infrared region of the spectrum has been of great interest to researchers for many
years with the mid- to far-IR frequency region from about 100 to 3800 cm
⫺1
of
particular interest. The vast majority of chemical substances have vibrational fund-
amentals in this region, and the absorption of light by these fundamentals provides a
nearly universal means for their detection. For many years, work in this area with
dispersive spectrometers was hampered by the low spectral brightness of blackbody
sources. This situation has changed dramatically with the widespread use of Fourier
transform infrared spectrometers with their multiplex advantage largely overcoming
the low source intensity problem. To a lesser, but still very significant and grow-
ing extent, the development of tunable infrared laser sources is contributing to the
development of infrared spectroscopy. The purpose of this review is to describe recent
research in tunable infrared laser spectroscopy.
This review covers recent developments in three areas: tunable infrared laser
sources, techniques for tunable laser infrared spectroscopy and applications of tunable
infrared laser spectroscopy. The focus time period of this review is the years 1996 to
2001. There has not been a recent review that has attempted to be as general as to
cover the topics listed above. However, there have been a number of recent reviews of
various aspects of these subjects and several databases of interest to workers in
the field. These reviews and databases will be referred to near the beginning of the
appropriate topic. Generally, these reviews will provide the reader with a more
in-depth discussion of that particular topic. The present review will focus primarily
upon cw infrared lasers and will pay scant attention to most tunable pulsed infrared
sources such as free electron lasers, pulsed OPOs, and Raman shifting IR sources.
These are important and valuable sources for spectroscopy, but outside our personal
areas of interest.
There is some ambiguity about what constitutes the infrared region of the
spectrum. Generally we define the region as the range from 50 to 12 000 cm
⫺1
. At the
low frequency end of this region, we will not consider work done with electronic
sources such as traveling wave tubes. At the high end, we will not consider electronic
transitions directly excited and detected by fluorescence or the detection or manipu-
lation of atoms. Nor will any LMR or Stark work involving fixed frequency IR
DOI: 10.1039/b111194a Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 219

sources, where the transition is tuned into resonance with the laser by external fields,
be considered.
2 Spectroscopic sources
A Diode lasers
The majority of current semiconductor lasers are constructed from n- and p-type
layers of different semiconductor materials in Groups 13 and 15 of the periodic table.
As research and development have progressed, many complicated lattice matched
structures such as AlGaAs, InGaAs, GaAlAs and GaAlAsP have evolved. As the
result of intense development efforts, extremely reliable, room temperature semi-
conductor lasers are now commercially available in the visible and near-infrared. To
date the best monolithic diode laser sources are those with distributed feedback
(DFB) structures. Mass produced 1.3 and 1.5 µm DFB lasers developed for communi-
cations technology provide high output powers of up to 25 mW, pure single frequency
emission at high modulation frequencies, continuous wavelength tuning and a lifetime
in excess of 10 years.
Soon after the first semiconductor diode laser was developed in 1962,
1
diode laser
spectroscopy began to be used for the detection, identification, and measurement of
molecular and atomic species in the gas phase. The applications of diode lasers to
spectroscopy have grown greatly as has the quality and variety of diode lasers avail-
able (see reviews on diode lasers
2,3
). The reasons for the wide acceptance of diode
lasers in the spectroscopy community are threefold. First, lasers offer better wave-
length resolution (linewidth) and spectral brightness (power emitted per unit line-
width) than conventional spectroscopic instrumentation. Second, among all lasers,
diode lasers offer a unique combination of tunability, excellent power conversion
efficiencies, small size with typical dimensions of less than 1 mm, and modulation
capabilities. Third, they are simply excited by electric current and, further, the laser
wavelength and the output power depend on the current. For small changes in
the injection current that dependence is nearly linear and instantaneous, allowing
predictable and fast wavelength control.
Diode laser spectroscopy can be divided into two categories according to
spectral region. Mid-infrared spectroscopy corresponds to the “fingerprint” region
from 3 µm to 25 µm where most molecular species exhibit fundamental absorp-
tions. The lead salt cw diode lasers operating at mid-infrared wavelengths typically
deliver output powers of about 100 µW and require cryogenic cooling (see Section
2.A.3). Near-infrared, or ‘overtone’, spectroscopy employs room-temperature,
single frequency diode lasers primarily developed for optical telecommunications
applications with output powers up to tens of mW at wavelengths below 2 µm. These
lasers access molecular overtone transitions that are typically a factor of 30 to
300 weaker than the fundamental transitions of the mid-IR. In trace gas monitoring
applications, mid-infrared spectroscopy offers higher sensitivity at the cost of com-
plexity while overtone spectroscopy is the method of choice in applications where
lower sensitivity can be tolerated, but where cost and room-temperature operation are
paramount.
220 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

1 VCSEL diode lasers. A new development in diode lasers is the creation of
low-power consumption, efficient vertical cavity surface emitting lasers (VCSELs).
VCSELs provide a unique gain medium, which has led to compact, low cost, tunable,
single frequency laser sources compatible with fiber-optics technology.
4–11
This is
accomplished by employing a very thin gain region sandwiched between two highly
reflecting Bragg mirrors consisting of tens of layers of λ/4 thickness. This results in a
very short laser resonator that is inherently a single-wavelength structure.
12
Further-
more VCELS emit a circular, relatively weakly diverging, and astigmatism-free beam,
in contrast to the highly divergent rectangular beams of end-emitting diode laser
designs.
2 Antimonide diode lasers. For reasons of sensitivity, laser sources in the mid-IR
are of considerable interest. Continuous wave lasing at room temperature at wave-
lengths above 2 µm with output optical powers up to 20 mW facet
⫺1
has been achieved
using structures grown by molecular beam epitaxy (MBE) on GaSb substrates and
employing compressively strained GaInSbAs quantum wells (QWs) between
Ga(Al)Sb(As) barriers in the active region. Narrow ridge Fabry-Perot GaInSbAs/
GaSb type II electrically pumped QW lasers emitting at 2.35 µm have been
reported.
13,14
These lasers emit in the fundamental spatial mode and exhibit single
frequency operation over a range of currents and temperatures. They emit in a
spectral region where lower overtone and combination absorption lines of such gases
as CO, CH
4
, NH
3
and NO
2
can be accessed. In efforts to extend coverage to the
fundamental region, several groups have reported the development of antimonide
diode lasers in the 2 to 3 µm spectral region
15
and InAsSb/InAs lasers between 3 to 5
µm.
16–20
3 Lead salt diode lasers. Lead-salt diode lasers have been developed for operation
at wavelengths from 3 to 30 µm. These lasers typically deliver 100–500 µW of output
power in a near-diffraction-limited beam and can be tuned by temperature or current
control. They are based upon (typically ternary) material combinations of Pb, S, Sn,
Se and Te and operate in the temperature range 15–80 K.
21–28
Therefore, a cryogenic
refrigerator or liquid nitrogen is required, together with a temperature controller, in
order to maintain the temperature of the diode within a few mK. Frequency jitter is of
the order of 1–10 MHz although the jitter can be increased by refrigerator vibrations.
Mid-infrared spectrometers employing lead-salt diode lasers have demonstrated
very high sensitivities. They have provided most impressive performances to date in
terms of minimum detectable gas concentration in trace gas detection,
19,24,29
and they
are extensively employed in studies of transient species,
30
as well as a variety of other
applications.
31
There are four reasons for this high sensitivity. First is the high line
strengths of fundamental molecular vibrational bands. For example, carbon dioxide
at a 3.3 ppm concentration in air (which is roughly 100 times lower than its typical
ambient level) will cause absorption of 15% at 4.23 µm over a 1 m path. Such macro-
scopic absorption signals can be easily measured, even without sophisticated signal
processing techniques. Second, cryogenically cooled InSb or HgCdTe detectors with
noise-equivalent powers in the range of 0.5–50 pW Hz
⫺1/2
are used in lead-salt diode
laser spectrometers. Although the lead-salt lasers produce less output power than their
near-infrared counterparts, they still provide enough light to render the detector noise
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 221

negligible, allowing near quantum-noise-limited detection. Third, near-diffraction-
limited beam characteristics of lead-salt diode lasers allow beam shaping and
propagation over long distances. This makes possible the use of multipass cells or
remote sampling in the open air. Fourth, lead-salt diode lasers have frequency-
modulation (FM) capabilities similar to those of near-infrared diode lasers, allowing
the use of harmonic detection and two-tone modulation techniques, which are
efficient methods of noise bandwidth reduction (see Section 4.C.2.b). These character-
istics add up to an impressive real-world instrument performance.
Lead-salt diode lasers have proved effective in scientific applications, but they are
less often used in industrial applications, because of several important practical draw-
backs. Perhaps the most serious is the difficulty in obtaining diodes at a specific
frequency because of the large manufacturing spread. The situation is aggravated by
the fact that each individual laser has a rather limited tuning range, typically 100 cm
⫺1
with temperature control, and tuning in that range is almost never free of mode hops.
Furthermore, the characteristics of the diode often change upon warming up to room
temperature. Thus obtaining and maintaining a laser diode that tunes to a specific
wavelength, as is required in applications in which there is a limited choice of absorp-
tion lines free from interference by other lines, e.g. detection of formaldehyde,
24,32
is
difficult.
Despite these technical difficulties, a number of spectrometers based on lead-salt
diode lasers have been developed for molecular spectroscopy,
31,33
trace gas monitoring
(see Section 4.C.3), chemical diagnostics,
34
and other applications.
31
4 Grating-tuned external-cavity and monolithic tunable semiconductor lasers. Many
of the problems in tuning diode lasers can be enormously reduced by incorporating
the diode into a grating-tuned external cavity (ECDL).
35
In an ECDL, one or both
faces of the laser chip are antireflection-coated to eliminate optical feedback. Instead
the feedback required for laser action is provided by a larger, external cavity. The
cavity acts as a wavelength selector, picking a specific wavelength out of the usually
broad gain spectrum of the semiconductor laser material. Several cavity configur-
ations have been developed that differ in the method of tuning, component count,
output beam characteristics, and output coupling efficiency. Mode-hop-free single
frequency tuning ranges of over 1000 GHz have been demonstrated for an ECDL.
36–38
Grating-tuned external cavity diode lasers with large tuning ranges have been com-
mercialized in the near-infrared from 0.9 to 1.6 µm.
This approach is ideally suited to achieving mode-hop free tuning for Fabry-Perot-
type diodes and QC lasers (see Section 2.D) in the mid-IR. A grating-coupled external
cavity has been used to obtain a wavelength tuning of hundreds of nanometres, or up
to 8% of the central wavelength in the 3–4 µm region for InAs/InAsSb or GaSb/
InAsSb heterostructure lasers with a few hundred mW peak power.
39
Fabry-Perot
quantum cascade lasers at 4.5 and 5.1 µm have been tuned with an external cavity.
40
The principal technical issue is the need to deposit a low loss anti-reflection coating on
one of the laser output facets.
A further potential future development will be broadly tunable monolithic inte-
grated multi-section diode laser chips employing gain, filter and Bragg tuning elem-
ents.
41
Unlike external grating controlled diode lasers these lasers
42–44
would offer fast
and versatile electronic wavelength tunabilty (∼70 nm at 1.56 µm).
222 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

B Tunable solid state lasers
A large and important class of tunable lasers is based on the vibronically broadened
transitions that can occur in certain gain media, such as color centers and certain
transition metal or rare-earth ions in crystalline hosts.
45–48
When such a medium is
placed in a tunable cavity and pumped above the laser threshold, stimulated emission
can be made to occur at any desired frequency within the emission band. Tunable laser
media based on 3d–3d transitions of transition-metal ions and 4f–5d transitions of
rare-earth ions cover almost the entire spectral range between 0.27 and 4.7 µm. Recent
spectroscopic studies have demonstrated that chromium-doped zinc selenide
(Cr
2⫹
:ZnSe) has several favorable characteristics as a mid-infrared solid-state material
near 2.5 µm.
49,50
These include room-temperature operation, broad tunability, the
possibility of direct diode-pumping and cw operation.
The Group II–VI host materials are generally characterized by high thermal con-
ductivity, chemical and mechanical stability, but with a relatively high thermal lensing
parameter. Cw output powers of 1.5 W, TEM
00
polarized cw operation with 660 mW
output power, and tunability over a 600 nm band have been demonstrated.
48,51
The tuning range of such lasers can be widely varied by the choice of the impurities
and by selecting different hosts to cover the red and near-infrared spectral range from
0.65 to 2.5 µm. Titanium-doped sapphire (Ti
3⫹
:Al
2
O
3
) has become the most useful
tunable solid-state laser material with a wide tuning range from 660–1100nm
and a large gain cross-section in laser spectroscopy largely replacing dye
lasers.
2
Near the border between the mid-infrared and the near-infrared region color
center lasers
53
have also been used extensively in laser spectroscopy for the past two
decades. However, in recent years color, alexandrite and dye lasers are being
replaced by more robust titanium sapphire lasers, OPOs, diode, and rare-earth doped
fiber lasers.
54
C Nonlinear optical frequency conversion devices
Difference-frequency generation (DFG) and optical parametric oscillators (OPOs)
provide a convenient means of extending the frequency range of available near-
infrared and visible robust laser sources to the mid-infrared.
1 Sources based on difference frequency generation (DFG). A number of IR DFG
sources have been constructed and used for spectroscopy; here
55–63
are some recent
examples. In the case of DFG, two laser beams (‘pump’ and ‘signal’) at different
frequencies combined in a nonlinear material with suitable dispersion characteristics
generate a beam at the difference-frequency (‘idler’). The narrow emission spectra of
the ‘pump’ (highest frequency) and ‘signal’ (middle frequency) translate into a simi-
larly narrow spectrum of the idler wave. Idler wavelength tuning is accomplished by
tuning the pump laser, or signal laser, or both. In order that the idler wave continue to
build up as the beams pass colinearly through the nonlinear material, the three waves
must stay in phase (the ‘phase matching condition’). This imposes a condition on the
refractive indices of the three waves.
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 223

