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Observation of saturation and rapid passage signals in the 10.25 micron spectrum of ethylene using a frequency chirped quantum cascade laser
Abstract
Rapid passage signals exhibiting saturation effects have been observed when a low-pressure sample of ethylene, within a multiple pass absorption cell, is subjected to radiation from a repetitively pulsed 10.25 micron quantum-cascade laser. Within each pulse the laser frequency sweeps 36 GHz from high to low frequency in a time of 140 ns. At the low gas pressures, less than 20 mTorr, in the absorption cell the sweep rate through a Doppler-broadened absorption line (ca. 0.5 ns), is much faster than the collisional relaxation time of the ethylene and this leads to rapid passage effects. Examples are given of the complex rapid passage signals observed in pure and nitrogen broadened spectra. The rapid passage effects, which lead to the variety of the observed signals, have been modelled by numerical solution of the coupled Maxwell-Bloch equations for four sets of two-level systems.
... Apparently, this is related to the fact that the set of Bloch equations, in which taking into account the influence of inhomogeneous broadening entails enormous calculation difficulties, was used in [10]. This circumstance was noted in [12]. ...
... It can be seen from Fig. 6 that, for indices of absorption k 0 z ≥ 5, dependence (12) saturates in the vicinity of the absorption line, while, at k 0 z ≥ 50, the difference between the curves is small even in the wings of the absorption curves. For quantitative esti-mate of the difference between the transmission curves, let us introduce quantity R defined as . ...
... However, in the vicinity of k 0 z ~ 100, this quantity is small, e.g., for (k 0 z) = 95 or (k 0 z) = 105, we have R = 6.7 × 10 -3 or R = 6.4 × 10 -3 . In practice, k 0 z can be found by comparing the experimental curve with those calculated from (12) and minimizing the value of (13). Consequently, to ensure 5% uncertainty of determining k 0 z in the region of high absorption (k 0 z ~ 100), the uncertainty of measuring intensity must be (this corresponds to the level of 1/3R), which is not always easy to do. ...
Effects caused by fast laser frequency sweeping across absorption lines are investigated. Oscillations in the time dependence of intensity of radiation generated by the medium, which are caused by beats between oscillations at variable excitation frequency and constant eigenfrequencies, are discovered. The time intervals between local maxima of the oscillations are inversely proportional to the difference between the eigenfrequency of the atomic oscillator and the instant frequency of the external radiation. The method of using fast frequency sweeping for determining indices of absorption at high optical densities (k0z ~ 100) is proposed.
... Furthermore, several studies reported observing absorption features with asymmetric line shapes when employing both the short pulse and the long pulse mode (see [3] and references therein). Additionally, the latter method suffers from non-linear absorption effects [78]. The majority of these issues are unknown in using cw QCLs and degrade the performance of pulsed QCL spectrometers. ...
... This is of particular importance at lower pressures, since under such conditions the effective laser line width of pulsed QCLs exceeds the typical Doppler line broadening. Non-linear effects also appeared more pronounced [26,78]. ...
The considerably higher power and wider frequency coverage available from quantum cascade lasers (QCLs) in comparison to lead salt diode lasers has led to substantial advances when QCLs are used in pure and applied infrared spectroscopy. Furthermore, they can be used in both pulsed and continuous wave (cw) operation, opening up new possibilities in quantitative time resolved applications in plasmas both in the laboratory and in industry as shown in this article. However, in order to determine absolute concentrations accurately using pulsed QCLs, careful attention has to be paid to features like power saturation phenomena. Hence, we begin with a discussion of the non-linear effects which must be considered when using short or long pulse mode operation. More recently, cw QCLs have been introduced which have the advantage of higher power, better spectral resolution and lower fluctuations in light intensity compared to pulsed devices. They have proved particularly useful in sensing applications in plasmas when very low concentrations have to be monitored. Finally, the use of cw external cavity QCLs (EC-QCLs) for multi species detection is described, using a diagnostics study of a methane/nitrogen plasma as an example. The wide frequency coverage of this type of QCL laser, which is significantly broader than from a distributed feedback QCL (DFB-QCL), is a substantial advantage for multi species detection. Therefore, cw EC-QCLs are state of the art devices and have enormous potential for future plasma diagnostic studies.
... Ce manque de puissance s'est bien sûr fait cruellement ressentir dans la qualité des spectres avec un rapport signal sur bruit restant très petit. l'ont démontré dans leur publication de 2006 [112]. Ces phénomènes apparaissent lorsque la durée de l'interaction du champ électromagnétique avec les molécules absorbantes est faible par rapport à la durée de vie des niveaux et sont similaires à ce qui est connu depuis longtemps dans le domaine de la résonance magnétique [113,114]. ...
... Nous l'avons vu, la pression joue un rôle majeur dans l'apparition des oscillations [101,112,120]. Notre étude nous a permis de déduire une pression minimale au-dessus de laquelle nous pouvons travailler sans apparition de rapid passage. ...
Les travaux présentés dans ce manuscrit concernent l’utilisation des lasers à cascade quantique dans l’infrarouge moyen en vue d’applications atmosphériques. Les deux premières parties du manuscrit présentent le contexte général de la thèse, le principe des lasers à cascade quantique (QCL), ainsi que l’étude de la bande _1 du SO2 autour de 9 μm avec un QCL en régime continu. Au cours de la troisième partie, nous présentons la première méthode que nous avons utilisée en régime pulsé qui est également la plus répandue utilisant des impulsions de courte durée (_10 ns). Nous avons pu alors mettre en évidence, dans le cadre de l’étude de la molécule de NH3, des problèmes liés à l’utilisation de cette technique qui en limite l’intérêt pour des applications en spectroscopie. Dans la quatrième partie, nous présentons la commande des QCL par impulsions longues (> 500 ns) permettant d’obtenir des spectres intra-impulsionnels avec des temps d’enregistrement de l’ordre de la microseconde. Nous avons pu appliquer cette technique pour l’étude du SO2 (à 9 μm) et comparer avec les résultats obtenus en régime continu. La comparaison des deux modes de fonctionnement précédents et leurs limitations nous a amené à proposer un nouveau mode de fonctionnement, utilisant des impulsions de durée intermédiaire (< 100 ns), présenté dans la cinquième partie du manuscrit. Cette méthode nous a permis de solutionner une grande partie des restrictions usuelles de ce type de laser et au cours de cette étude, nous avons également pu mettre en avant un certain nombre de points importants en ce qui concerne les phénomènes de rapid passage
... A theoretical description based on optical Bloch equations with special attention paid to inhomogeneous (Doppler) broadening, which is negligible in magnetic resonance experiments, has been proposed [48,51,54]. Molecular alignment caused by the linearly polarized laser beam has to be considered in the calculations [48]. ...