This condition can often be satisfied with a birefringent nonlinear crystal by having
some of the three waves polarized along an ordinary axis and some polarized along a
direction that includes the extraordinary axis. If the polarization direction that
includes the extraordinary axis is not parallel to it (angle tuning), the three waves will
not propagate in the same direction (double refraction) and the beams will separate as
they pass through the crystal (‘walk-off’) limiting the overlap region and the DFG
power. In order to satisfy phase-matching keeping all waves exactly parallel or per-
pendicular to the optic axis (‘90⬚ phase matching’), the refractive indices must be
tuned as the difference frequency is tuned by varying the temperature of the crystal or
by tuning pump and signal simultaneously.
In the early demonstration of the DFG method by Pine,
64
single mode argon-ion
and dye laser outputs were combined in bulk lithium niobate crystal to produce
narrow-band (15 MHz) radiation tunable from 2.2 to 4.2 µm by temperature tuning
the crystal. Simultaneous tuning of both signal and idler has been used
65,66
in AgGaS
2
to provide tunable single frequency radiation from 3.5 to 9 µm. For DFG radiation
longer than 5 µm it is possible to use birefringent bulk nonlinear optical materials,
such as AgGaS
2
,
67,68
AgGaSe
2
69
and GaSe.
70–72
Another approach to phase matching is to introduce periodic short (about 10–
30 µm wide) regions in which the sign of the third order susceptibility alternates, thus
bringing the three waves back into the right phase relationship. This is called quasi-
phase matching. It is most easily achieved in the ferroelectric material lithium niobate
where the direction of the extraordinary axis can be reversed locally by the application
of an external electric field at elevated temperatures. The resulting material is called
periodically-poled lithium niobate (PPLN).
The development of PPLN now permits near-infrared diode or fiber lasers
to be used
61,62,73–78
instead of much bulkier cumbersome dye or Ti:sapphire lasers,
making it feasible to construct compact mid-infrared spectrometers that operate
at room temperature and can generate cw output powers up to 1 mW.
79
Thus the
convenience and practicality of near-infrared diode laser and optical fiber tech-
nology are combined to achieve the analytical power of mid-infrared spectroscopy
in a single instrument. Such an instrument inherits the single-frequency operation
and high modulation speed capabilities of diode lasers, and takes advantage
of their relatively wide tuning range. For example, a typical 780 nm diode laser
can be grating tuned over 20 nm, or 2.6% in wavelength, without appreciable
changes in output power. When the output of such a laser is down-converted by
mixing with a 980 nm diode laser, the tuning range in frequency units remains the
same, in this case a very significant spectral coverage: 3.6–4.1 µm, or 13% in
wavelength.
The implementation of diode-pumped mid-infrared frequency conversion received
a significant boost from the development of novel periodically poled nonlinear
materials, such as lithium niobate (PPLN),
80
lithium tantalate (LiTaO3), and potas-
sium titanyl phosphate (KTiOPO4, or KTP) at wavelengths in the 2.5–5.2 µm spectral
region.
81–83
The quasi-phase matching properties of each of these crystals can be
engineered for interaction of any pump and signal wavelengths within the transpar-
ency range of the crystal, allowing significant flexibility in the choice of laser
sources.
84–86
In the future, quasi-phase matched GaAs,
87
should become available,
greatly extending the long wavelength region covered by DFG.
224 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

A detailed quantitative theory of this nonlinear optical process is beyond
the scope of this review chapter (see the paper by Zondy
88
and the references
contained therein). Therefore, we will simply state that the maximum idler power
generated in a given crystal is proportional to the product of crystal length, pump
power, signal power, and the squared second-order nonlinear coefficient of the
crystal. Maximum DFG output power is achieved by means of optimal focusing.
There is an optimal focus because as the focus is tightened a point is reached at
which any further increase in beam intensity through tighter focusing is offset by a
decrease in interaction length due to diffraction, resulting in loss of output power.
Although this type of source is routinely used for spectroscopy and gas detection,
DFG in bulk nonlinear (either birefringent or quasi-phase matched) crystals is
characterized by low conversion efficiency, typically
89
in the range 0.002–0.05 %
W
⫺1
cm
⫺1
The tradeoff between beam size and interaction length can be eliminated altogether
in guided-wave DFG. Tight optical confinement of pump and signal radiation near
the waveguide core creates a region of high intensity and good modal overlap that can
be maintained throughout the length of the waveguide. Thus the interaction length at
tight focusing is now limited by the length of the waveguide, and not by diffraction.
Guided-wave parametric processes, such as OPO, SHG, and DFG, have been demon-
strated
90
in periodically poled LiNbO
3
, LiTaO
3
, and KTP. In LiNbO
3
, for example, a
waveguide can be formed by titanium in-diffusion, or by a Li
⫹
–H
⫹
ion exchange,
typically followed by several hours of annealing at elevated temperature to create a
graded index distribution.
A DFG waveguide designed to carry a single spatial mode at the idler wavelength is
necessarily multimode at the shorter, pump and signal wavelengths. The presence of
multiple spatial modes complicates waveguide phase matching characteristics. For
example, a TM
00
(fundamental) mode at the signal wavelength will interact with TM
02
and TM
10
modes at the pump wavelength, but not with TM
01
or TM
11
modes. Efficient
and reproducible fundamental-mode excitation of a DFG waveguide was first
achieved by Chou et al.,
90,91
using a combination of a mode filter and an adiabatic
taper. An improved device featuring separate inputs for the pump and signal beams
followed by a directional coupler has also been demonstrated. DFG waveguides have
been used to build viable sources of mid-infrared radiation for spectroscopic
purposes.
92–94
2 Tunable optical parametric oscillators (OPO). Optical parametric oscillators
(OPOs) are progressing as useful spectroscopic tools for the generation of coherent
radiation that is continuously tunable over large spectral ranges.
95–101
Unlike DFG the
nonlinear crystal is placed in a cavity and is used to generate output beams at two new
frequencies (signal and idler) ν
1
and ν
2
from a single pump beam at ν
3
. Energy conser-
vation requires ν
1
⫹ ν
2
= ν
3
. How the frequency is divided between the new waves,
signal and idler is determined by the phase-matching condition. The development of
pulsed OPOs is mature, and these devices are available commercially. On the other
hand, cw OPOs still have some practical problems in terms of requiring high power
pump sources, efficient and high quality nonlinear crystals, low loss broadband optics,
and mode-hop-free-operation with good frequency stability in order to realize their
potential as spectroscopic sources.
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 225

In order to have sufficient pumping power and/or sufficient feedback, the OPO is
configured so that, in addition to the signal frequency being inside a resonant cavity,
the pump frequency is also resonant (singly resonant) or in addition the idler can also
be made resonant (doubly resonant). In principle, both doubly resonant and singly
resonant OPO configurations can be used although as a practical matter doubly
resonant OPOs are extremely difficult to construct and very hard to tune.
Quasi-phase matching in periodically poled ferroelectric crystals offers several dis-
tinct advantages for their use in cw OPOs, such as non-critical phase matching and a
high effective nonlinear coefficient, d
eff
. A particularly significant development was the
demonstration of the use of PPLN as the parametric gain medium, in which case the
oscillation threshold of externally pumped cw singly resonant OPOs can be reduced
to the few watt level, hence making it feasible to use diode pumped solid-state pump
lasers.
100,102–107
Pump powers as low as 800 mW were used to pump a PPLN singly
resonant OPO in which both the pump and signal were resonated.
108
The parametric process can also be used in optical parametric amplifiers (OPAs) to
boost infrared output powers. Continuous wave OPOs amplified by pulsed OPAs offer
a competitive alternative to other tunable lasers in the 1 to 5 µm spectral region in
terms of linewidths, wavelength tunability and output powers.
D QC lasers
Quantum cascade (QC) lasers are unipolar semiconductor injection lasers based on
intersubband transitions in a multiple quantum-well heterostructure. They are
designed by means of band-structure engineering and grown by molecular beam
epitaxy. The emission wavelength of a QC laser depends on the thickness of the
quantum well and barrier layers of the active region rather than the band gap of
diode lasers. These lasers operate either as cw or pulsed devices. There are several
papers
109–112
providing details of their design and operating characteristics.
Quantum cascade lasers grown in the InGaAs/AlInAs lattice matched to the InP
material system have been fabricated for emission wavelength from 3.5 to 24 µm.
Quantum cascade lasers have excellent spectroscopic properties in terms of power and
linewidth, but their tuning range is limited and their beam divergence is large. Devices
with 100 stages (quantum well gain regions in series) have peak powers of 0.6 W at
room temperature. Until recently room temperature operation was only feasible for
pulsed operation, but CW operation up to temperatures of 312 K was reported in
2001.
113
Single frequency operation is achieved through the integration of a Bragg
grating into the laser waveguide, resulting in a distributed feedback (DFB) laser.
The latest generation of QC-DFB lasers is based on a “top-grating” approach that
takes advantage of the characteristics of a mid-infrared waveguide. For mid-infrared
wavelengths below 15 µm, a dielectric waveguide built from low-doped semiconductor
layers that have appropriate refractive index modulation is used.
114
At longer wave-
lengths, the waveguide is overlaid with metal. In this case the radiation is guided not
only by the dielectric but also by the surface plasmon mode.
Continuous wavelength tunability without mode hops is achieved through the tem-
perature dependence of the waveguide parameters. The temperature can either be
varied by a temperature change of the heat sink on which the device is mounted or
226 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

more rapidly by dissipative heating through changing the direct current through the
device. Characteristic total tuning ranges per current sweep are typically around 0.4%
of the emission wavelength. For many spectroscopic purposes, the spectral linewidth
of the laser emission is as important as continuous tunability. The linewidth of cw QC
lasers ranges from a few MHz through current stabilization to a few kHz with fre-
quency stabilization, but exceeds 150 MHz in pulsed operation. Device reliability and
long-term stability are excellent as a result of using robust materials, such as InP and
GaAs based hetero-structures.
E Line tunable gas lasers
Line-tunable carbon dioxide and carbon monoxide lasers are available at wavelengths
near 10 µm and 5–8 µm, respectively with output powers of several watts and line-
widths of less than 100 MHz.
115
Their laser transitions coincide with pressure-
broadened absorption lines of numerous chemical compounds. The disadvantage of
the limited number of available laser transitions can partly be compensated in the case
of the CO
2
laser by selecting CO
2
isotopes that provide additional laser wavelengths.
Laser emission can occur in the bands of 10.2–10.8 µm using a large number of
rotational lines. Individual P and R branch lines can be selected using a Littrow-
mounted grating. Fine tuning corresponding to the CO
2
Doppler width can be
accomplished for ∼50 MHz at every line. To avoid the tuning gaps between adjacent
laser lines, pulsed continuously-tunable high pressure (at atmospheric and up to
10 atm) pulsed CO
2
lasers have been developed.
116
At 10 atm. the pressure broadening
is ∼2 cm
⫺1
and the individual rotational–vibrational lines, that are typically separated
by 1–3 cm
⫺1
, merge. Monochromatic cw laser action tunable over <1 GHz can be
obtained in waveguide lasers operating near 1 atm.
For high resolution spectroscopy, a more satisfactory way
117–122
to achieve
tunability is to mix the output of a tunable microwave source with the CO
2
laser
frequency in a non-linear medium, thereby adding microwave sidebands on the CO
2
laser carrier frequency.
Other useful line tunable molecular IR lasers include discharge excited CO lasers,
which yield a large number of lines in the 5.1–5.6 µm region. However, in order to
achieve efficient operation, the CO discharge tube must be cooled to low temperatures,
which complicates their practical use. CO lasers can also be operated near 3 µm on
CO overtone lines.
123,124
The tunability of CO lasers can also be extended by intro-
duction of microwave sidebands.
125–127
Also significant are HF and DF chemical
discharge lasers, which operate in the 2.8–4 µm spectral region.
128
Tunable far-IR
radiation can be generated by mixing a fixed frequency far-IR laser with a microwave
source.
129
F Free electron lasers
Free electron lasers
130
involve major facilities. They produce a rapid sequence of
short, intense pulses useful for photolysis and spectroscopy where high resolution is
not important.
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 227

3 Spectroscopic techniques
A Multipass cells for long optical pathlength spectroscopy
Sensitive laser absorption spectroscopy often requires a long effective pathlength of
the probing laser beam in the media being analyzed. Traditionally, this requirement is
satisfied using an optical multipass cell. There are three types of multipass cells in use:
White cells,
131,132
Herriott cells,
133
and astigmatic mirror cells.
135
For all three types of
cells, focusing mirror curvature, applied to the beam at each reflection, keeps the beam
from diverging, as if in a cavity.
The White cell
131,132
is the oldest arrangement. It has two semicircular mirrors
called the “D” mirrors closely spaced along a common diameter facing a third
notched mirror in a nearly confocal arrangement. The probe beam enters through one
notch and emerges through the other. The number of passes is varied by changing a
D mirror angle.
The Herriott cell
133
has two identical spherical mirrors separated by slightly less
than their diameter of curvature (a nearly concentric arrangement) facing each other.
A probe beam, launched through a hole in one of the mirrors at an angle to the optical
axis, completes a certain number of passes between the mirrors, and exits through the
same hole (or a hole in the other mirror). The beam bounce pattern and pathlength
are controlled by adjusting the mirror separation.
For both the White and the Herriott configurations, the number of passes, if not
limited by attenuation of light, is limited by the overlapping of spots on the mirrors.
Spot overlapping creates an etalon causing the base line to oscillate with optical
interference fringes. Astigmatic mirror cells
134
are variations on the Herriott cell that
spread the light spots over the mirror surfaces, greatly increasing the number of spots
achievable without overlapping spots and therefore the number of passes. With this
type of cell, the number of passes can routinely exceed 100, thus providing a com-
mensurate improvement in signal strength. The three multipass arrangements can be
enclosed in a vacuum housing, for measurement in static gas samples or controlled gas
flows, or open to ambient air (for trace gas monitoring applications).
Certain design rules must be followed to ensure that the beam exits the cell after a
controlled number of passes, especially in the case of astigmatic cells. It is important
to recognize that mirrors are not perfect, and a small portion of the probe light is lost
to absorption and scattering each time the beam bounces. Optical throughput of the
cell and peak-to-peak absorption both decrease exponentially with the number of
passes. In a system where sensitivity is limited by detector noise, for small absorptions
and maximum sensitivity, the optimum number of reflections corresponds to the
output light being 1/e times the input light. However, as long as the noise is deter-
mined by laser intensity fluctuations rather than by detector noise, additional passes
will improve S/N.
Performance of long pathlength multipass cells, especially those with dense beam
patterns, suffers from optical interference fringes caused by light scattering by the cell
mirrors even in the absence of spot overlaps. The fringe magnitude is sensitive to
optical alignment and is typically o the order of 0.01–0.1% full-scale transmission.
Mirror drift and vibration can also become a problem, as they modulate the cell
transmission. However, deliberate vibration of a mirror can reduce the effective
228 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

magnitude of interference fringes, as it scrambles the baseline variations caused by
accidental etalons within the cell.
B Cavity enhanced spectroscopies
1 Cavity ring-down spectroscopy. Cavity ring-down spectroscopy (CRDS) is a
direct absorption technique, which can be performed with pulsed or continuous light
sources and has a significantly higher sensitivity than obtainable in conventional
absorption spectroscopy.
135–142
The CRDS technique is based upon the measurement
of the rate of absorption rather than the magnitude of absorption of a light pulse
confined in an ultra-high finesse optical cavity consisting of two highly reflective
concave mirrors. The advantage over traditional absorption spectroscopy results from
intrinsic insensitivity to the probing laser intensity fluctuations and the extremely long
effective pathlengths (up to many km can be achieved in cell lengths less than 1 m) that
can be realized in stable optical cavities. CDRS spectroscopy has the additional
advantage that the absorption is measured on an absolute scale compared to other
absorption techniques, such as modulation techniques. CDRS is especially effective in
gas-phase spectroscopy for measurements of weak absorptions of abundant species.
Today, CDRS is used extensively in the visible and near-infrared. It is being investi-
gated for use in the mid-infrared with progress being determined by the availability
and cost of ultra-low loss cavity mirrors. Good mirrors give ring-down times exceed-
ing 10 µs. Although CDRS is significantly more sensitive than most absorption
spectroscopies, it cannot compete with background free techniques such as laser
induced fluorescence (LIF) or resonant enhanced multiphoton ionization (REMPI)
With pulsed lasers, CRDS requires that a short laser pulse be injected into a high
finesse optical cavity to produce a sequence of pulses leaking out through the end
mirror from consecutive traversals of the cavity by the pulse. The intensity of pulses in
such a pulse train decays exponentially with a time constant
where α is the absorption coefficient of the intracavity medium, l is the cavity length
and R is the mirror reflectivity (both mirrors are assumed to have the same reflectiv-
ity). By measuring the ringdown time, τ, without and with the absorbing gas present,
the value of α can be determined. This technique is simple and immune to laser power
fluctuations. The energy of the first pulse transmitted in the ringdown sequence is
(1⫺R)
2
times smaller than the exciting laser pulse. The mirrors for CRDS experiments
typically have R > 99.95%. This makes the measurements with the ∼1 nJ IR pulses
(P ∼ 100 mW, pulse length 10 ns) available from most cw IR sources practically
impossible.
With narrow band low power cw sources, the laser and cavity can be slowly tuned
into coincidence, filling the cavity with radiation on resonance. After the cavity is filled
with radiation, the laser emission can be interrupted and the ringdown decay meas-
ured in the same manner as is done with a pulsed laser. This cw modification of CRDS
(1)
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 229