... The linear correlation in Figure 4b, which is valid over about one order of magnitude, reveals that power saturation can also be neglected. If present, this effect would usually be identifiable from a nonlinear and reduced gradient [54]. ...
The recent availability of thermoelectrically cooled pulsed and continuous wave quantum and inter-band cascade lasers in the mid-infrared spectral region has led to significant improvements and new developments in chemical sensing techniques using in-situ laser absorption spectroscopy for plasma diagnostic purposes. The aim of this article is therefore two-fold: (i) to summarize the challenges which arise in the application of quantum cascade lasers in such environments, and, (ii) to provide an overview of recent spectroscopic results (encompassing cavity enhanced methods) obtained in different kinds of plasma used in both research and industry.
... There have been additional experimental and theoretical developments for this case [17]. Note also that chirp asymmetry has also been observed in isolated molecular transitions during chirped excitation in which they record resonance width changes but no line center shift at fast chirp rates [18]. In that regime they do record 'ringing' (oscillations in the response), indicative of coherence effects not present in the basic leptogenesis model, which from an optical standpoint is essentially a population model. ...
The effective conjugation symmetry that arises in the rotating wave frame is the analogue of the discrete symmetry in field theory. Breaking this effective conjugation symmetry leads to asymmetries between up- and down- chirped excitation in quantum optical systems. We use semiclassical quantum optics theory to describe these processes and experimentally characterize the asymmetry in the optical response in chirped, two-color saturated absorption spectroscopy (SAS) in an atomic vapor cell. Doing so demonstrates a theoretical and phenomenological correspondence to the simplest model of leptogenesis, the process by which our universe purportedly went from equal amounts of matter and antimater to its present matter excess. The understanding of the asymmetry as due to a broken discrete symmetry under chirp appears to illuminate the underlying processes responsible for other chirp asymmetries previously noted in the literature.
... High-resolution spectroscopic techniques such as saturation spectroscopy benefit from these narrow linewidths, and are tools for establishing accurate frequency markers and laser stabilization [5,6]. The high coherence and intensity of the electric field produced by these sources also allows the observation of coherent transient effects when the laser frequency is swept over a strong molecular transition of a velocity selected sample on a timescale that is short compared to its relaxation time [7][8][9]. These nonlinear optical effects, which include Rabi cycling and optical nutation, are highly dependent on the linewidth of the laser that prepares the velocity selected sample. ...
We report on the broadening of the optical bandwidth of a distributed feedback quantum cascade laser (QCL) caused by the application of radio frequency (RF) noise to the injection current. The broadening is quantified both via Lamb-dip spectroscopy and the frequency noise power spectral density (PSD). The linewidth of the unperturbed QCL (emitting at ∼ 5.3 μm ) determined by Lamb-dip spectroscopy is 680 ± 170 kHz , and is in reasonable agreement with the linewidth of 460 ± 40 kHz estimated by integrating the PSD measured under the same laser operating conditions. Measurements with both techniques reveal that by mixing the driving current with broadband RF noise the laser lineshape was reproducibly broadened up to ca 6 MHz with an increasing Gaussian contribution. The effects of linewidth broadening are then demonstrated in the two-color coherent transient spectra of nitric oxide.
... From our point of view, this can be explained by the computational complexity in accounting for the inhomogeneous broadening in the Bloch equations used for the theoretical analyses in [10]. The latter circumstance is mentioned and discussed in [12]. ...
The self-radiation of an extended absorbing medium under the action of a laser field with a fast-changing frequency is studied. The oscillations in the time-dependent intensity of the medium radiation are found. These oscillations are caused by the beatings between the vibrations of the oscillator at the fast-changing exciting and constant resonance frequencies. The time duration between local peaks of the oscillations is inversely proportional to the difference between the resonance frequency of the oscillator and the instantaneous laserfrequency.
... An appropriate theory was constructed. Later, in the 2000s, numerous experimental studies were stimulated by the invention of tunable quantum cascade lasers (see, e.g., [5][6][7][8][9][10]). ...
We consider the possibility of using spectroscopy for the diagnostics of optically dense media by recording the time-dependent intensity of transmitted light under the action of a laser field with fast-changing frequency. We compare advantages of this approach with the traditional technique based on the Beer-Bouguer-Lambert law. Reasonably accurate extinction measurements of the order of k(ω 0)l ≈ 102 and greater are realizable with the help of tunable diode lasers.
... In this study, a low pressure dc discharge with a pressure of p = 1.33 mbar and 5 ms duration has been used to investigate the influence of nonlinear absorption phenomena, which cause strong distorted absorption structures in the spectrum of NO. This is called the rapid passage effect and has been taken into account by a calibration routine based on tabulated line strengths [6,[8][9][10][11]. Different mean plasma currents have been applied to the discharge leading to gas temperature values ranging from about 300 K up to about 500 K. ...