was first introduced by Romanini et al
143,144
using cw near-infrared DFB diode
lasers. CH
4
and HF detection at 1.65 and 1.31 µm, respectively yielded a sensitivity of
∼ 10
⫺7
cm
⫺1
for atmospheric concentrations in the range 0.5 to 200 ppmv and HF
concentrations from 0.1 to 50 ppmv.
The first work on CRDS measurements with a QC-DFB laser was published in
2000.
145
The authors used a cw laser generating 16 mW at 8.5 µm. The measured
ringdown time of the empty three-mirror cavity was 0.93 µs. An acousto-optic modu-
lator was used to interrupt the cavity injection for ringdown time measurements. The
system was tested on dilute ammonia mixtures, and a noise-equivalent sensitivity of
0.25 ppbv achieved. An estimated 1.0 × 10
⫺9
cm
⫺1
absorbance was reported as the
limit of detection. Kosterev et al.
146
describe a spectroscopic gas sensor for NO
detection based on a cavity ringdown technique. A cw QC-DFB laser operating at
5.2 µm was used as a tunable single-frequency light source. The technique used has the
following features:
1. The laser frequency is slowly scanned across the absorption line of interest.
2. One of the cavity mirrors is dithered back and forth to ensure periodic, random
coincidences of the laser frequency with a cavity mode.
3. Once such a resonance occurs and the cavity is filled, the laser beam entering the
cavity is abruptly interrupted or set off-resonance, and the decay rate of the exiting
light is measured.
The noise-equivalent sensitivity was estimated to be 0.7 ppbv for an 8 sec data
acquisition time.
Interesting work
147
has been done combining a novel OPO with cavity ringdown
spectroscopy.
2 Integrated cavity output spectroscopy. A simpler (as compared to CDRS) way
to exploit a high finesse optical cavity for increasing the sensitivity to absorption
has been developed
148–153
called “integrated cavity output spectroscopy” (ICOS) or
“cavity-enhanced absorption” (CEA spectroscopy). Here, laser light is coupled into
the high finesse cavity via accidental coincidences of the light with the cavity eigen-
modes. The time-integrated intensity radiation leaking out of such an optical cavity,
averaged over many cavity modes, can then be used to characterize the absorption of
the intracavity medium. Effectively this is equivalent to a time integration of the
ringdown curve. Just as in cavity ringdown spectroscopy, an effective optical path-
length of several km can be obtained in a very small volume. However, in its most
naïve application, the noise levels are made quite high by the fact that the laser has
many transverse modes that may be excited with poorer transmission than TEM
00
.
Further when the laser and cavity are running free, the peak transmission as the two
resonances pass over each other varies radically.
Presently a variety of techniques exist to perform high-sensitivity absorption
spectroscopy in a high finesse optical cavity.
139,154–156
One of the most advanced
is called the “noise-immune cavity-enhanced optical heterodyne spectroscopy”
(NICE-OHMS) technique.
157,158
In this technique, the laser frequency is locked to the
frequency of a cavity mode. This method has the potential to provide shot-noise
limited sensitivity with an effective pathlength determined by the cavity ring-down
time. Ma et al.
157
reported a sensitivity of 10
⫺14
cm
⫺1
. This spectacular sensitivity is
superior to that achieved with CRDS. The first implementation of this technique
230 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

using a QC-DFB laser has been reported.
159
However, this approach is technically
highly sophisticated and probably is only suitable for very specific applications at the
present time.
C Optothermal
Optothermal spectroscopy is a method invented by Gough et al. for obtaining very
high resolution molecular beam spectra of molecules cooled to about 1 K by super-
sonic expansion.
160
In brief, a laser beam is crossed with a molecular beam at 90⬚. On a
molecular absorption line, the molecule is excited. When the molecular beam impinges
on a cryogenically cooled bolometer, the molecular excitation increases the bolometer
temperature and this temperature rise is detected.
With typical linewidths of a few MHz and molecules cooled to a few K, opto-
thermal spectroscopy is capable of providing very high resolution rotationally
analyzable spectra of both fairly large molecules
161
and smaller molecules with large
amplitude motions.
162,163
In a recent paper,
164
optothermal spectroscopy has been
combined with Stark state mixing to obtain information about the normally invisible
gerade levels of acetylene in the overtone region in the near-infrared.
Weakly bound complexes are readily formed in supersonic expansions of gas mix-
tures. Generally the rotational constants of such complexes are rather large because
of the large internuclear separations of van der Waals bonds. Thus optothermal
spectroscopy is a powerful means for investigating such complexes. There are several
recent publications
165–171
where the spectra of complexes were obtained optothermally
and rotationally analyzed.
Optothermal spectroscopy provides a means of investigating the dynamics of
the infrared photodissociation of complexes. In the simplest experiment, mode
specific line broadening of rotational components is observed. A better approach
is to position the bolometer detector so that it is not detecting the main beam, but
instead is detecting photofragments recoiling out of the beam.
168,172
Miller has
developed the method of studying the dissociation dynamics of complexes by
translational spectroscopy extensively and has written a recent review
173
of this
subject.
Optothermal spectroscopy is also useful for studying IVR in stable molecules
through observation of the mode mixings.
174–179
The energy region of greatest interest
in this work is that where the spacing between states interacting with intensity bearing
state is becoming comparable with the interaction matrix elements. For molecules
with three to five atoms that are heavier than hydrogen, this typically is the region of
XH stretching fundamentals (X᎐
᎐
C,N,O,Si)
175,177,178
or of the first overtone of the XH
stretch.
174,176,179
Miller and Wight have developed a method for studying scattering of molecules by
surfaces using optothermal spectroscopy and have used this to observe rainbow scat-
tering of methane from a LiF surface.
180
They are able to label the molecules with
vibrational excitation prior to their scattering from the surface, thereby being able to
investigate the retention of vibrational excitation as a function of scattering angle.
Since the molecules being scattered have very few rotational states populated after the
supersonic expansion, they can investigate rotational excitation by the scattering
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 231

event, by labeling the molecules with vibrational energy with rotational resolution
after they are scattered off the surface.
D Surfaces—sum frequency generation
Three-wave mixing processes such as sum frequency mixing, difference frequency
mixing and second harmonic generation are forbidden by symmetry in isotropic
media. Thus when a visible laser and an infrared laser impinge on an interface, any
light generated at the sum frequency is a specific probe of the anisotropy introduced
by the interface. This makes such a sum frequency technique a very valuable means for
observing the vibrational spectra of molecules adsorbed at the interface. Because the
nonlinear effect is proportional to the product of the two laser intensities and there
are few molecules in a monolayer, intense laser sources are required. As very high
resolution is not required, the tunable infrared sources employed are pulsed and high
power. The main sources used are optical parametric oscillators, difference frequency
generation, Raman shifting, or free electron lasers. The theory of this technique, its
apparatus and methodologies, and its applications have been very recently reviewed by
Buck and Himmelhaus.
181
The investigation of molecules at interfaces by sum frequency generation is a vast
subject. Since 1996 there have been at least fifteen reviews,
182–196
in addition to the one
cited above, of various aspects of this technique. The reader is referred to these
reviews for further information on this subject.
E Photoacoustic spectroscopy
Photoacoustic spectroscopy (PAS) has found its principal uses in sensitive trace gas
detection. It is based on the photoacoustic effect, in which acoustic waves result from
the absorption of radiation. Its history goes back to the work of Alexander Graham
Bell in 1880. In its application to laser spectroscopy, the laser beam impinges on a
selected target gas in a specially designed cell.
197–206
In contrast with other mid-IR absorption techniques, PAS is an indirect technique
in which the effect on the absorbing medium and not direct light absorption is
detected. Light absorption results in a transient temperature rise in the absorbing
medium via non-radiative relaxation processes, which then translates into a pressure
rise or sound wave. This is detected with a sensitive microphone or microphones.
There are two modes of operation for PAS, either the exciting light can be modulated
at a frequency away from any cell resonance or it can be adjusted to coincide with an
acoustic resonant frequency. The in-resonance mode is usually employed with the
low-power pump lasers to provide larger signals. PAS is ideally a background-free
technique: the absorbing gas generates the signal and, in the absence of an absorbing
gas, there is no acoustic signal. In real PAS experiments background signals can
originate from nonselective absorption by the gas cell windows (coherent noise) and
from outside acoustic (incoherent) noise. PAS signals are proportional to the pump
laser intensity and therefore PAS has mostly used high-power laser sources, in particu-
lar CO
2
and CO lasers. (see Section 2.E). In addition, diode lasers, solid-state lasers,
232 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

DFG and OPOs in the infrared have been applied to photoacoustic trace gas
detection.
The key features of the photoacoustic technique include (1) excellent detection
sensitivities down to sub-ppbv concentrations with powers in the watt range, (2) a
large dynamic range, (3) PAS detector responsivity almost independent of the pump
wavelength and (4) PAS signal usually directly proportional to the absorbed radiation
intensity. However, in trace gas monitoring applications, PAS is limited to point
monitoring due to the requirement of an absorption cell.
F Thermal lenses, gratings and deflection
The index of refraction of a material depends upon its temperature. The deposition
of energy into a substance by laser irradiation at a vibrational transition results
in heating. Consider an IR beam copropagating through a sample with a larger
diameter visible laser. The local heating in the center of the visible laser beam creates a
lens through the refractive index change that defocuses the visible beam. Thus thermal
lensing can be used to detect molecular absorptions. Similarly, if the two beams
are of about the same diameter and are crossed at a small angle, the heating by
the IR laser on a molecular absorption, with its consequent refractive index change,
can lead to a deflection of the visible beam. Or by creating a standing IR wave,
a thermal grating can be created in the medium and a visible laser beam diffracted
from it.
The thermal lensing methodology goes back to the middle 1970s in the work of
Albrecht and co-workers, who used it to detect overtone transitions of aromatic com-
pounds.
207
This methodology has never become popular. Since 1996, there have
appeared only two papers
208,209
using mid-IR generated thermal gratings although the
thermal grating method has been included in a review on the application of laser
spectroscopy to combustion systems.
210
Thermal deflection spectroscopy
211,212
is applied extensively to the study of thin
films. In this technique, the thermal expansion of the sample upon light absorption is
detected. This subject is rather remote from the main thrust of this review, and will not
be covered. An interesting extension
213
is the use of thermal deflection spectroscopy to
investigate electrochemical surfaces. In the gas phase, photothermal deflection has
been used to monitor ethylene emission by fruit.
214
Photothermal changes in refractive
index have been used to detect ammonia.
215
G Double resonance
Double resonance experiments were proposed
216
and carried out
217
by Bitter and
Brossel. Over the intervening years, double resonance has blossomed into a variety of
techniques of molecular spectroscopy and molecular dynamics. The introduction of a
second photon provides enormous flexibility. In the following sentences, some of this
flexibility will be described. Example papers using a tunable infrared photon as part
of the double resonance scheme will be cited. Because of the choice to cite recent
work, the primary inventors and original descriptions of the techniques to be outlined
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 233

are unfortunately likely to be slighted. Also here experiments using more than two
photons are lumped into the category of double resonance.
Double resonance methods can be classified into several categories by: (1) the
spectral regions of the two light sources (here at least one will be in the IR), (2)
the number of energy levels involved (normally three, but with collisional energy
transfer possibly four), (3) the method of detection, (4) the purpose of the experiment
(e.g. access to spectroscopic information or investigation of dynamics), and (5)
whether time resolved (pump–probe) or not.
With one light source fixed to be in the infrared, the other, in principle, could be
anywhere in the electromagnetic spectrum. In practice, the other photon involved
must be where reasonably powerful light sources are available. This in effect rules out
the soft X-ray and shorter wavelength regions. All other regions are fair game with the
proviso that if one wants to have a powerful tunable source in the far-IR access to a
free electron laser is almost mandatory.
Although more photons and energy levels may be used to detect the spectroscopic
signal, most double resonance schemes involve one intermediate level accessed by two
photons of different frequencies and can thus be categorized as three level schemes. If
the pump and probe photons belong to spectroscopic transitions with no common
level, then collisional energy transfer is used to connect the two level schemes. Such
four level schemes can be used to study collisional energy transfer,
218–227
a tech-
nique pioneered by Oka around 1970. A more unusual type of four level double
resonance scheme uses a collision to transfer population to states not easily directly
accessible, to permit spectroscopic study of transitions from these states. Either the
molecule of interest may be laser excited and the population allowed to distribute by
collision,
228
or a different chemical species may be excited and energy levels of
the molecule of interest may be then populated by near-resonant V–V energy
transfer.
229
A wide variety of detection schemes are used in double resonance experiments: (1)
absorption of one of the frequencies,
219,225,228–237
(2) LIF,
218,220–223,227,238–253
(3) detection
of fluorescence from photofragments,
238,241–245
(4) detection of ions, either parent or
product,
247,254–263
and (5) optothermal detection.
162,174,177,178,264–268
In addition to these
methods a couple of specialized techniques have been developed: (1) detection by time
of flight of a photofragment
269
and (2) detection by deflection in a molecular beam
through dressed states with optogalvanic detection.
270
This last method is somewhat
different from the relatively standard molecular double resonance method where the
two focusing fields of molecular beam electric resonance are replaced by two laser
beams that move populations with the C field being a microwave field (There appear
to be no recent examples of the standard technique using tunable IR lasers). In the
deflection experiment
270
cited above both infrared and microwave are applied simul-
taneously and the effect of the double resonance is observed with a single focusing
field. What is going on is too complex to describe here.
One purpose for carrying out double resonance experiments is to
obtain spectroscopic information, i.e. assigned energy
levels.
162,177,178,231–235,237,239–241,247–250,252–255,258,259,261,263,264,267,270
This information can then
be interpreted in terms of molecular structure. Or it can provide information about
the molecular environment as, for example the environment inside a helium drop-
let.
266,267
Or it can be interpreted in terms of intramolecular vibrational energy
234 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

redistribution (IVR) through analysis of the coupling of bright states to background
levels.
174,177,178,222,231,238,243,245,249,264,265
Typical purposes of double resonance experiments carried out in a time resolved
manner are to determine rates of collisional energy transfer
218–227
or to determine state
resolved rates of unimolecular decay.
239,242,244
Most of these time resolved experiments
have been done in the gas phase, but there is one recent example of vibrational energy
transfer between O
3
molecules in rare gas matrices.
236
Finally, double resonance can be used for analytical chemistry purposes. An
example is the use of double resonance in determining OH concentrations in the
troposphere.
251
H Miscellaneous methods
There is extensive work on laser IR multiphoton photolysis. This work is usually
done with fixed frequency CO
2
lasers and will not be covered here. Sometimes tunable
lasers are used. Thus silicon isotopes have been separated by tunable FEL
photodissociation.
271
4 Applications
There are a wide variety of applications of tunable infrared laser spectroscopy. The
principal categories of application are (1) obtaining information about molecular
structure, (2) investigating dynamics and (3) analytical applications such as trace gas
monitoring. Each of these applications will be taken up in turn.
A Spectroscopy and molecular structure
One of the traditional roles of molecular spectroscopy is to provide information about
molecular structure through the determination of molecular geometries from the
rotational constants obtained by rotational analysis and the force constant inform-
ation obtained from vibrational frequencies. As a general purpose tool for high
resolution spectroscopy, tunable laser infrared spectroscopy has severe competition
from Fourier transform infrared spectroscopy (FTIR). A large FTIR instrument is
capable of providing resolution at or very near the Doppler widths of individual gas
phase molecular absorption lines while simultaneously providing wide spectral cover-
age and rather good sensitivity. Because lasers provide a higher spectral intensity at a
given frequency than any practical blackbody sources and a very well collimated
beam, the true advantage of tunable laser sources for spectroscopy is in providing high
sensitivity and short response times. This leads naturally to the application of tunable
infrared laser spectroscopy to highly reactive systems such as free radicals, carbon
clusters, ions, various reactive metal compounds, and weakly bound complexes,
or to situations where the laser power in a small frequency region is required by
the detection scheme such as optothermal, photoacoustic, or helium droplet
spectroscopies.
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 235