With a time resolution of 33 µs, the gas temperature in a pulsed dc air plasma admixed with 0.8% NO has been measured by quantum cascade laser absorption spectroscopy (QCLAS). For this purpose, the temperature dependent intensity ratios of two absorption structures of NO at 1900 cm−1 (5.26 µm) have been used. The QCLAS system worked in the Intra Pulse Mode with a pulse repetition frequency of 30 kHz leading to a spectrum recorded each 33 µs. In a low pressure discharge, the influence of nonlinear absorption phenomena causing strong distorted absorption structures of NO has been taken into account by a calibration routine based on tabulated line strengths. Different mean plasma currents have been applied to the discharge leading to gas temperature values ranging from about 300 K up to about 500 K.
... c 2009 IOP Publishing Ltd laser pulse which can be used to acquire a full absorption spectrum within the pulse width of typically a few hundred nanoseconds [15]. However, at low pressure conditions the measured absorption features are affected by an obstacle known as the "rapid passage effect" accompanied by potential power saturation effects [16,17]. Nevertheless, if a sensible normalisation [12] or calibration [9,11] is performed quantitative results can be obtained. ...
The recent advent of quantum cascade lasers (QCLs) enables room-temperature mid-infrared spectrometer operation which is particularly favourable for industrial process monitoring and control, i.e. the detection of transient and stable molecular species. Conversely, fluorocarbon containing radio-frequency discharges are of special interest for plasma etching and deposition as well as for fundamental studies on gas phase and plasma surface reactions. The application of QCL absorption spectroscopy to such low pressure plasmas is typically hampered by non-linear effects connected with the pulsed mode of the lasers. Nevertheless, adequate calibration can eliminate such effects, especially in the case of complex spectra where single line parameters are not available. In order to facilitate measurements in fluorocarbon plasmas, studies on complex spectra of CF4 and C3F8 at 7.86 μm (1269 – 1275 cm-1) under low pressure conditions have been performed. The intra-pulse mode, i.e. pulses of up to 300 ns, was applied yielding highly resolved spectral scans of ~ 1 cm-1 coverage. Effective absorption cross sections were determined and their temperature dependence was studied in the relevant range up to 400 K and found to be non-negligible.
Using a low power modulated quantum cascade laser, collective coherent effects in the 5 µm spectrum of NO have been demonstrated by the observation of sub-Doppler hyperfine splitting caused by a large AC Stark effect.
Using a mW power chirped pulse quantum cascade laser propagating in a 60 m pathlength Herriott cell, delayed rapid passage and transient gain signals have been observed in the 8 micron spectrum of acetylene.
Over the past few years mid-infrared absorption spectroscopy based on quantum cascade lasers operating over the region from 3 to 12 µm and called quantum cascade laser absorption spectroscopy or QCLAS has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of QCLAS because most of these compounds and their decomposition products are infrared active. QCLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a microsecond, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from QCLAS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of QCLAS techniques to industrial requirements including the development of new diagnostic equipment. The recent availability of external cavity (EC) QCLs offers a further new option for multi-component detection. The aim of this paper is fourfold: (i) to briefly review spectroscopic issues arising from applying pulsed QCLs, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas and at surfaces, (iii) to describe the current status of industrial process monitoring in the mid-infrared and (iv) to discuss the potential of advanced instrumentation based on EC-QCLs for plasma diagnostics.
A rapidly swept quantum cascade laser (QCL) has been used to probe a small region of the ν
6 band of formaldehyde around 8 μm by direct absorption spectroscopy. Two of the strongest transitions accessible within the operating region of the QCL are assigned as (1,1,1)←(2,0,2) and (10,1,9)←(9,2,8). In this paper, we report pressure-induced broadening and shift coefficients for these transitions with a variety of gases (He, Ne, Kr, Ar, N2, O2, and CO2). The pressure broadening coefficient measurements are shown (in some cases) to be chirp rate dependent, even under conditions where rapid passage effects are negligible, highlighting the marked difference that can occur between studies with rapidly swept fields and those employing conventional diode laser devices.
This paper demonstrates how a quantum cascade laser (QCL) in its intrapulse mode can provide a simple method for probing the products of a photolysis event. The system studied is the 266 nm photodissociation of CF3I with the CF3 fragments subsequently detected using radiation at approximately 1253 cm(-1) generated by a pulsed QCL. The tuning range provided by the frequency down-chirp of the QCL operated in its intrapulse mode allows a approximately 1 cm(-1) segment of the CF3 nu3 band to be measured following each photolysis laser pulse. Identification of features within this spectral region allows the CF3 ( v = 0) number density to be calculated as a function of pump-probe delay, and consequently the processes which populate and deplete this quantum state may be examined. Rate constants for the population cascade from higher vibrational levels into the v = 0 state, k 1, and for the recombination of the CF3 radicals to form C2F6, k2, are measured. The returned values of k1 = (2.3 +/- 0.34) x 10(-12) cm(3) molecule(-1) s(-1) and k2 = (3.9 +/- 0.34) x 10(-12) cm(3) molecule(-1) s(-1) are found to be in good agreement with reported literature values.
A widely tunable pulsed external cavity quantum cascade laser operating around 8 μm has been used to make rotationally resolved measurements of rapid passage effects in the absorption spectrum of N2O. Rapid passage signals as a function of laser power and N2O pressure are presented. Comparisons are drawn with measurements performed on the same transition with a standard distributed feedback quantum cascade laser. The initial observations on rapid passage effects induced with an external cavity quantum cascade laser show that such high power, widely tunable radiation sources may find applications in both nonlinear optics and optical sensing experiments.
The application of quantum cascade lasers in the intrapulse operation mode for low-pressure plasma spectroscopy is hampered by the observation of rapid passage effects, leading to lower quantitative accuracy. We demonstrate that accurate densities and rotational temperatures of CH4 within a CH4 plasma can be obtained by characterizing the rapid passage effects in gas phase conditions prior to carrying out the plasma measurements. Furthermore, we show that the ratios of the integrated absorption of two transitions are not affected by the rapid passage effect and, thus, rotational temperatures of species can be obtained.