The discussion below is classified by the type of system being investigated. The
categories are: (1) stable molecules, (2) radicals, (3) carbon clusters, (4) ions, (5) metal
compounds, (6) complexes and (7) condensed phase systems.
1 Stable molecules. Most of the detection methods already considered have been
used to obtain the high resolution spectra of stable molecules. It is hard to categorize
the various purposes of high resolution investigations of stable molecules. These can
range from the measurement of rotational constants, to obtaining data for under-
standing large amplitude motions, to finding suitable absorption lines for monitoring
a substance, or to obtaining highly accurate transition frequencies for metrology pur-
poses. Rather than listing the various investigations on stable molecules by method or
purpose, we choose to list them by chemical substance with notes about method and
laser source. Table 1 lists investigations of stable molecules by tunable lasers in the
mid-IR.
There have been almost as many publications involving the investigation of
vibrational overtones in the near-infrared as in the mid-IR. One reason for this is an
Table 1 Tunable mid-infrared laser investigations of stable molecules 1996–2001
Substance Method Laser source Reference
PH
3
Absorption Sideband CO
2
272
CH
2
FCN Absorption Diode 273
CH
2
BrF Absorption Diode 274
CH
2
FCl Absorption Diode 275–277
CHBrClF Absorption⫹others Diode 278
CS
2
Absorption Diode 279
CH
3
I Double resonance 235
HCCH Absorption Diode 280
ICN Absorption Diode 281,282
OCS Abs, sub-Doppler Sideband overtone CO 283–285
CH
2
᎐
᎐
(CHCl) Absorption Diode 286,287,298
C
2
F
6
Abs. jet-cooled Diode 288
C
6
H
6
Abs. jet-cooled Color center 289
SF
6
Double resonance Sideband CO
2
290
NH
3
Double resonance 233,234
H
2
CCO Double resonance jet-cooled 241
CHCl
3
Abs. jet-cooled Diode 291
C
2
H
6
Optothermal Color center 163
H
2
FCCH
2
F Optothermal electric resonance Sideband CO
2
161
HF
2
CCH
2
F Optothermal electric resonance Sideband CO
2
161
HF
2
CCHF
2
Optothermal electric resonance
double resonance
Sideband CO
2
292
Cis-HClC᎐
᎐
CFH Absorption Diode 293–295
Cis-CHF᎐
᎐
CHF Absorption Diode 296,297
CH
3
OH Double resonance Sideband CO
2
237
CH
3
OH Optothermal electric resonance
double resonance
162
Absorption
CD
3
OD Absorption Waveguide CO
2
299
F
2
C᎐
᎐
CHF Absorption Diode 300,301
NO Abs, sub-Doppler Sideband CO 302
CO Absorption Sideband CO 303
236 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

interest in IVR, which will be discussed later. Perhaps another is that the development
of near-IR laser diodes is far more advanced than that of mid-IR laser diodes making
work in the near-IR easier. In Table 2 are listed the publications in the near-IR on the
overtone spectra of stable molecules not aimed specifically at obtaining information
about IVR as IVR is a subject for later discussion.
2 Radicals. Molecular species that are highly reactive or ionic have always fascin-
ated spectroscopists. Starting almost two hundred years ago, the electronic spectra of
very many diatomic molecules have been investigated by their emission in the visible
or ultraviolet. Because the excited electronic states of polyatomic molecules can usu-
ally find a way to dissociate, a much smaller number of polyatomics have been studied
in emission. The electronic absorption spectra of quite a few polyatomic molecules
have been studied, mostly in the UV or visible. However, absorption spectroscopies
of vibrational or rotational transitions in the infrared and microwave offer a very
significant advantage over spectroscopy of electronic transitions, because dissoci-
ation of the upper state is impossible energetically. Thus there has been much work in
these spectral regions on the spectra of transient species: radicals, ions, and high
temperature molecules.
There has been a recent (2000) general review of this area by Hirota
322
in this series.
For constantly updated information, Jacox maintains a database Vibrational and
Electronic Energy Levels of Polyatomic Transient Molecules online as part of the NIST
Webbook (http://webbook.nist.gov/), This is based on her original monograph,
323
which she also updates in print regularly.
324
In Table 3, publications on radicals since
Hirota’s review are listed. The criteria for inclusion in Table 3 are much more restrict-
ive than Hirota’s review. To be listed in Table 3, a paper must be a tunable laser
spectroscopic investigation of a nonmetallic radical and not be an ion or a carbon
cluster.
Here we will focus primarily on methodologies for investigating free radical spectra,
as the work of Hirota
322
has provided such excellent coverage of the literature through
Table 2 Spectroscopic investigations of stable molecules in the near-infrared
Molecule Method Special aspects Reference
HCCH Optothermal Stark effects 164
H
3
SiD Photoacoustic Also FTIR 304
H
3
CNO
2
(Also D substituted) Photoacoustic Also FTIR 305–307
C
6
H
5
CH
3
(D substituted species) Photoacoustic Also FTIR 308,309
H
2
O (D substituted species) Photoacoustic
311
D
2
O;
312
HDO 310,311
ClH
2
CCH
2
Cl Photoacoustic Also FTIR 312
HCCCN Double resonance 231
C
5
H
5
N Photoacoustic Also liquid and non-laser 313
ClCN Photoacoustic Ti:sapphire source 314
HSiF
3
Photoacoustic Ti:sapphire source 315
HCN Photoacoustic Ti:sapphire source 316
SbH
3
Photoacoustic Ti:sapphire source 317
CH
3
C
5
H
4
N Photoacoustic Also non-laser 318
CO
2
Photoacoustic 319
OCS Photoacoustic Ti:sapphire source 320
2,6-Difluorotoluene Photoacoustic Ti:sapphire source 321
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 237

1999. Generally there are four methods for the production of radicals for spectro-
scopic investigation:
1. passing an electrical discharge through a gaseous mixture of a buffer gas contain-
ing suitable precursors,
2. producing a flow of highly reactive species usually an atom (F is most used) by a
microwave gaseous discharge, which is then mixed with a suitable precursor to
produce the radical by a chemical reaction (typically hydrogen atom extraction),
3. flash photolyzing a suitable precursor causing bond rupture,
4. pyrolysis of more stable compounds.
As can be seen from Table 3, the first three methods are used almost exclusively for
normal cell absorption spectroscopy. If pyrolysis is used in conjunction with an
absorption cell, the pyrolysis is done outside the cell with a fast flow system carrying
the pyrolysis products into the cell, since flash pyrolysis of large volumes is difficult.
This restricts its use to longer-lived species. However, jet pyrolysis is proving to be a
powerful technique.
336
The production of radicals for spectroscopic study by electric discharge has been
used effectively and extensively for many years. As a means for the production of
radicals in cells, it remains very much a hit and miss proposition. While much is
known about the nature of glow discharges in gases, the neutral chemistry in the
discharge, which is driven most often by dissociative electron bombardment, is very
rarely understood. In contrast, the production of ions for spectroscopic study tends to
be better understood as a result of decades of investigations of ion–molecule
reactions.
Discharge flow methods using F atom abstraction of hydrogen for radical produc-
tion usually produce predictable radical products. The products of O atom
reactions are somewhat less predictable, but this method for radical production is
highly effective and was used in the investigations of ClBO
329
and BO
329
cited in Table
3.
The use of flash photolysis to produce radicals goes back to the pioneering work of
Norrish and Porter
337
in the late 1940s. The flash photolysis method, which we think is
most appropriately called “kinetic spectroscopy” was used extensively by Herzberg,
Table 3 Infrared radical publications 2000–2001
a
Species Production Method Laser source Reference
D
3
Discharge Absorption DFG 325
HOPO Discharge Absorption Diode 326
CH
3
CH
2
Discharge Jet abs DFG 327
ClBO Rxn with disch. prod. Absorption Diode 328
BO Rxn with disch. prod. Absorption Diode 329
HCCN Flash photolysis Absorption DFG 330
CN Discharge Absorption Ti:sapphire 331
HCBr Flash photolysis Absorption Ti:sapphire 332
CCN Discharge Absorption Diode 333
CH
2
Flash photolysis Absorption Ti:sapphire 334
BBr Discharge Absorption Diode 335
a
The year 2001 is incomplete.
238 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

Ramsay and their coworkers to investigate the electronic spectra of many small
radical species in the decades of the 1950s through the 1980s. The adaptation of
kinetic spectroscopy to use with tunable infrared lasers was first carried out by Kim
et al.
338
and Laguna and Baughcum
339
at Los Alamos National Laboratory. Since we
will be returning to this technique in conjunction with the use of tunable infrared laser
spectroscopy for kinetics investigations (see Section 4.B.1), it seems appropriate to
discuss it in somewhat more detail here.
In the original version of kinetic spectroscopy,
337
the absorption spectrum of
radicals was obtained on a photographic plate by firing a photographic flash at known
time delay after the photolysis flash. Thus spectra at all relevant wavelengths at
a single time after the flash were obtained in a single sequence of two flashes. To use a
tunable monochromatic cw infrared laser as the spectroscopic probe, data at a single
wavelength but all relevant times are collected with a transient digitizer in a single
flash. The photolysis laser is usually an excimer laser permitting the repetition of the
experiment at 10–100 Hz as the probe laser is slowly scanned for a spectroscopic
investigation. Alternatively the probe laser may be fixed on the peak of an absorption
line and the time behavior of the absorption signal may be acquired with averaging
over repeated flashes for chemical kinetics studies. Radical chemical kinetics will be
discussed in Section 4.B.1.
Instead of using excimer laser flash photolysis, mercury sensitized photolysis may
be carried by introducing a small amount of mercury vapor into the absorption cell
and flash photolyzing with a mercury flash lamp.
340
a Spectroscopy of jet-cooled radicals. The observation and analysis of the high
resolution mid-IR spectra of transient free radical molecules provides a powerful and
nearly universal means for obtaining information about their molecular structure
and a definitive means for monitoring their concentrations. However, the rotational
assignment of the infrared spectra of free radicals becomes increasingly difficult as the
size of the species increases. Typically, for rigid species at dry ice temperature, assign-
ment is just feasible for molecules containing three first row atoms. If large amplitude
motions such as low barrier internal rotations or tunnelings are present, even mole-
cules with only two first row atoms, e.g. CH
3
CH
2
or H
2
COH, produce dry ice temper-
ature mid-IR spectra that are complex and difficult to analyze. As is well known, the
rotational congestion causing this problem can be greatly reduced by supersonic
expansion cooling to a few K, thereby greatly reducing the number of rotational levels
thermally populated.
Radicals can be produced in the nozzle of a slit jet, cooled by supersonic expansion,
and their infrared spectra observed by absorption spectroscopy. Electric discharge,
341
flash photolysis,
342
or pyrolysis
336,343
may be used to produce the radicals. The radical
of interest is produced in relatively high concentration (∼10
15
cm
⫺3
) at or just inside
the nozzle of a slit jet expansion. Only a moderate loss of the radicals by chemical
reaction typically takes place during the expansion process. Expansion greatly reduces
the concentration of the radicals, but the adiabatic cooling of the supersonic expan-
sion process is simultaneously reducing the number of rotational states populated.
For small quantities of radicals and precursor seeded into a rare gas expansion, the
temperature is proportional to n
⫺2/3
(where n is the number density) by the thermo-
dynamics of reversible adiabatic expansion. For a nonlinear polyatomic radical, the
rotational partition function is proportional to T
3/2
. Therefore, the number of radicals
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 239

in the lowest rotational state is unchanged by the expansion. Reasonable signals can
be expected and are indeed observed. With an absorption pathlength of 50 cm
obtained by using a slit 2 cm in length and 25 passes with a Herriott cell, an absorp-
tion of 5% has been observed
344
for CH
3
produced by flash photolysis of CH
3
I.
Each of the production methods has advantages and disadvantages. The work of
Nesbitt
327,341,345–347
uses a very short discharge region (∼1 mm) just prior to the expan-
sion so that the gas is subjected to the hostile environment of the discharge for only
1–2 µs. Nesbitt has often been able to find an I- or Br-substituted precursor where
the radical formation is likely be the result of dissociative electron attachment in the
discharge.
where X = Br or I. The duty cycle of the system is relatively high with the discharge
being on for ∼1 ms per gas pulse and a repetition rate of ∼10 Hz. The radical
concentration may be AC modulated at up 100 kHz, greatly increasing sensitivity.
347
The principal advantage of flash photolysis radical preparation over the electrical
discharge is that the photolysis process is usually well understood and can be rather
reliably tailored to produce the species of interest. Excimer laser flash photolysis
typically produces a somewhat higher signal than electric discharge.
344
However, the
duty cycle is quite low as the radical is present for only about 3 µs for each gas pulse.
However, because the detector level just before the absorption signal appears is sub-
tracted from the detector level when the signal is present, low frequency noise is
effectively eliminated.
Pyrolysis can also be a very clean source of the species of interest.
343
As a radical
preparation method, it has been developed to a very high degree by Chen
348
for mass
spectrometric investigations. Because the final jet temperature is proportional to the
pre-expansion temperature, the temperature in the probe region tends to be higher,
e.g. 45 K for a nozzle temperature of 1500 K, as compared with 15–20 K for discharge
preparation and as low as 8 K for flash photolysis preparation.
At present there are only a few publications from the time period after the Hirota
review, most of which are concerned with technique and all of which have already
been cited. There are only three radicals that have been extensively investigated:
allyl,
346
chloroketene
343
(if chloroketene can be counted as a radical), and ethyl.
327
However, carbon clusters have been produced and observed with jet cooling and will
now be discussed.
3 Carbon clusters. The spectra of carbon clusters, carbon cluster ions, and slightly
hydrogenated carbon clusters have been the subject of intense investigations for more
than a decade. Much of this work has been in the visible and UV (John Maier) or in
the microwave (Gottlieb, Thaddeus, and McCarthy) and therefore is not covered by
the present review. Saykally, who has been at the forefront of this research in the IR,
has recently reviewed the field.
349
Subsequent to this review by Vanorden and Saykally,
there has been only one IR gas phase carbon cluster paper
350
on linear C
10
.
4 Ions. The vibrational spectra of ions are covered by the Hirota review
322
and the
Jacox database. If we exclude ions in clusters or complexes with neutral molecules,
RX ⫹ e
⫺
R ⫹ X
⫺
240 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

only a few papers on ions have been published since 1999. Several are on H
3
⫹
, includ-
ing a review.
351
There is a paper on combination bands of H
3
⫹
,
352
and two on H
3
⫹
kinetics in the discharge.
353,354
There is one paper
355
on the near-IR electronic transi-
tion of CS
⫹
studied in absorption with a Ti:sapphire laser source. There are two
papers
263,356
on special spectroscopic techniques for investigating ions. In one,
356
ions
of the aromatic compounds, naphthalene, phenanthrene, anthracene, and pyrene, are
created by excimer laser ionization and stored in a quadrupole ion trap. They are then
irradiated with a free electron laser and undergo multiphoton fragmentation. The
fragment ions are detected by mass spectrometry, thereby producing the infrared
spectrum of the parent ions. In the second paper,
263
high Rydberg states are produced
by two color excitation. These are then vibrationally excited with an infrared laser and
the IR absorption detected by the consequent autoionization.
A number of papers on clusters or complexes between ions and neutrals have been
published. These will be discussed in the sections on complexes.
5 Metal compounds. Five experimental spectroscopy papers using tunable infrared
lasers on metal compounds have been published in the period 2000 to 2001. The first is
a diode laser absorption investigation at 1000 K of the vibrational spectrum of
RbCl
357
in which vibrational states up to ν⬙=8 and J⬙=160 have been observed and
assigned. Two
358,359
are jet cooled spectra of transition metal carbonyl compounds
from the Davies group: iron pentacarbonyl
358
and tungsten hexacarbonyl.
359
The final
papers describe spectra observed through thermionic emission produced by multi-
photon excitation of nanocrystals with a free electron laser. In one, the spectra of
zirconium oxide nanocrystals were observed.
360
In the other, vanadium carbide nano-
crystals from V
14
C
12
to V
32
C
32
, were observed.
361
In addition to spectroscopic investi-
gations, reaction kinetics of metal compounds can also be investigated using infrared
kinetic spectroscopy (see Section 4.B.1).
6 Complexes. For the reader deeply interested in complexes, in late 2000 an
entire issue of Chemical Reviews (vol. 100, Issue 11, November 2000) was devoted
to van der Waals molecules. Two species can be bound together by forces weaker
than a normal chemical bond. The energy of the bonding can range from those of
very weak van der Waals attractions of a few cm
⫺1
to nearly the strength of a
chemical bond from the interaction between a Lewis acid and a Lewis base. The
types of forces can be enumerated as (1) van der Waals, (2) Lewis acid–base, (3)
ion–molecule interactions. (4) charge transfer interactions, (5) multipole inter-
actions and (6) H-bonded interactions. The criteria used for a van der Waals
molecule in the special issue of Chemical Reviews appears to include most of these
interactions.
a Van der Waals, multipole, and Lewis acid–base bonding complexes. The field of
complexes bound by van der Waals, multipole (e.g. dipole–dipole) or Lewis acid–base
bonding remains active. Table 4 gives the titles and references to tunable infrared laser
studies in this area.
b H-bonded. In Table 5 publications from 1996 to 2001 on neutral hydrogen bond-
ing complexes are listed. Of particular interest is Saykally’s mammoth project, which
accounts for about half of Table 5, to determine the potential function for liquid
water through studying water clusters.
378
Most of his work has used tunable
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 241

far-infrared radiation obtained by mixing a tunable microwave source with a fixed
frequency far-IR laser.
c Ion–molecule clusters. High resolution investigations of clusters containing ions
have been reviewed in 2000 by Hirota
322
and by Bieske and Dopfer.
404
Publications in
this area in 2000 that may have been published too late for the review of Bieske and
Dopfer
404
and 2001 publications are listed in Table 6.
In addition to the high resolution investigations included in these two reviews, lower
resolution studies of ion complexes are also carried out. In 1997, Lisy reviewed the
work on solvated alkali metal ions.
406
Table 7 lists lower resolution tunable infrared
laser publications on ion complexes.
7 Condensed phase systems. Typically, condensed phase systems have phonon
broadened lines. Tunable IR lasers therefore offer no advantages in terms of
resolution over FTIR. Normally very high sensitivity is not required for condensed
systems so that the potentially higher sensitivity of laser methods is unimportant.
However, there are a few special circumstances where tunable infrared lasers prove
valuable.
a Hole burning and isomerization spectroscopies. In the solid phase, infrared lasers
may be used to burn holes in heterogeneously broadened lines. Strauss has used such
hole burning to investigate various types of orientational isomerization in a number
Table 4 Tunable infrared laser studies of van der Waals molecules
Article title Reference
Near-infrared laser spectroscopy of the Ar–C
2
HD complex—Fermi resonance
assisted vibrational predissociation
362
Infrared spectra of the Kr–CO and Xe–CO van der Waals complexes 363
Slit jet infrared absorption spectroscopy of N
2
–HCl complexes 364
Vibrational predissociation of an inert gas cluster containing an active molecule—
the ν
3
spectrum of Ar
3
–HF
365
A study of the intermolecular ν
5
1
vibration in OC–(HCl)–Cl-35 based on
near-infrared spectroscopy
366
High-resolution infrared diode laser spectroscopy of Ne–N
2
O, Kr–N
2
O, and
Xe–N
2
O
367
Pulsed molecular beam infrared absorption spectroscopy of HCCH–CO 368
Gentle recoil synthesis of free-radical clusters via unimolecular photolysis of closed
shell complexes
369
Identification of the OC–IH isomer based on near-infrared diode laser
spectroscopy
370
Isotopic probing of very weak intermolecular forces: Microwave and infrared
spectra of CO–He isotopomers
371
Rovibrational spectroscopy of the C
2
H
2
–Ar van der waals complex, using a
fluorescence depletion infrared–ultraviolet double resonance technique
239
The infrared spectrum and internal rotation barrier in HF–BF
3
166
High resolution infrared spectroscopy and structure of CO–N
2
O 372
Spectroscopy and structure of the open-shell complex O
2
–N
2
O 373
Intermolecular state dependence of the vibrational predissociation of N
2
HF 374
Free-jet IR spectroscopy of SiF
4
–N
2
and SiF
4
–CO complexes in the 10 µm region 376
High resolution mid-infrared spectroscopy of Ar–H
2
O—the ν
2
bend region of H
2
O 377
The infrared spectrum of the Ar–CO complex—comprehensive analysis including
van der Waals stretching and bending states
377
242 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

salts and polymers.
428–441
A different kind of experiment with a similar flavor has been
carried out by Roubin et al.
442
Here the rotational isomerization induced by free
electron laser irradiation of CH
2
D–CH
2
D isolated in rare gas matrices was
investigated.
Table 5 Tunable infrared laser publications of hydrogen bonded non-ionic complexes
Title Reference
Probing hydrogen bond potentials via combination band spectroscopy—
a near-infrared study of the geared bend van der waals stretch intermolecular
modes in (HF)
2
379
Electronic and infrared spectra of jet-cooled 4-aminobenzonitrile–H
2
O. Change of
NH
2
from proton acceptor to proton donor by CN substitution
380
Far-infrared laser vibration–rotation–tunneling spectroscopy of water clusters in
the librational band region of liquid water
381
High symmetry effects on hydrogen bond rearrangement: The 4.1 THz vibrational
band of (D
2
O)
4
382
Vibrational spectroscopy of single methanol molecules attached to liquid water
clusters
383
Infrared spectrum of the water–carbon monoxide complex in the CO stretching
region
385
Characterization of the (D
2
O)
2
hydrogen-bond-acceptor antisymmetric stretch by
IR cavity ringdown laser absorption spectroscopy
386
4.5 µm diode laser spectrum of (HI)
2
386
The infrared spectroscopy and dynamics of OCO–HCl and SCO–HCl—
an example of mode specific intermolecular energy transfer
168
The far-infrared vibration–rotation–tunneling spectrum of the water tetramer-d
8
387
Quantifying hydrogen bond cooperativity in water—VRT spectroscopy of the
water tetramer
388
Far-infrared VRT spectroscopy of two water trimer isotopomers—vibrationally
averaged structures and rearrangement dynamics
389
Characterization of a cage form of the water hexamer 390
Vibration-rotation tunneling spectra of the water pentamer—structure and
dynamics
391
Terahertz laser vibration–rotation–tunneling spectroscopy of the water tetramer 392
Terahertz laser vibration–rotation–tunneling spectroscopy and dipole moment of a
cage form of the water hexamer
393
Direct measurement of water cluster concentrations by infrared cavity ringdown
laser absorption spectroscopy
394
Pseudorotation in water trimer isotopomers using terahertz laser spectroscopy 395
Terahertz laser vibration–rotation–tunneling spectrum of the water pentamer-d
10
—
constraints on the bifurcation tunneling dynamics
396
Infrared cavity ringdown spectroscopy of water clusters: O–D stretching bands 397
Quantitative characterization of the water trimer torsional manifold by terahertz
laser spectroscopy and theoretical analysis. II. (H
2
O)
3
398
Infrared cavity ringdown spectroscopy of the water cluster bending vibrations 399
Quantitative characterization of the (D
2
O)
3
torsional manifold by terahertz laser
spectroscopy and theoretical analysis
400
Terahertz laser spectroscopy of the water dimer intermolecular vibrations. I. (D
2
O)
2
401
Terahertz laser spectroscopy of the water dimer intermolecular vibrations.
II. (H
2
O)
2
402
Terahertz vibration–rotation–tunneling spectroscopy of water clusters in the
translational band region of liquid water
403
Far-infrared laser vibration–rotation–tunneling spectroscopy of water clusters in
the librational band region of liquid water
382
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 243

b Para-H
2
matrix isolation. In recent years Oka and Momose have developed the
field of matrix isolation spectroscopy using para-H
2
as the matrix. Momose has pub-
lished a review of some of this work.
443
This extremely soft matrix permits rotation of
the isolated molecules for larger species than the harder rare gas matrices traditionally
used. Usually FTIR is used for these investigations, but often the resolution provided
by FTIR is insufficient. The papers published since 1996 in this area using tunable IR
lasers are given in Table 8.
Table 6 High resolution studies of ion–molecule clusters in 2000 and 2001
Title Reference
Microsolvation of the water cation in argon: II. Infrared photodissociation spectra
of H
2
O
⫹
–Ar
n
(n = 1–14)
406
Observation of the infrared spectrum of the ν
3
band of the argon–ammonium ionic
complex
407
Potential energy surface and infrared spectrum of the Ar–H
2
Cl
⫹
ionic complex 408
The intermolecular potential of NH
4
⫹
–Ar II. Calculations and experimental
measurements for the rotational structure of the ν
3
band
409
Linear and centrosymmetric N
2
ⴢAr
⫹
ⴢN
2
410
High resolution infrared direct absorption spectroscopy of ionic complexes 411
Infrared spectrum and ab initio calculations of the HNH
⫹
Ne open-shell ionic
dimer
412
Intermolecular potential energy surface of the proton-bound H
2
O
⫹
–He dimer:
Ab initio calculations and IR spectra of the O–H stretch vibrations
413
Microsolvation of the water cation in neon: Infrared spectra and potential energy
surface of the H
2
O
⫹
–Ne open-shell ionic complex
414
Spectroscopic and theoretical characterisation of the ν
2
band of ArⴢDN
2
⫹
415
Table 7 Lower resolution studies of ion–molecule clusters 1996-2001
Title Reference
Infrared spectroscopy of NH
4
⫹
(NH
3
)
n⫺1
(n = 6–9)—shell structures and collective ν
2
vibrations
416
Electronic structures of cobalt cluster cations—photodissociation spectroscopy of
Co
n
⫹
Ar (n = 3–5) in the visible to near-infrared range
417
Pump–probe photodepletion spectroscopy of (C
6
H
6
)
2
⫹
—identification of spectrum
in the charge resonance band region
418
Characterization of the hydrogen-bonded cluster ions 419
Hydrogen bonding in metal ion solvation—vibrational spectroscopy of
Cs⫹(CH
3
OH)
(1–6)
in the 2.8 µm region
420
Vibrational predissociation spectroscopy of Cs
⫹
(H
2
O)
(1–5)
421
Microsolvation of the ammonium ion in argon—infrared spectra of NH
4
⫹
–Ar
n
complexes (n = 1–7)
422
Energy partitioning following the infrared photofragmentation of SF
6
ⴢ(C
6
H
6
)
⫹
cluster ions
423
Structures and isomeric transitions of NH
4
⫹
(H
2
O)
(3–6)
—from single to double rings 424
Infrared spectroscopy of resonantly ionized (Phenol)(H
2
O)
n
⫹
425
Infrared spectroscopy of jet-cooled neutral and ionized aniline–Ar 260
The infrared spectrum of the benzene–Ar cation 426
Charge transfer interaction in the acetic acid–benzene cation complex 427
244 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

c Helium droplets. Another sort of matrix medium is nanodroplets of helium
formed into a molecular beam and probed optothermally. The probing radiation can
be in any part of the electromagnetic spectrum. Thus only part of the research in the
area of helium nanodroplets also involves tunable infrared laser spectroscopy. There is
much work, for example, in electronic spectroscopy. A special topics section of The
Journal of Chemical Physics has just appeared (vol. 115, Issue 22, December 8, 2001)
containing several minireview articles about the general subject. One
453
of these
reviews is directly concerned with tunable infrared laser spectroscopy of helium drop-
lets. There are also several rather recent reviews of the general area.
454–458
A fascinating
aspect of these systems is that often the impurity species in the droplet being probed
exhibits rotational structure, but with smaller rotational constants, which are 70% (or
more) of those of the free molecule for hydrides, but otherwise often much smaller
than those of the free molecule. Because this subject has been covered so extensively
so recently, it will not be considered further here.
d Surfaces. Tunable infrared lasers are used in a variety of ways to probe surfaces
and interfaces. By far the most important methodology is sum frequency generation at
the surface. The basis for this method has been described and references to fifteen
reviews related to this subject have been given in a previous Section on techniques
(3.D). As the subject is so vast that it demands an entire review to itself, no further
discussion of the general area will be given here. However, a promising develop-
ment
459
in surface imaging by near-field microscopy combined with sum frequency
generation demands to be mentioned.
Apart from the area of sum frequency generation, there are other interesting appli-
cations of tunable infrared laser spectroscopy to surface science. These can be broken
down into experiments probing the surface itself or experiments probing gas phase
molecules. The gas phase molecules can be either scattering from the surface,
180
desorbing from the surface after chemical reaction
460
or subliming from the surface
without chemical reaction.
461
Alternatively the sticking coefficients of gas phase
molecules, laser excited vibrationally, may be explored.
462
The vibrational spectra of
molecules on the surface may be investigated using various non-sum frequency laser
techniques.
463–466
Table 8 Papers using tunable infrared lasers and para-H
2
matrices since 1996
Title Reference
High-resolution infrared spectroscopy of isotopic impurity Q
1
(0) transitions in
solid parahydrogen
444
Infrared spectroscopic study of rovibrational states of methane trapped in
parahydrogen crystal
445
High-resolution laser spectroscopy of methane clusters trapped in solid
parahydrogen
446
Impurity molecules in parahydrogen crystal—high resolution infrared spectroscopy 447
Tunneling chemical reactions in solid parahydrogen—a case of CD
3
⫹H
2
CD
3
H⫹H at 5 K
448
High-resolution infrared spectroscopy of the J = 1 H
2
pair in parahydrogen crystals 449
High-resolution spectroscopy of ions in gamma-ray irradiated solid parahydrogen 450
Observation of the high-resolution infrared absorption spectrum of CO
2
molecules
isolated in solid parahydrogen
451
Sharp spectral lines observed in gamma-ray ionized parahydrogen crystals 452
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 245