A high power continuous wave quantum cascade laser operating around 1900 cm(-1) has been used to conduct Lamb dip spectroscopy on a low pressure sample of NO. The widths of the Lamb dips indicate that the laser linewidth is 800 ± 60 kHz and the power sufficient to induce significant population transfer of up to 35%. While the Lamb dip signals are symmetric at low laser chirp rates, they become increasingly asymmetric as the chirp rate increases, further confirming the significant degree of population transfer. In addition rapid passage structure on the Lamb dip signal is observed after the weak probe beam is swept through the line center. This structure is sensitive to both the probe chirp rate and the underlying hyperfine structure of the rovibrational transition, and is accurately modeled using the optical Bloch equations.
Rapid passage signals have been observed in which a delayed switch from absorption to emission is observed as the optical density of acetylene in a long path length cell is increased. The delayed switch results in the generation of a large, narrow, transient gain signal. Using numerical solutions of the Maxwell-Bloch equations we have demonstrated that this effect is due to constructive interference between the incident laser field and the field generated by the response of the gaseous medium. This occurs owing to the minimal collisional damping at the low gas pressures within the gas cell, so that the chirp rate of the laser is faster than the collisional reorientation time of the molecules.
The kinetics of stable species has been studied in situ in pulsed CF4/H2 radio frequency discharges by means of time resolved quantum cascade laser absorption spectroscopy. The absorption spectra were usually recorded with a time resolution of 5 ms and required a multi-species analysis, because of interfering complex absorption features of CF4 and C3F8. For this reason, measurements were carried out at two different spectral positions. High resolution spectroscopic data were established by calibrating effective absorption cross sections and their relative temperature dependences for the relevant low pressure conditions (10 Pa). During the discharge a decrease in the CF4 density by ~12% was observed. The off-phase was characterized mainly by the gas exchange. The C3F8 density in the off-phase was found to be of the order of the detection limit (3 × 1013 cm−3). Spectra acquired during the plasma-on phase showed a rapid temperature-induced increase in the absorption signal and, additionally, suggested the influence of a short-lived broadband absorbing species. The reasonable assumption of the presence of CF4 hotbands has not yet enabled a further quantification.
A continuous wave quantum cascade laser (cw-QCL) operating at 10 μm has been used to record absorption spectra of low pressure samples of OCS in an astigmatic Herriott cell. As a result of the frequency chirp of the laser, the spectra show clearly the effects of rapid passage on the absorption line shape. At the low chirp rates that can be obtained with the cw-QCL, population transfer between rovibrational quantum states is predicted to be much more efficient than in typical pulsed QCL experiments. This optical pumping is investigated by solving the Maxwell Bloch equations to simulate the propagation of the laser radiation through an inhomogeneously broadened two-level system. The calculated absorption profiles show good quantitative agreement with those measured experimentally over a range of chirp rates and optical thicknesses. It is predicted that at a low chirp rate of 0.13 MHz ns(-1), the population transfer between rovibrational quantum states is 12%, considerably more than that obtained at the higher chirp rates utilised in pulsed QCL experiments.
Rapid passage signals showing the effects of molecular alignment have been observed when low pressure samples of nitrous oxide are interrogated by radiation from a pulsed 7.84 µm quantum cascade laser. These effects occur when the sweep rate of the laser through a Doppler broadened absorption line is much faster than the collisional relaxation time, and when the power density of the linearly polarised laser radiation is sufficient to cause optical pumping. Using a laser pulse of duration 1.3 microseconds, the frequency sweeps approximately 90 GHz. The variation of the laser tuning rate during the laser pulse, from about100 MHz/ns at the beginning to about 20 MHz/ns at the end, allows the relationship between sweep rate and collisional damping to be investigated. We show, by comparing the experimental signals with those calculated by coupled Maxwell-Bloch equations, how the rapid passage effects in nitrous oxide are influenced by the number density, transition cross-section and reorientation lifetime.
Pulsed quantum-cascade-laser spectrometers are usually used to detect atmospheric gases with either the interpulse technique
(short pulses, typically 5–20ns) or the intrapulse technique (long pulses, typically 500–800ns). Each of these techniques
has its drawbacks. Particularly the gas absorption spectra are generally distorted. We have previously developed another technique
called intermediate-size pulses (typically 50–100ns) technique for gas detection using pulsed QCL spectrometers. In this
paper, infrared spectra of ammonia recorded with this technique in the 10μm region are presented. For the NH3 spectra recorded at low pressure (i.e. in the mbar range), the spectra show typical oscillations after the absorption. The
Beer–Lambert law cannot explain these oscillations, termed the rapid-passage effect. Comparisons between experimental and
calculated spectra will be realized. This phenomenon is not satisfactory from a spectroscopic point of view and spectra must
be recorded at higher pressures. For the NH3 spectra recorded at higher pressure (i.e. in the 50mbar range), the oscillations disappear and the Beer–Lambert law could
be reused. This paper will demonstrate that the intermediate-size technique gives reliable measurements for NH3 detection. Moreover the typical apparatus function (0.003cm−1 HWHM) is far lower from the typical apparatus function of the interpulse QCL spectrometers (0.015cm−1 HWHM).
The same quantum cascade laser spectrometer working around 9μm was used in both continuous-wave and pulsed modes to compare
their own characteristics. The laser emitting in continuous-wave mode was mainly used to study some spectroscopic parameters
of SO2 ro-vibrational lines. This work demonstrates the necessity to use new calculations previously developed instead of conventional
databases such as HITRAN. In addition, the same laser emitting in pulsed mode with long pulses (600 to 900ns) was used to
record SO2 spectra with the intrapulse technique. This work permits us to make comparisons about those two modes of emission for the
development of future spectrometers.
The coupling of quantum cascade lasers (QCLs) with an off-axis cavity enhanced absorption (CEA) spectrometer and an astigmatic
multiple pass absorption (MPA) spectrometer are described in this paper. A continuous wave (cw) liquid nitrogen cooled distributed-feedback
QCL at 5.7μm and a cw room temperature mode-hop-free external-cavity QCL at 6.1μm were employed as the light source. For
the CEA spectrometer, the effects of mirror size and laser scan rate were evaluated. For the MPA spectrometer, a pair of astigmatic
mirrors with a 55cm mirror distance was aligned to the 366-pass configuration. The jet-cooled samples were generated using
a homemade pulsed slit jet nozzle assembly. Two LabVIEW programs were written to automate and synchronize the timing of the
laser scan, the pulsed slit jet molecular expansion, and the data acquisition. Infrared spectra of jet-cooled methyl lactate
and the Ar–H2O complex and room temperature N2O and NH3 samples were measured using both the rapid scan and the wavelength modulation methods to evaluate the sensitivity and resolution
of the CEA and MPA spectrometers. The combination of the MPA spectrometer with the external-cavity QCL using the rapid scan
method was found to be the best suited combination to measure high resolution jet-cooled infrared spectra.
Quantum cascade lasers (QCLs) have attracted considerable interest as an alternative tuneable narrow bandwidth light source
in the mid-infrared spectral range for chemical sensing. Pulsed QCL spectrometers are often used with short laser pulses and
a bias current ramp similar to diode laser spectroscopy. Artefacts in the recorded spectra such as disturbed line shapes or
underestimated absorption coefficients have been reported. A detailed time-resolved high-bandwidth analysis of individual
pulses during a laser sweep has been performed. Quantitative results for CH4 absorption features around 1347cm−1 (7.42μm) fell short of the expected values for reasonable operating conditions of the QCL. The origin of the artefacts using
short pulses was identified to be partly of the same nature as in the case of long laser pulses. A complex combination with
the tuning principle was found, leading to an apparently increased instrumental broadening (effective line width) and underestimated
concentrations at low-pressure conditions.
We report observations of rapid passage signals induced in samples of N2O and CH4 present in a multipass cell with an optical path length of 5m. The effect of laser power and chirp rate upon the signals
has been studied by utilising two different chirped quantum cascade lasers operating around 8μm. The rapid passage signals
exhibit an increasing delay in the switch from absorption to emission as a function of increased gas pressure (up to 8Torr
of gas). By comparing a selection of transitions in N2O and CH4, we show that, unlike ammonia, this ‘pressure shift’ is independent of the transition dipole moment, spectroscopic branch
probed and laser chirp rate. As the transition dipole moment is much larger in nitrous oxide than methane, we believe that
this indicates that N2O–N2O collisions are more efficient at removing coherence from the polarised sample than CH4–CH4 collisions. We have also observed this pressure shift in a short path length of 40cm, although with a much reduced value,
indicating that propagation effects are important in this optically thick minimally damped system.
Two 5 µm continuous wave quantum cascade lasers are used to perform a counterpropagating pump and probe experiment on a low pressure sample of nitric oxide. The strong pump field excites a fundamental rovibrational transition and the weaker probe field is tuned to the corresponding rotationally resolved hot band transition. When both light fields are in resonance, rapid passage is observed in the hot band absorption lineshape arising from a minimally damped and velocity-selected sample of molecules in the v=1 state. The measured rapid passage signals are well described by a two-level model based on the optical Bloch equations.
We review the use of both pulsed and continuous wave quantum cascade
lasers in high-resolution spectroscopic studies of gas phase species. In
particular, the application of pulsed systems for probing kinetic
processes and the inherent rapid passage structure that accompanies
observations of low-pressure samples using these rapidly chirped devices
are highlighted. Broadband absorber spectroscopy and time-resolved
concentration measurements of short-lived species, respectively
exploiting the wide intrapulse tuning range and the pulse temporal
resolution, are also mentioned. For comparison, we also present recent
sub-Doppler Lamb-dip measurements on a low-pressure sample of NO, using
a continuous wave external cavity quantum cascade laser system. Using
this methodology the stability and resolution of this source is
quantified. We find that the laser linewidth as measured via the
Lamb-dip is ca. 2.7 MHz as the laser is tuned at comparably slow rates,
but decreases to 1.3 MHz as the laser scan rate is increased such that
the transition is observed at 30 kHz. Using this source, wavelength
modulation spectroscopy of NO is presented.
In this Letter, a 10 microm quantum cascade laser operating in the intrapulse mode is used observe rapid passage (RP) effects within a 40 cm single-pass gas cell containing low pressures of NH(3). The laser tuning range allows the rotational states J=2 with K=0, 1, and 2 to be probed. We show that the RP structures change as a function of optical density and that the magnitude of the delay in the switch from absorption to emission as a function of increased gas pressure is dependent upon the initial value of K. These measurements are qualitatively well modeled using the Maxwell-Bloch equations.
Intrapulse quantum cascade laser spectrometers are able to produce both saturation and molecular alignment of the gas sample. This is due to the rapid sweep of the radiation through the absorption features. The intrapulse time domain spectra closely resemble those recorded in coherent optical nutation experiments. In the present paper, the frequency down-chirped technique is employed to investigate the nitrous oxide-foreign gas collisions. We have demonstrated that the measurements may be characterized by the induced polarization dominated and collision dominated measurement limits. The first of these is directly related to the time dependence of the long range collision cross sections. Among the collisional partners considered, carbon dioxide shows a very unusual behavior of rapid polarization damping, resulting in the production of symmetrical line shapes at very low gas buffer pressures. In the collision dominated regime, the pressure broadening parameters, which we have derived, are comparable at slow chirp rates, with those derived from other experimental methods. By comparing the pressure broadening coefficients of Ar, N(2), and CO(2) with those of He, making use of the chirp rate independence of the pressure broadening by helium, we have shown that at higher chirp rates there is clear evidence of the chirp-rate dependence of the pressure broadening parameters of N(2) and CO(2).