B Dynamics
Tunable infrared laser spectroscopy can be applied to the study of various dynamic
processes. These can either be the chemical kinetics studies of overall rates or various
types of state-to-state dynamics.
1 Chemical kinetics. The kinetics of chemical reactions can be probed with infra-
red kinetic spectroscopy (IRKS) as described briefly in the techniques section. This
topic has been the subject of a quite recent (2001) review.
467
Some additional
articles
468–471
in the area appeared in 2001. The Taatjes and Hershberger review
467
does not cover kinetics work on metal compounds.
472–479
2 State-to-state dynamics. For the past thirty-five years, many physical chemists
have been fascinated with unraveling the dynamical behavior of molecules at the level
of individual quantum states or at least of a fairly restrictive collection of quantum
states. These processes can take the form of dynamical processes taking place in a
single isolated molecule or they can result from collisions between molecules. For the
single molecule the processes are (a) the closely related processes of intramolecular
vibrational redistribution (IVR) and unimolecular decomposition and (b) photolysis.
Collisional processes include (i) pressure effects on spectroscopic lines, (ii) energy
transfer and (iii) its competition with unimolecular decay, and (iv) reactive collisions.
a IVR and unimolecular decomposition. In its most simplistic form, the normal
means for applying spectroscopy to IVR is through the Fourier transform theorem.
By taking high resolution infrared spectra of a polyatomic molecule, the energy
eigenstates that couple with the radiation field can be observed and their intensity
measured. For small molecules or low photon energies, the density of states at the
level of the upper state of the spectroscopic transition is small and the observed
transitions can be readily characterized by the usual language of normal modes,
fundamentals, overtones, combination bands, and the semirigid rotor. However, at
higher energies, where the density of states is large, the ‘bright’ spectroscopic states
that carry the oscillator strength begin to interact with the ‘dark’ states that carry
negligible oscillator strength through anharmonic terms in the Hamiltonian and
Coriolis interactions. By characterizing the frequency spread over which a bright state
shares its intensity and, statistically, the number of new transitions observed, inform-
ation can be garnered about the strength of the coupling of the bright states to a
subset of the background states and the number of actively coupling background
states. Through the Fourier transform theorem coupling frequency behavior to
time behavior, this provides information about the initial flow of energy from the
vibrational state excited into other nearby states, and thus about IVR.
In reality the subject is far more complicated than the simple description
given above and there has been considerable progress in it. Although there are a
number of recent reviews of various aspects of the area, the most recent reviews
480–482
that appear to be fairly directly related to infrared laser spectroscopy date back several
years.
In the area relating IVR to unimolecular decomposition, there is a very recent
review by Callegari and Rizzo
483
that describes a method for using a near-infrared
laser to select a state that is then further pumped up by a second laser to a vibrational
246 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

state above the dissociation threshold with the OH reaction products observed and
probed by state by pumping them in the UV and observing the fluorescence.
There are a number of papers in the area that are listed in Table 9.
The rate of unimolecular decay can be measured by measuring linewidths. An
example is the investigation of the decay of CH
4
-OH complexes by an infrared–
Table 9 Publications in IVR and unimolecular decomposition relevant to tunable IR laser
spectroscopy since 1996
Title Reference
Molecular-beam infrared–infrared double-resonance spectroscopy study of the
vibrational dynamics of the acetylenic C–H stretch of propargyl amine
177
Transient electronic absorption of vibrationally excited CH
2
I
2
: Watching energy
flow in solution
484
Secondary time scales of intramolecular vibrational energy redistribution in CF
3
H
studied by vibrational overtone spectroscopy
238
Intramolecular energy transfer in highly vibrationally excited methanol. I. Ultrafast
dynamics
485
Intramolecular energy transfer in highly vibrationally excited methanol.
II. Multiple time scales of energy redistribution
243
Intramolecular energy transfer in highly vibrationally excited methanol.
III. Rotational and torsional analysis
486
Eigenstate resolved infrared–infrared double-resonance study of intramolecular
vibrational relaxation in benzene—first overtone of the CH stretch
264
The spectroscopy and intramolecular vibrational energy redistribution dynamics
of HOCl in the v(OH) = 6 region, probed by infrared-visible double resonance
overtone excitation
245
Intramolecular vibrational redistribution in aromatic molecules. I. Eigenstate
resolved CH stretch first overtone spectra of benzene
265
Doorway state enhanced intramolecular vibrational energy redistribution in the
asymmetric ᎐
᎐
CH
2
hydride stretch of methyl vinyl ether
178
Chromophore dependence of intramolecular vibrational relaxation—Si–H stretch
second overtone versus C–H stretch first overtone in methylsilane
179
Vibrational relaxation of OH and OD stretching vibrations of phenol and its clus-
ters studied by IR–UV pump–probe spectroscopy
487
High-resolution spectrum of the 3ν
1
band of cyanoacetylene obtained via infrared/
infrared double resonance
231
Molecular beam infrared spectrum of nitromethane in the region of the first C–H
stretching overtone
176
Relaxation within and from the (3
1
/2
1
4
1
5) and (3
1
4
1
/2
1
4
1
5
1
) Fermi dyads in
acetylene—vibrational energy transfer in collisions with C
2
H
2
, N
2
and H
2
222
State-to-state studies of intramolecular energy transfer in highly excited
HOOH(D): Dependencies on vibrational and rotational excitation
249
Intramolecular vibrational dynamics of the asymmetric ᎐
᎐
CH
2
hydride stretch of
isobutene
175
Eigenstate resolved infrared and millimeter-wave–infrared double resonance
spectroscopy of methylamine in the N–H stretch first overtone region
174
Sub-doppler infrared spectra and torsion–rotation energy manifold of methanol in
the CH-stretch fundamental region
162
Sub-doppler infrared spectra of the OH-stretch fundamental of C-13-methanol 488
State-to-state studies of intramolecular dynamics and unimolecular reactions 489
Hydrogen bond breaking dynamics of the water trimer in the translational and
librational band region of liquid water
490
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 247

UV double resonance method.
252
As mentioned previously, the optothermal method
may be used to investigate unimolecular dissociation of complexes.
173,491
b States produced by photolysis. Individual states produced by photolysis can be
investigated by infrared absorption spectroscopy. A recent example is the determin-
ation of the branching ratio into O(
1
D) in the photolysis of NO
2
at 193 nm.
492
Zhang
and Hossenlopp
493
have observed the bending excitation of CO
2
when anhydrides are
photolyzed using IR diode laser probing. The vibrational excitation of CD
3
from the
flash photolysis of d
6
-acetone has been estimated.
494
with IR diode laser probing.
Alternatively, a particular rovibrational state of a molecule
269,495
or cluster
496
can be
prepared with IR laser pumping, the system UV photodissociated and final products
probed by LIF.
c Collisional processes. There are several processes that result from molecular
collisions: pressure effects on spectroscopic lines (e.g. line broadening or narrowing),
intermolecular vibrational energy transfer, rotational energy transfers, and chemical
reaction. Tunable infrared laser spectroscopy has been used to investigate all of these
processes.
i Pressure effects on spectroscopic lines. There have been several recent investi-
gations of pressure effects on lineshapes.
497–510
Some investigate pressure broadening
of individual lines
505,508,510
including, in some cases,
508,510
theoretical treatments. There
is also a recent study showing collisional line narrowing
509
and several investigations
of line mixing phenomena have been reported.
506,507,510
Finally there is a report
237
of
pressure effects on the microwave line shape in microwave optical double resonance
investigations of methanol using, as the IR source, microwave sidebands of a CO
2
laser.
ii Vibrational and rotational energy transfer. There is a not very recent (1996)
review in this area.
511
Vibrational energy transfer of small molecules in the gas
phase has been investigated
512
for very highly excited OH (ν=9). Vibrational relax-
ation can also be investigated in matrix isolation by pump–probe methods as has
been done for ozone in rare gas matrices.
236
There have been several studies, by two
different groups, of vibrational energy transfer including rotational state changes in
acetylene using IR–UV double resonance.
220–224,227,513
Infrared–UV double reson-
ance has also been employed to investigate rotational changes in vibrational
energy transfer in NO.
218,226
Infrared–infrared double resonance has been used to
investigate vibrational and rotational energy transfer in ozone.
225
Nesbitt has
developed a method for studying rotational energy transfer in a time and fre-
quency resolved manner.
514–516
He has extended the method to resolution of
translational energies using the Doppler effect.
517,518
Several other ingenious methods
for investigating collisional energy transfer have been developed: Hermans et al. use
light-induced drift;
519
Millot and Roche populate a particular state by SEP and
then follow the population shifts upon collision with IR laser absorption spectro-
scopy;
520
Kalinin et al. investigate cooling in a jet by diode laser absorption
spectroscopy.
521
Flynn has developed a method for investigating collisional energy transfer between
large molecules and small marker or probe molecules.
522–525
He excites the large
molecule electronically by a UV laser pulse and then observes the excitation of the
probe molecule, typically CO
2
, using a tunable diode laser to observe the rotationally
resolved spectrum of the probe. This approach can yield the collision probability
248 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

function, P(E,E⬘), for transitions from energy E to energy E⬘.
522
This method is used
by other groups.
526
iii States produced by photodissociation. The competition between vibrational
deactivation and photodissociation can also be studied by the method of Flynn and
co-workers.
527–530
He excites pyrazine and diazabenzenes in the UV and observes both
the photoproduct, HCN, and vibrational excitation of the CO
2
bath gas with a
tunable diode laser.
iv Reactive collisions. Nesbitt has used crossed supersonic jets to investigate
the state distributions of the products of the reaction
531–534
F⫹H
2
HF ⫹ H and
the reaction
535,536
F ⫹ CH
4
CH
3
⫹ HF. The rovibrational energy distribution of the
HF was obtained by probing HF with a single frequency laser. For the latter reaction,
the HF Doppler profiles provided information on the angular distribution and the HF
kinetic energy distribution.
536
MacDonald et al.
537,538
have investigated the nascent rovibrational distribution of
HCN from the reaction of CN with H
2
.
C Analytical applications
The analytical applications of infrared spectroscopy constitute a huge field. Some
recent general texts on the subject are the book edited by Stuart and Ando
539
and the
handbook on near-IR spectroscopy edited by Burns and Ciurczak.
540
There have been
a number of recent collected conference proceedings on atmospheric monitoring
541–547
and on industrial applications
548
and process control.
549
These books are by no means
limited to tunable laser spectroscopies, but are cited to provide a general background.
1 Principles of quantitative gas phase absorption spectroscopy. All analytical
applications share a common background. The substance to be analyzed is typically
a mixture of several to a number of substances. For all linear spectroscopy, the
Beer–Lambert law is expected to hold so that the base e absorbance of a substance can
be expressed as
where A is the absorbance at frequency ν, σ
i
(ν) is the absorption cross-section of
substance i at frequency ν, N
i
is the concentration of i in molecules per unit volume,
and L is the pathlength. The simplest case is for one σ
i
(ν)N
i
to be much larger than
any other term in the expression so that the absorbance at frequency ν is effectively
determined by a single substance. This simple case is rarely realized except in high
resolution gas phase spectroscopy of small molecules on carefully selected absorption
lines. If the simple case is not realized, it is still possible to determine the concen-
tration of all species present by measuring the absorbance at a number of frequencies
and inverting the resulting system of equations, provided that the σ
i
(ν) are all known
at each frequency.
In the gas phase, the absorption cross-section lineshape for a single rovibrational
line depends upon the pressure, but the integral of the cross-section over the line
profile does not. The lineshape is usually rather well approximated by the Voigt
A(ν) = (σ
1
(ν) N
1
⫹ σ
2
(ν)N
2
⫹ ⴢⴢⴢ)L
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 249

function, a transcendental function (identical with the imaginary part of the plasma
dispersion function) obtained from the convolution of a Gaussian with a Lorentzian
function. In order to compute the absorption cross-section σ(ν) at frequency ν for a
single line of a single substance, one must know the values of ν
0
(the line center
frequency), S (the integrated line strength) and γ (the pressure broadening coefficient).
These parameters have been determined for many lightweight gas molecules across the
infrared spectrum, and compiled into extensive databases such as HITRAN
550
and
GEISA.
551
Numerically accurate absorption spectra can be computed based on these
tabulated data, not only for single gas species but also for gas mixtures.
In trace gas sensing applications, self-broadening and broadening against other
trace gases can be neglected in calculations, and dry air-broadening alone will suffice.
At atmospheric pressure, γP Ⰷ ∆ν
D
so that the Doppler contribution to the overall
linewidth can often be neglected, and the line shape be treated as pure Lorentzian.
Likewise, at pressures low enough to ensure γP Ⰶ ∆ν
D
, the line shape can be treated as
pure Gaussian. In either case, calculation of the line profile is simplified considerably.
At intermediate total pressures where γP ∼ ∆ν
D
, which for most lightweight gases is in
the range from 5 to 200 Torr, calculation of the Voigt profile is necessary to obtain
numerically accurate absorption spectra. Methods for approximate calculation of the
Voigt profile, and the related plasma dispersion function, are now a well-developed
subject. The approximations published by Humlicek
552
are particularly useful for this
purpose.
2 Detection Techniques. a Balanced and balanced-ratiometric detection. Laser
intensity noise and drift may limit the sensitivity of absorption measurement. In this
situation, balanced detection may be used to recover small absorption signals. The
detected noise of an equal-intensity replica of the probe beam, such as that created by
a variable-ratio beamsplitter, is subtracted from noise detected in the probe beam,
thus leaving only the uncompensated weak absorption signals of interest. The
variable-ratio beamsplitter can be made by placing a polarization rotator (a half-wave
plate) in series with a polarizing beamsplitter cube. With the input polarization
rotated about 45⬚, the beams emerging from the beamsplitter cube carry equal
amounts of power P, and power noise ∆P. In the absence of absorption, the photo-
currents generated by the (identical) signal and reference detectors can be subtracted
to cancel laser amplitude fluctuations. When one of the beams is attenuated due to
small absorption a, by a gas, the balance of photo-currents is disturbed, and a signal is
seen at the output of the amplifier. Adjustment of the beam splitting ratio (‘zeroing’)
is obviously necessary, as the signal and reference detectors will have unequal
responses, and there are inevitably differences in the optical transmissions of the
signal and reference arms. Furthermore, the detectors must have essentially identical
frequency response characteristics at all relevant frequencies in order to cancel noise.
An implementation of this method that avoids the need for exact balancing the
signal and reference light was proposed by Hobbs
553
and commercialized by New
Focus and Physical Sciences, Inc. It is known as balanced ratiometric detection
(BRD). It employs electronic circuitry to compute a log ratio of photo-currents,
rather than their difference, and to cancel noise currents at the same time. This analog
divider uses logarithmic conformance and tight symmetry of base-emitter curves of a
matched transistor pair. This scheme provides nearly perfect cancellation of noise
250 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

currents, even when the reference beam carries twice the power of the signal beam.
Since the signal versus reference current balancing is performed by means of electronic
feedback, no physical adjustment of the beam splitting ratio is necessary. The BRD
differential response to absorption signals depends on the ratio of the signal and
reference currents, which changes when the signal beam is partially absorbed. It
also depends on temperature because the transistor base-emitter voltage does, and
additional compensation circuitry is needed to produce a useful output voltage V
out
that is linearly proportional to the absorbance. Noise-equivalent absorbances as low
as 2 × 10
⫺7
Hz
⫺1/2
have been demonstrated by Allen and co-workers,
554
close to the
limit imposed by shot noise.
b Frequency- and wavelength-modulation spectroscopy. The ability of a diode laser
to change its emission wavelength with injection current, and to do so very rapidly,
permits frequency modulation spectroscopy.
555
The diode tuning response depends on
the modulation rate and ranges from 2–3 GHz mA
⫺1
at low frequencies to under
300 MHz mA
⫺1
at high frequencies. Frequency modulation is always accompanied
by amplitude modulation, as the injection current also controls the laser output
power. Sine-wave modulation of the diode laser has the effect of creating multiple
side-bands in its otherwise nearly monochromatic emission spectrum. Each side-band
is separated from the carrier by an integer multiple of the modulation frequency Ω.
In frequency-modulation spectroscopy Ω significantly exceeds both the laser line-
width that is typically several tens of MHz and is comparable to the absorption
linewidth. Only the two first-order side-bands of the laser frequency ω, ω ⫹ Ω and ω
⫺ Ω, have appreciable magnitude.
556,557
After frequency independent attenuation, such
as is encountered in non-resonant optical systems or media (e.g. imaging optics or
vacuum), the side-bands upon detection combine coherently with the carrier and
balance each other to produce an unmodulated output signal. If the attenuation
strongly depends on frequency, however, as is the case with most gases, one of the
side-bands will be absorbed to a greater extent than the other leading to the appear-
ance of Ω (and its harmonics) in the detected laser signal. The strength of absorption
determines the magnitude of these harmonics, which may be measured separately and
with high noise immunity, by using a lock-in amplifier for example.
This detection technique was first applied by Bjorklund to a cw dye laser.
558
It
proved extremely powerful and is widely used in diode laser spectroscopy today, some-
times in modified form such as two-tone frequency-modulation (TTFM),
559
or
amplitude-modulated phase-modulation (AMPM) spectroscopy.
560
Wavelength modulation (WM) is really another form of FM spectroscopy, in which
the modulation frequency Ω is smaller than the laser linewidth, and the modulation
indices are large.
561
The aforementioned side-bands are then present to a very high
order and, by virtue of their small separation from each other, merge into a continu-
ous spectrum. Detection is again performed at the first, second, or higher harmonics
of Ω as the laser carrier frequency ω is scanned in the vicinity of a gas absorption line.
WM spectroscopy dominates applications that rely on relatively low-speed detectors,
and its sensitivity is usually limited by the laser amplitude 1/f noise.
562
3 Trace gas monitoring. Mid-IR and near-IR absorption spectroscopy methods
for monitoring trace gases are making important contributions in several areas.
The monitoring of urban air pollution, investigations of the stratosphere, medical
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 251

applications and industrial process monitoring are the most often cited. Tunable
infrared laser spectroscopy is improving the sensitivity and speed of monitoring in all
these areas.
The principal alternative methods employed for these purposes generally use
sampling, followed by either a general analytical method such as gas chromato-
graphy-mass spectroscopy or by chemical processing to create species that can be
detected by chemiluminescence or fluorescence, but if remote sensing is required
only spectroscopic methods are suitable. Spectroscopy in the infrared offers the
possibility of detecting a wide range of species as most molecules have infrared
absorption bands. Infrared spectroscopic monitoring can be done either by laser
methods or by Fourier transform infrared spectroscopy (FTIR). The latter offers
broad spectral coverage at the expense of some selectivity because of the poorer
spectral resolution of small instruments and also generally at the expense of
sensitivity.
A general problem faced by both infrared spectroscopic methods (laser or FTIR) is
interference by absorptions of water vapor and other constituents of the atmosphere.
Long pathlengths do not help here as they increase not only the signal of interest but
the interfering absorption as well. Multi-component spectral fitting algorithms have
been developed that can resolve weak absorption lines of interest in the presence of
heavy interference by a known gas. The problem becomes far more severe, however,
when the identity of the interfering species is unknown.
a Laser remote sensing. Two major kinds of optical techniques applicable in active
remote-sensing of the atmosphere, long path differential optical absorption spec-
troscopy (DOAS) and LIDAR, are well developed and the topic will not be treated in
detail in this review (see the book chapters by Platt
563
and by Svanberg
128
). DOAS
allows the quantitative determination of atmospheric trace gas concentration by
making use of the characteristic absorption structures of trace gas molecules along a
path of known length in the atmosphere. As a spectroscopic technique DOAS
has the merits of inherent calibration, high sensitivity (sub-ppb to ppt) and precision
(1–10%), good specificity and the capability for remote measurements. The DOAS
principle can be applied in a wide variety of optical configurations. The most basic
arrangement consists of a telescope looking into the light beam emitted from a light
source (laser or incoherent light source coupled to a dispersive device) followed by a
detector. A variation of this design uses a corner-cube retroreflector to return the light
from a source located next to the detector. DOAS applications include studies in
urban and rural air, observations of the troposphere, volcanic plume emissions as well
as investigations of the distribution of stratospheric ozone and species leading to its
destruction.
546,565–569
Atmospheric sensing by infrared LIDAR relies on two key factors: the strength of
infrared nonresonant and resonant scattering from aerosols and industrial particulate
emissions and the ease with which many molecular species can be monitored by means
of their absorption spectra. IR LIDAR is a mature technological field with many
excellent literature resources (see for example the book chapter by Orr
570
) Examples
can be found in the work of Saito et al.
571
in forest measurements and Quagliano et
al.
572
Meteorology drives much of the interest in LIDAR, as LIDAR is able to provide
information on aerosols, atmospheric chemistry, clouds, winds and pollution. In the
case of aerosols, LIDAR can be used to measure and map their characteristic spatial,
252 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

size and shape distributions, as well as their chemical and physical composition. The
temperature-inversion mixing layer that often traps urban pollution can be monitored
by ground based LIDAR, as can localized plumes of particulates and chemicals from
industrial sites. Airborne and space-borne LIDAR platforms can provide coverage of
large areas quickly.
573,574
b Architectures and examples of point measurement trace gas sensors. Point sensing
may be done by sampling into a multipass cell (see Section 3.A). Five types of laser
based spectroscopic sensors, which currently embody state-of-the-art technology will
be described.
i Overtone band detection with near-infrared diode lasers. Near-infrared diode
lasers, because of their excellent characteristics, are ideal light sources for spectro-
scopic gas analysis. Diode lasers, as reported in Sections 2.A.1 and 2.A.4, have distinct
advantages in terms of output power, beam quality, linewidth, tunability, size, cost
and lifetime. Near-infrared diode laser based analyzers can achieve detection limits
comparable to that of FTIR systems. A number of near-infrared spectrometers
employing commercially available diode lasers with emission wavelengths from
780 nm to 2 µm have been described in the literature.
18,19,575–587
Gas detection at near-IR wavelengths is based on the molecular vibration over-
tone and combination-overtone bands that are significantly less intense than the
fundamental bands. For example, the lines of the first overtone of the CH stretching
vibration of methane centered at 1.6 µm is roughly 160 times weaker than the fund-
amental. Ambient methane would cause a 0.005% absorption over a 1 m pathlength at
this wavelength at room temperature.
550
Reliable measurement of such low absorption
is difficult, so that long optical pathlengths or special measurement techniques are
necessary to obtain satisfactory signal-to-noise ratios for trace level monitoring
applications.
Near-IR radiation can be detected with a silicon photo-diode. The near-IR diode
laser source–Si detector combination is very fast, with modulation bandwidths of
over 1 GHz, allowing rapid scanning, fast frequency modulation, and leading to gas
detection in real time. Near-infrared diode lasers often come in compact sealed
packages that include a convenient fiber-coupled output. Thus the probe light can
be delivered from a single source to several sampling locations. Likewise, the radi-
ation passing through the sample can be returned to a detector via a fiber, some-
times even the same fiber. Overtone spectrometers usually have adequate optical
probe power. Near-infrared diode lasers emit anywhere from 1 to 100 mW of single-
frequency radiation with low excess noise: 15 to 35 dB over the shot noise limit is
typical.
High output power levels have two important benefits. First, detector noise can be
neglected and the measurement of absorption can be performed near the optical shot-
noise limit. For example, Allen et al.
554
report a detection sensitivity of 2 × 10
⫺7
Hz
⫺1/2
absorption units with the use of a 1.3 µm diode laser and a balanced ratiometric
detector. Second, with high initial power available in a beam, one can employ a
multipass cell to propagate the beam back and forth through a gas sample, achieving
long effective pathlengths and thus increasing the observed absorption signal.
Although the throughput of such a cell decreases exponentially as the number of
passes increases (see Section 3.A ), there is sufficient light after many passes to permit
measurements that are not limited by detector noise.
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 253

Near-infrared overtone spectrometers, such as that developed by Uehara and Tai,
588
are usually built to detect one or a few specific gases, for two reasons. First, the near-
infrared wavelength region is not covered completely. Diode lasers are only available at
some wavelengths, and each diode has a limited tuning range when operated without
an external cavity. Second, these near-infrared diodes are relatively inexpensive,
making it economical to have several dedicated lasers, each detecting one gas species,
in a single instrument. This configuration is also attractive because different gas spe-
cies can be measured in parallel. For example, several groups have developed a gas
sensor that is capable of monitoring several substances using one common beam path
through the sample and one detector.
589
Alternatively, a single external-cavity diode laser (ECDL) with a large tuning range
can be used (see Section 2.A.4). A spectrometer based on such a widely tunable laser is
a very useful tool in that it can acquire spectra of an entire molecular band in a single
electronically-controlled scan in a matter of seconds. Oh and Hovde,
590
for example,
used a widely tunable 1.5 µm ECDL to record a spectrum of the ν
1
⫹ν
3
stretch–
vibration combination band of acetylene.
ii Gas sensors based on lead salt mid-IR diodes. In recent years air monitoring
employing lead salt lasers in tunable diode laser absorption spectroscopy (TDLAS)
has met the major requirements for atmospheric trace gas monitoring; sub-ppb
sensitivity, high detection speed and the potential for simultaneous measurements
of several species. A number of groups have reported excellent performance of
such sensors.
19,24,27,28,29,582,591–593
For example, Fried et al.
24
achieved a CH
2
O detec-
tion limit of 31 ppt for integration times of 25 s. This sensitivity corresponds to a
minimum detectable absorbance of 1.0 × 10
⫺6
and this is within a factor of two of
that reported by Werle et al.
592
Scott et al.
594
describe the design and operation of a
fully automatic sensor for the measurement of five trace gases in the lower strato-
sphere on board an airplane. Their instrument houses four lasers and four detectors
mounted on the same liquid-nitrogen-cooled platform, beam shaping optics, a com-
pact multi-pass absorption cell with 80 m pathlength, analog electronics, and a
computer-controlled data storage system. The instrument employs wavelength modu-
lation and second harmonic detection to achieve high sensitivity. It is capable of
detecting optical absorption as small as 10
⫺5
. This sensitivity provides detection limits
in the range of several tens of ppt for species such as HCl, NO
2
, HNO
3
, CH
4
, and
N
2
O.
iii Gas sensors based on difference frequency generation absorption spectroscopy.
Further to our discussion in Section 2.C.1, it is possible to possible to utilize various
DFG architectures based on birefringent and quasi-matched nonlinear optical
materials pumped by various pump sources such as Ti:sapphire lasers, diode and
fiber amplified diode lasers and fiber lasers. Several groups worldwide have
reported
56,57,59,61,71,75,86,595
different architectures and applications (See Section 2.C.1).
For example Rehle et al.
596
have used a laser spectrometer based on difference
frequency generation in periodically poled LiNbO
3
(PPLN) to quantify atmospheric
formaldehyde with a detection limit of 0.32 ppbv. To achieve this sensitivity,
specifically developed data processing techniques and high power Yb and Er/Yb fiber
amplifiers were used. With state-of-the-art fiber coupled diode laser pump sources at
1083 nm and 1561 nm, tunable narrow-linewidth (<60 MHz) difference frequency
radiation can be generated in the 3.53 µm (2832 cm
⫺1
) spectral region at power
254 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

levels of greater than 0.5 mW. This significantly higher power capability allows the
use of an optical-noise-reducing dual-beam absorption configuration that
employs two DC-coupled Peltier-cooled HgCdTe (MCT) detectors and a 100 m
absorption pathlength in a low-volume (3.3 l) astigmatically compensated Herriott
gas cell.
With this device, formaldehyde in ambient air in the 1 to 10 ppbv range has been
detected continuously for nine and five days at two separate field sites in the Greater
Houston area operated by the Texas Natural Resource Conservation Commission
(TNRCC) and the Houston Regional Monitoring Corporation (HRM). The acquired
spectroscopic data were compared with results obtained by a well-established wet-
chemical o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA) technique with
good agreement. While the accuracy of the determined concentrations is comparable
with results from conventional wet-chemical techniques, the described DFG sensor
offers a time response of a few seconds and permits unattended continuous operation
for long periods of time. The inherent maintenance-free design of a tunable infrared
DFG based diode laser spectrometer and the capability of remotely controllable com-
puterized operation makes such instrumentation a convenient, robust tool for mobile
trace gas detection.
Further reduction in size, cost and performance improvements can be realized by
utilizing the latest developments of optical fiber and amplifier technology and surface
mounted electronics. For example, Richter et al.
79
describe a compact modular
mid-infrared DFG source at the 0.1mW-level.
iv Detection of trace gases with cw QC-DFB lasers. Gas sensing with a cw
QC-DFB laser was first reported by Sharpe et al.
597
The laser was scanned via a
sawtooth current ramp (5–11 kHz) and the absorbance of the sample was measured
directly. The laser could be swept rapidly over a frequency range of up to 2.5 cm
⫺1
.
Absorption spectra of pure NO and NH
3
gases at low pressure were acquired at
5.2 µm and 8.5 µm, respectively.
The direct absorption approach with cw QC-DFB lasers has been further
developed
598
and the first successful application of a single-frequency QC-DFB
laser to the analysis of trace gases in ambient air was reported in 2000.
599
This work
was later extended to higher sensitivity.
600
A QC-DFB laser designed for cw operation
at cryogenic temperatures in the 7.9 µm spectral region was used. Absorption in air
was detected in a 100 m multipass cell at a reduced pressure of 20–40 Torr. A “zero-
air” background subtraction technique
25
was used in order to suppress the influence
of interfering effects. Spectra of ambient air and pollutant-free “zero air” were
acquired alternatively. The zero-air signal (as a function of a datapoint number)
was subtracted from the ambient air sample signal and normalized to the zero-air
signal.
Detection of more complex organic molecules with congested unresolved
ro-vibrational spectra sets a challenge to QC-DFB laser based chemical sensors.
Traditionally, the spectral recognition of such species is performed by acquiring
medium-resolution absorption spectra in a wide spectral region (thousands of
wavenumbers) identifying absorption bands rather than isolated lines. This approach
cannot be realized with QC-DFB lasers because of their limited tunability. The
maximum demonstrated spectral range covered by a single-frequency QC-DFB laser
is ∼ 30 cm
⫺1
, achieved in pulsed operation when the heat sink temperature varied from
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 255