Information about intermolecular potentials is usually obtained through the analysis of the absorption line shapes recorded in the frequency domain. This approach is also adopted to study the effects of motional narrowing and speed dependence of the pressure broadening coefficients. On the other hand, time domain measurements are directly related to molecular collisions and are therefore frequently employed to study molecular relaxation rates, as well as the effects of velocity changing collisions and the speed dependence of the absorption cross sections. Intrapulse quantum cascade laser spectrometers are able to produce both saturation and molecular alignment of the gas sample. This is due to the rapid sweep of the radiation through the absorption features. In the present work the frequency down-chirped radiation emitted by an intrapulsed quantum cascade laser operating near 7.8 mum is employed to investigate the collisional relaxation processes, and the collisional narrowing, in the 15(0,15)<--16(1,16) and 15(1,15)<--16(0,16) doublet in the water vapor nu(2) band. The effects of He, Ne, Ar, N(2), and CO(2) as collisional partners are investigated. The experimental results clearly indicate the dependence of the collisional cross sections upon the chirp rate. They also demonstrate that by using different chirp rates it is possible to gain information about the intermolecular processes driving the molecular collisions and the related energy transfer.
We have designed and characterized a mid-IR spectrometer built around a pulsed distributed-feedback quantum cascade laser using the characteristic frequency down-chirp to scan through the spectral region 6.5 cm(-1) spectral region. The behavior of this chirp is extensively measured. The accuracy and detection limits of the system as an absorption spectrometer are demonstrated first by measuring spectra of acetylene through a single pass 16 cm absorption cell in real time at low concentrations and atmospheric pressure. The smallest detectable peak is measured to be approximately 1.5 x 10(-4) absorbance units, yielding a minimum detectable concentration length product of 2.4 parts per million meter at standard temperature and pressure. This system is then used to detect acetylene within an ethylene-air opposed flow flame. Measurements of acetylene content as a function of height above the fuel source are presented, as well as measurements of acetylene produced in fuel breakdown as a function of preinjection fuel temperature.
Pulsed quantum-cascade-laser (QCL) spectrometers are usually used to detect atmospheric gases with either the interpulse technique (short pulses, typically 5-20 ns) or the intrapulse technique (long pulses, typically 500-800 ns). Each of these techniques has many drawbacks, which we present. Particularly the gas absorption spectra are generally distorted. We demonstrate the possibility to use intermediate pulses (typically 50-100 ns) for gas detection using pulsed QCL spectrometers. IR spectra of ammonia recorded in the 10 microm region are presented in various conditions of pulse emission. These experiences demonstrate the large influence of the pulse shape on the recorded spectrum and the importance to use our alternative method for gas detection with pulsed QCL spectrometers.
A direct-absorption spectrometer, based on a pulsed, distributed feedback, quantum cascade laser with a 10.26-microm wavelength and an astigmatic Herriott cell with a 66-m path length, has been developed for high-resolution IR spectroscopy. This spectrometer utilizes the intrapulse method, an example of sweep integration, in which the almost linear wavelength up-chirp obtained from a distributed feedback, quantum cascade laser yields a spectral microwindow of as many as 2.5 wave numbers/cm(-1). Within this spectral microwindow, molecular fingerprints can be monitored and recorded in real time. This system allows both the detection of carbon dioxide and ethylene and the real-time observation of the evolution of these gases in the exhaust by-products from several cars.
A semiconductor injection laser that differs in a fundamental way from diode lasers has been demonstrated. It is built out
of quantum semiconductor structures that were grown by molecular beam epitaxy and designed by band structure engineering.
Electrons streaming down a potential staircase sequentially emit photons at the steps. The steps consist of coupled quantum
wells in which population inversion between discrete conduction band excited states is achieved by control of tunneling. A
strong narrowing of the emission spectrum, above threshold, provides direct evidence of laser action at a wavelength of 4.2
micrometers with peak powers in excess of 8 milliwatts in pulsed operation. In quantum cascade lasers, the wavelength, entirely
determined by quantum confinement, can be tailored from the mid-infrared to the submillimeter wave region in the same heterostructure
material.
The P(3) and P(4) manifolds of the ν3 band of SF6 have been observed in a supersonic beam with a bolometric detection. The influence of the laser beam divergence on the excitation efficiency has been studied. Rabi oscillations are observed when the wavefront is flat in the interaction region whereas only adiabatic rapid passage occurs when the molecules see a curved wavefront.
We report high-resolution saturated-absorption spectra recorded by use of a few microwatts of radiation generated in a single pass by difference-frequency mixing. These results were obtained without the use of buildup cavities for the nonlinear mixing or for the saturation spectroscopy. We show high-quality saturated-absorption signals for the fundamental rovibrational band of CO(2) near 4.3 mum. Convenient sources and frequency-conversion devices open new possibilities for sub-Doppler spectroscopy in the infrared.
We report what we believe are the first spectroscopic measurements to be made with a room-temperature quantum-cascade distributed-feedback laser. Using wavelength modulation spectroscopy, we detected N(2)O and CH(4) in the chemical fingerprint wavelength range near 8microm . The noise equivalent absorbance for our measurement was 5 parts in 10(5), limited by excess amplitude modulation on the laser output, which corresponds to a 1-Hz bandwidth detection limit of 250 parts N(2)O in 10(9) parts N(2) in a 1-m path length.
We report what we believe to be the first systematic study of Doppler-free, nonlinear absorption by use of cavity ringdown spectroscopy. We have developed a variant of cavity ringdown spectroscopy for the mid-infrared region between 9 and 11 microm, exploiting the intracavity power buildup that is possible with continuous-wave lasers. The infrared source consists of a continuous-wave CO2 laser with 1-mW tunable infrared sidebands that couple into a high-finesse stable resonator. We tune the sideband frequencies to observe a saturated, Doppler-free Lamb dip in the nu7, 11(1,10) <-- 11(2,10) rovibrational transition of ethylene (C2H4). Power studies of the Lamb dip are presented to examine the intracavity effects of saturation on the Lamb-dip linewidth, the peak depth, and the broadband absorption.
Quantum cascade lasers operating in the 3.5 to 24 micron spectral range can be used for trace gas detection in ambient air based on absorption spectroscopy. Recent advances in spectroscopic detection techniques have been employed to achieve minimum detectable absorption coefficients of 10<sup>-9</sup> cm<sup>-1</sup> in several real world applications.
There is an increasing need in many chemical sensing applications
ranging from industrial process control to environmental science and
medical diagnostics for fast, sensitive, and selective gas detection
based on laser spectroscopy. The recent availability of novel pulsed and
CW quantum cascade distributed feedback (QC-DFB) lasers as mid-infrared
spectroscopic sources address this need. A number of spectroscopic
techniques have been demonstrated worldwide by several groups. For
example, the authors have employed QC-DFB lasers for the monitoring and
quantification of several trace gases and isotopic species in ambient
air at ppmv and ppbv levels by means of direct absorption, wavelength
modulation, and cavity enhanced and cavity ringdown spectroscopy
A spectrometer using a pulsed, 10.25-μm-wavelength, thermoelectrically cooled quantum-cascade distributed-feedback laser has been developed for sensitive high-resolution infrared absorption spectroscopy. This spectrometer is based upon the use of the almost linear frequency downchirp of up to 75 GHz produced by a square current drive pulse. The behavior of this downchirp has been investigated in detail using high-resolution Fourier-transform spectrometers. The downchirp spectrometer provides a real-time display of the spectral fingerprint of molecular gases over a wave-number range of up to 2.5 cm−1. Using an astigmatic Herriott cell with a maximum path length of 101 m and a 5-kHz pulse repetition rate with 12-s averaging, absorption lines having an absorbance of less than 0.01 (an absorption of less than 1%) may be measured.
Using a tunable diode-laser spectrometer, we have measured self-broadening coefficients for a few transitions in the ν7 fundamental band of C2H4 at 298 and 174K. The studied transitions J′, Ka′, Kc′←J, Ka, Kc with 3⩽J⩽17, 1⩽Ka⩽4, and 1⩽Kc⩽14 are located in the spectral range 919–982cm−1. The collisional widths are measured by fitting each spectral line with Voigt, Rautian, and speed-dependent Rautian profiles. The latter model provides larger broadening coefficients than the Rautian profile and still larger coefficients than the Voigt profile. An approximate semiclassical calculation performed by considering only electrostatic interactions leads to reasonable agreement with the experimental data. By comparing the results obtained at room and low temperatures, the temperature dependence of the self-broadening has been determined both experimentally and theoretically.
Fifty Doppler-broadened absorption lines of ethylene have been measured within large profiles CO2 or N2O lase lines. These laser lines are produced by a high pressure waveguide laser and have a full width between 200 and 900 MHz. Eleven absorption lines, the more intense ones, have been assigned to the nu7 band of C2H4. The other absorption lines must belong to hot bands or to the nu7 band of H2 12C13CH2.
I report the observation of population inversion by optical adiabatic rapid passage. These observations, on an ${\mathrm{NH}}_{3}$ infrared transition with all the relevant parameters known, agree with theoretical expectations. The pressure dependence of ${T}_{1}$, obtained by this technique, makes possible the first estimate of the collision-induced rotational lifetime of ${\mathrm{NH}}_{3}$ in an excited band, which is significantly longer than the rotational lifetime in the ground vibrational band.
This report deals with non-linear effects produced in molecules by strong laser fields. The molecules experience these laser fields during their passage through the laser waists. We present results on rapid adiabatic passage processes which move the molecules up and down the energy ladder, the latter due to stimulated emission. Experimentally, stimulated emission is observed by opto-thermal detection of a molecular beam where de-excitation by stimulated emission leads to negative signals as compared to straightforward excitation processes. Two-level, three-level and multi-level systems are covered by the following discussion.
The theoretical and experimental analysis of fast passage phenomena in rotational spectroscopy is described in the pressure broadened limit. The phenomenon of Stark sweeping fast passage is carefully distinguished from the phenomena of transient absorption and transient emission that were treated earlier. We show that the observed response at the detector after Stark sweeping a molecular resonance through a fixed oscillator is a combination of off-resonance absorption and the beat between the fixed oscillator and the emission from the fast passage induced polarization.
The recent development of sub-Doppler spectroscopy using microwave modulation sidebands on CO2 laser radiation has enabled us to perform ultrahigh resolution tunable infrared spectroscopy and study the Stark and Zeeman effects in ethylene. Clear Stark splittings were observed in low J vibration–rotation transitions of the ν7 band by applying an electric field of up to 50 kV/cm. The Stark shift is caused by the polarizability of ethylene as well as by the second order Stark effect between the accidentally degenerate ν7 and ν8 vibrational states (ν07−ν08 =8.91 cm−1). Analysis of the observed Stark patterns gives the polarizability anisotropies for the ground state to be αaa− 1/2 (αbb+αcc) =1.91(13) A˚3 and αbb−αcc =0.07(19) A˚3 and for the ν7 state to be αaa− 1/2 (αbb+αcc) =1.78(11) A˚3 and αbb−αcc =0.15(14) A˚3. The dipole moment along the a axis of ethylene induced by the ν7 and ν8 vibrations was determined to be ∂2μa/∂q7 ∂q8 =0.0791(4) D. Zeeman splittings of ethylene in a magnetic field of approximately 2.5 kG were also observed. Since the infrared radiation was right-handed circularly polarized and the axial magnetic field was along the direction of the beam, only the Δm=+1 transitions were observed. The tensor components of the rotational g factors were determined to be gaa =−0.424(8), gbb =−0.123(8), and gcc =0.037(6). A qualitative discussion on the relationship of the electronic structure of ethylene and the rotational g factors is given.