50 K to 300 K.
109
In practice the tunability of a QC-DFB laser in a chemical sensor is
usually limited to 1–3 cm
⫺1
.
The feasibility of detecting and quantifying volatile organic compounds (VOCs)
with cw QC-DFB lasers has been explored
600
using ethanol (C
2
H
5
OH) as an example.
The ethanol vapor absorption spectra (ethanol at 1 Torr pressure with ambient air
added to a total pressure of 36.6 Torr) was acquired with the QC-DFB laser in a
0.43 m long gas cell. The rovibrational structure is reasonably well resolved in this
spectrum, in contrast with the spectrum in the C–H and O–H stretch spectral regions
(∼3 µm). The resolved spectral features clearly distinguish the absorption of ethanol
from other species. However, the high density of the pressure-broadened spectral lines
makes the technique of individual line fitting with a Voigt profile (local approach)
inapplicable. Instead, some kind of global approach should be used in order to take
advantage of the whole spectral fingerprint. The technique used
600
is principally based
on finding the correlation between previously acquired reference spectra and a sample
spectrum (test spectrum) under the same line-broadening conditions (i.e. same air
pressure and temperature). The test spectrum was considered as a linear combination
of four reference spectra of the absorbing species. The resulting concentrations were
found to be mutually consistent and in agreement with the concentrations of H
2
O,
N
2
O and CH
4
extracted from the spectra by the traditional single-line fitting
approach. This work establishes that, for one case at least, high-resolution IR absorp-
tion data acquired in a limited 1–3 cm
⫺1
range with a QC-DFB laser can be success-
fully used for quantification of complex organic compounds as trace gas components.
Non-DFB multimode QC lasers (Fabry-Pérot devices) may find application in dif-
ferential absorption lidar (DIAL) systems.
601
There is a wide-ranging interest in
extending DIAL-based chemical sensing from the detection and quantification of
major species to the characterization of dilute contaminants, e.g., organic chemical
vapors that do not possess spectrally resolved vibrational transitions. In general, the
8–12 µm wavelength spectral region is the most desirable wavelength range for per-
forming sensitive, selective detection of organic species, but it is not yet known
whether the weak absorptions by water vapor and other atmospheric species in this
region will limit the DIAL range too severely.
Several QC lasers could be simultaneously used in a DIAL system to probe the
broad absorption features at selected wavelengths. A pseudo-random code modu-
lation scheme has been proposed to distinguish signals produced by different lasers.
601
v Gas sensors based on pulsed QC lasers. One of the most attractive features of QC
lasers is that they can be operated near room temperature. This option is presently
restricted primarily to pulsed QC-DFB devices. Pulsed operation poses a number of
specific performance issues. The most fundamental limitations are reduced average
power (peak power is essentially the same as in a cw mode, but the duty cycle is <1%)
and laser line broadening caused by the frequency chirping during the pump current
pulse. The first work on spectroscopic chemical sensing with a pulsed QC-DFB laser
was reported in 1998 by Namjou et al.
602
preceding the first cw QC-DFB laser based
publication.
597
In this work, a technique for fast-tuning the optical frequency of the
laser pulses by applying a subthreshold current (STC) ramp was introduced. Wave-
length modulation (WM) spectra of diluted N
2
O and CH
4
samples were acquired near
λ = 8 µm. The laser was excited by a 1 MHz train of 11 ns wide current pulses. The
laser linewidth was estimated to be 720 MHz. Detection was performed using a slow
256 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

HgCdTe detector in essentially the same manner as in cw experiments. A similar
approach to detection was used by Sonnenfroh et al.
603
and is referred to as ‘quasi-cw’.
An alternative approach to data acquisition with pulsed QC-DFB lasers is to use a
fast detector and individually measure the peak power of every pulse with gated
electronics. In this mode of measurement the detected signal is much higher than the
detector noise and does not depend on the repetition rate. Time-gating permits sup-
pression of the light that occurs earlier or later than the informative signal. This
approach was utilized in the first successful application of a pulsed QC-DFB laser to
the trace gas detection in ambient air.
599,600
The flexibility provided by the digital
frequency control employed in this study was utilized for linearization of the wave-
length scan and for wavelength modulation. The sensor was used to measure CH
4
,
N
2
O and HDO concentrations in ambient air and a precision of 9 ppbv, 4 ppbv and
120 ppbv was achieved for each of these species, respectively.
High sensitivity detection of NH
3
is also of interest in the control of deNOx chem-
istry, industrial safety and medical diagnostics of kidney related diseases. A compact
mobile ammonia sensor based on a thermoelectrically cooled pulsed QC-DFB laser
operating at ∼10 µm was described by Kosterev et al.
604
The optical configuration of
this sensor was similar to that described earlier,
599
but the multipass cell was replaced
with a simple 50 cm long double pass gas cell, and no zero air was employed. This
sensor was applied to real-world measurements at NASA-JSC, namely the continuous
monitoring of NH
3
concentration levels present in bioreactor vent gases. A sensitivity
of better than 0.3 ppmv was estimated which was sufficient to quantify expected
ammonia levels of 1 to 10 ppmv.
Multimode pulsed Fabry-Pérot QC lasers can be used in analytical applications that
do not require or would not benefit from high spectral resolution.
40
One example is a
spectroscopic analysis of liquids, where the spectral lines are broad. Such an appli-
cation was demonstrated by Lendl et al.
605
to quantify phosphate in Coke samples.
A fiber-coupled injection cell with a pathlength of ∼100 µm in the aqueous sample
was used. The results were in a good agreement with the values found by ion
chromatography.
vi Photoacoustic detection of trace gases. Trace gas detection by photoacoustic
spectroscopy (PAS) has found wide ranging applications in environmental science,
industry and medicine (see Section 3.E). As the photoacoustic signal is proportional
to the intensity of the light sent through the sample, this method was, until recently,
used in conjunction with intense infrared gas lasers. Diode laser based PAS is a very
promising new development.
204,606–608
A sensitivity of 8 ppmv was demonstrated
608
with only 2 mW of modulated diode laser power in CH
4
overtone region.
Implementation of a QC-DFB laser in the fundamental absorption region has a
potential for considerably improved sensitivity. Ammonia and water vapor photo-
acoustic spectra were obtained using a cw cryogenically cooled QC-DFB laser with a
16 mW power output at 8.5 µm as reported by Paldus et al.
609
A PAS cell resonant at
1.66 kHz was used. The QC-DFB was used for frequency scans using temper-
ature tuning and for real-time concentration measurements with a fixed laser tempera-
ture. Measured concentrations ranged from 2200 ppmv to 100 ppbv. A detection limit
of 100 ppbV ammonia (∼10
⫺5
noise-equivalent absorbance) at standard temperature
and pressure was obtained for a 1Hz bandwidth and a measurement interval of
10 min.
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 257

Recently, Hofstetter et al.
610
reported PAS measurements of ammonia, methanol
and carbon dioxide using a pulsed 10.4 µm QC-DFB laser operated at 3–4% duty
cycle with 25 ns long current pulses (2 mW average power) and close to room
temperature with Peltier cooling. Temperature tuning resulted in a wavelength range
of 3 cm
⫺1
at a linewidth of 0.2 cm
⫺1
. This sensor used a 42 cm long PAS cell with a
radial 16 microphone array for increased detection sensitivity. In addition the cell was
placed between two concave reflectors resulting in 36 passes through the cell (with an
effective pathlength of 15 m). The laser beam was mechanically chopped at a resonant
cell frequency of 1.25 KHz, which resulted in PAS signal enhancement by a Q factor
of 70. A pyroelectric detector recorded the QC laser power to normalize the PAS
signal. Detection of ammonia concentrations at the 300 ppbv level with a SNR of 3
was achieved at a pressure of 400 mbar.
c Laser spectroscopy in medicine. Laser spectroscopy is finding increasing appli-
cations in medicine and biology.
611–614
A particular role for spectroscopy is in monitor-
ing simple small molecules that have been shown to be important. The role of
simple molecules such as nitric oxide (NO) in physiological processes has received
considerable attention in recent years and was a subject of the 1998 Nobel Prize in
Medicine.
One application is the measurement of NO in human breath samples, since exhaled
air is an indicator of several processes taking place in the human body. To observe
NO in breath, a cavity enhanced absorption spectroscopy (CES) sensor
153
with a cw
QC-DFB laser operating at 5.2 µm with an output power of 80 mW was used. A direct
performance comparison was carried out between a sensor configuration where the
CES optical cavity was replaced with a 100 m pathlength multipass cell. It was found
that, in spite of having an effective pathlength of 670 m, CES had a lower absorption
sensitivity because of baseline noise of ∼1% (averaging 10
4
QC laser scans). These
baseline fluctuations are intrinsic to CES and result from the mode structure of the
cavity transmission spectrum. Some improvement in CES baseline noise can be
achieved with a recently developed off-axis technique.
151
Recent work indicates that other gases, such as carbon monoxide (CO), can play
also a very significant physiological role. CO is produced from heme catabolism by the
enzyme heme oxygenase. Previous work has shown that CO promotes blood flow by
inhibiting vascular tone and platelet aggregation and that neuronal CO production
may modulate the NO-cGMP (guanosine 3⬘,5⬘-cyclic monophosphate) signaling
system, demonstrating important biochemical interactions between the two diatomic
gases. The extremely low levels of gas production in living cells and the relatively short
lifetime of cell cultures have complicated the detailed understanding of the kinetic, or
time-dependent processes responsible for their generation. A typical production rate
of CO, for example from vascular smooth muscle cells (VSMCs), is 1 to 10 pmol
min
⫺1
per 10
7
cells. Instrumentation for in vivo measurement of gas production should
have sensitivities on the parts per billion (ppb) level in order that the dynamics of gas
production can be followed with laboratory-scale cell sample populations.
615
Because
of low CO production rates from biological tissues, measurements of CO concen-
trations have been limited to gas chromatography and radioisotope counting tech-
niques. Although these methods are highly sensitive, they cannot measure CO directly,
requiring several time-consuming intermediate steps requiring ∼15 min, and may be
affected by interferences from water, oxygen, and carbon dioxide.
258 Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272

Infrared laser absorption spectroscopy is an attractive alternative approach for the
detection of biological CO at the ppb level in real time.
616,617
Morimoto et al.
616
meas-
ured endogenous CO production from vascular cells using a mid-IR laser based on
difference frequency generation (DFG) of two near-IR lasers as a spectroscopic
source. The CO absorption was detected in the fundamental vibration band near
4.6 µm. In this work, an extractive technique was used with gas samples taken from
the flask containing the cell culture to an 18 m pathlength optical multipass cell so that
the measurements could be performed at a reduced pressure of 100 Torr.
Kosterev et al.
617
reported improved design and performance of an optical mid-IR
CO sensor intended for continuous monitoring of cell culture activity at ambient
atmospheric pressure. The same fundamental absorption band region was used for
CO detection, but a quantum cascade laser with a distributed feedback structure
(QC-DFB)
109
was employed instead of the DFG source. The high output power of
the QC-DFB laser and an advanced data analysis approach made it possible to detect
biological CO and CO production rates with ∼1 m optical pathlength folded above a
standard culture flask of VSMCs.
A further improvement of the pulsed QC-DFB based sensor was reported by
Kosterev et al.
618
The laser beam was split into two channels, one being used to probe
the gas absorption and the other as a reference to measure the laser pulse energy.
The subsequent normalization eliminated pulse-to-pulse energy fluctuations as
an error source, which was the predominant cause of error previously.
617
This
automated sensor was also used for continuous monitoring of CO in ambient air
detected by its R(3) absorption line at 2158.300 cm
⫺1
(λ∼4.6 µm). A noise-equiv-
alent detection limit of 12 ppbv was experimentally demonstrated with a 1 m optical
pathlength. This sensitivity corresponds to a standard error in fractional absorbance
of 3 × 10
⫺5
.
All the measurements were carried out at atmospheric pressure, and hence it was
not possible to periodically acquire a baseline with an evacuated sample container. In
order to keep the baseline (which included weak unwanted interference fringes from
optical elements) stable during multi-hour measurements, the slow drifts of the laser
frequency were actively compensated by computer-controlled corrections to the sub-
threshold current. A constant CO production rate of 44 ppbv h
⫺1
was observed,
taking into account the 0.5 l volume of the cell culture container. This corresponds to
a net CO production rate of 0.9 nmol per 10
7
cells h
⫺1
, which is in agreement with
previous measurements
616
obtained with similar cells and treatment regimes
d Isotope ratio composition measurements. The accurate measurement of isotopic
abundance ratios is an extremely important research tool in a wide variety of scientific
fields. Isotopic signatures have been successfully used to analyze the biochemical cycles
of materials, in particular greenhouse gases and other areas of applications, such as
in non-invasive medical diagnostics. In the environmental sciences it is well known
that most naturally occurring compounds are isotopically labeled by isotopic
fractionation effects. Hence, the isotopic signature can often be used as an indication
of the compound’s formation pathway under the prevailing environmental conditions.
Laser spectroscopy is being developed as a new methodology for precision isotopic
ratio measurements of gases. This technique joins other available tools such as
traditional high precision isotopic ratio mass spectrometers (IRMS),
619
FTIR and
NMR.
620
Annu. Rep. Prog. Chem., Sect. C, 2002, 98, 219–272 259

The isotope abundance ratio in an environmental compound depends on the area
where the sample was collected, thereby reflecting different production processes of
the compound and different transportation histories in the atmosphere and ocean.
For instance, the
13
C/
12
C ratio (∼1/100) in CH
4
differs by up to a few percent from one
production process to another
621
and the
15
N/
14
N ratio in N
2
O dissolved in oceans
varies depending on the region and depth.
The standard means for measuring isotope ratios is mass spectrometry. Very high
mass resolution is required for discrimination of species with small difference, such as
13
CH
4
and
12
CH
3
D and it is impossible to separate
15
N
14
N
16
O and
14
N
15
N
16
O, which
have the same mass. In laser absorption spectroscopy, however, different isotopic
molecular species can be distinguished easily irrespective of their masses if appropri-
ate absorption lines are selected.
In absorption spectroscopy, the abundance ratio is determined by comparing the ratio
of the absorbances for the selected absorption lines in a sample gas with that in the
standard gas of known isotopic composition. There have been many demonstrations
or proposals of isotopic analyses by absorption-spectroscopic methods, using
lead-salt semiconductor lasers in the mid-infrared region,
622–624
a DFB diode laser in
the near-infrared,
621,625
a color center laser,
626
and a quantum cascade laser in the 8 µm
region.
598
In these methods, the absorbances of adjacent lines of different isotopic
species were compared by scanning the wavelength of a spectroscopic source. Usually
a weaker line was selected for the more abundant species or the laser beams are made
to travel different distances in an absorption cell in order to compensate the large
abundance difference in order to balance the signal levels. Uehara et al.
621
demon-
strated a precision of isotopic analysis of ±3 × 10
⫺4
(±0.3 ml
⫺1
) for
13
CH
4
/
12
CH
4
.
Hence laser spectroscopy can complement mass spectrometry and is potentially an im-
portant tool for precise analysis of biogeochemical cycles of environmental substances
In another field, a precise measurement of the ratio of the two stable carbon iso-
topes of CO
2
(
13
CO
2
and
12
CO
2
) can provide valuable information about CO
2
exchange processes in the volcanic plume.
627
Changes in the isotopic ratio can be
explained either in terms of variations in the volcanic source, mixing with or anthro-
pogenic sources (e.g., agricultural or industrial) as well as photosynthesis,
628
or could
manifest global and local changes. The strong vibrational–rotational absorption
bands of
12
CO
2
and
13
CO
2
centered at 4.28 and 4.37 µm, respectively, also provide
several line pairs that are suitable for isotope ratio determination by direct absorption
spectroscopy for this species.
Acknowledgements
This work was supported by the Robert A. Welch Foundation and the National
Science Foundation under Grant CHE-0111125 and Texas Advanced Technology
Program.
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