Short pulses of narrow-line low-intensity dye-laser light nearly resonant with the Zeeman-split 2P1/2 resonance line (7948 Å) of rubidium were observed to propagate through dilute rubidium vapor as slowly as (1/14)c. These slow pulse velocities showed that most of the energy in the propagating wave was contained by the vapor as coherent atomic excitation. The observed pulse velocities vp are in good agreement with the equation vp=dω/dk for the group velocity obtained from linear-dispersion theory. Also, the experimental results are quantitatively explained by adiabatic following, in which the pseudomoments of the atoms remain aligned along the effective field of the laser light. The adiabatic-following model allows for a direct comparison of our results with the work on self-induced transparency. For high-intensity light, adiabatic following predicts a nonlinear pulse velocity and the possibility of observing self-steepening.
Laser spectroscopy has found many industrial applications, e.g., control of automotive exhaust and process monitoring. The midinfrared region is of special interest because it has stronger absorption lines compared to the near infrared (NIR). However, in the NIR high quality reliable laser sources, detectors, and passive optical components are available. A quantum cascade laser could change this situation if fundamental advantages can be exploited with compact and reliable systems. It will be shown that, using pulsed lasers and available fast detectors, lower residual sensitivity levels than in corresponding NIR systems can be achieved. The stability is sufficient for industrial applications. © 2003 American Institute of Physics.
Rotational structure has been resolved and analyzed in two of the infrared-active perpendicular bands of C2H4 vapor: the Type b fundamental band, ν10, at 826 cm—1, and the Type c fundamental band, ν7, at 949 cm—1. Many of the individual PP and RR branch lines have been observed. The analysis has been confined to values of the quantum number K≥3, for which energy levels ethylene shows no detectable deviations from a symmetric-top rotational structure. The analysis reveals a Coriolis interaction between ν7 and ν10, and between ν4 and ν10, and values of the Coriolis constants ζ7,10z and ζ4,10y are obtained; these are related to normal coordinate calculations for the appropriate symmetry species, and force constants are derived to fit the observed zeta constants. The band center of ν10 has been revised from the original figure of 810 cm—1 to the new value, 826 cm—1, and the inactive frequency ν4 is estimated to lie at 1023±3 cm—1, in good agreement with the previous estimate of 1027 cm—1.
The change in the value of ν10 leads to a suggested change in the value of the Raman-active fundamental ν6 from 1236 to 1222 cm—1. New combination bands have been observed at 2174 cm—1, assigned as ν3+ν10; and at 2252 cm—1, assigned as ν4+ν6; also rotational structure has been resolved and analyzed in the ν6+ν10 band at 2048 cm—1.
The new data obtained for the C2H4 molecule are summarized in Table XII, with all of the other data presently available on the vibrational and rotational constants.
Practical considerations arising from Doppler distributions and beam divergence typically limit the amount of internal excitation that can be produced as a molecule passes across a cw laser beam. For coherent excitation this population transfer is furthermore limited by coherent population return (CPR), in which transiently excited molecules, off resonance, adiabatically evolve so as to return to the initial state at the end of a smooth pulse. We illustrate this effect with simulations, and show that in principle the presence of incoherence in the laser radiation (i.e., a finite bandwidth) can help suppress CPR, thereby substantially improving the averaged population transfer. For cases of practical relevance, the velocity-averaged population transfer can be optimized by a proper choice of the laser beam waist along the molecular beam direction. © 2003 American Institute of Physics.
We describe transient spectroscopic measurements with a current-switched semiconductor diode laser. Following a fast pulse modulation in the injection current of the diode laser, the diode laser frequency is switched by a few hundred MHz and, then, is scanned over 10 GHz range in a short-time evolution (<0.5 µs). Free-induction decay based on the frequency switching and transient absorption spectroscopy of rubidium D1 lines based on the fast frequency-scan are experimentally demonstrated. These fast spectroscopic measurements are accomplished in a single-shot pulse modulation and are, thus, applicable to short-lived samples such as in flowing or combustion systems.
Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detection of trace gases
in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than 1nmole/mole (<1ppbv) using
a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26μm and coupled to a 210-m path length multiple-pass
absorption cell at reduced pressure (50Torr). The sensitivity of the system is enhanced by operating under pulsing conditions
which reduce the laser line width to 0.010cm-1 (300MHz) HWHM, and by normalizing pulse-to-pulse intensity variations with temporal gating on a single HgCdTe detector.
The system is demonstrated by detecting nitric oxide in outside air and comparing results to a conventional tunable diode
laser spectrometer sampling from a common inlet. A detection precision of 0.12ppb Hz-1/2 is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of 1×10-5Hz-1/2 or an absorbance precision per unit path length of 5×10-10cm-1 Hz-1/2. A precision of 0.3ppb Hz-1/2 is obtained using a thermoelectrically cooled detector, which allows continuous unattended operation over extended time periods
with a totally cryogen-free instrument.
We present a review of the use of diode lasers in atomic physics with an extensive list of references. We discuss the relevant characteristics of diode lasers and explain how to purchase and use them. We also review the various techniques that have been used to control and narrow the spectral outputs of diode lasers. Finally we present a number of examples illustrating the use of diode lasers in atomic physics experiments.
Population inversion by adiabatic rapid passage (ARP) utilizing narrow saturation resonances is observed for an infrared transition of NH 3 . The population change produced by sweeping the frequency of a strong saturating laser field through the center of a Doppler‐broadened absorption line is probed by a weak counterpropagating field as in a Lamb‐dip experiment. When the ARP conditions are satisfied, inversion of population is detected as amplification of the probe wave near the line center.
FTS and TDL spectra of ethylene in the region have been observed, measured, calibrated, assigned and intensities have been measured. The ultimate goal of this work is the production of a planetary modeler's atlas. A spectrum taken in double-pass configuration at the McMath–Pierce FTS instrument at Kitt Peak National Observatory has been frequency calibrated using CO2 laser bands. Results of a previous analysis (Cauuet et al., J Mol Spectrosc 1990;139:191) have enabled the assignment of the FTS spectrum wherein we have measured over 500 line intensities in the 900– region. These FTS intensities have been calibrated against 13 isolated transitions, taken as secondary intensity standards. These standard lines have been measured independently using TDL (tunable-diode-laser) spectrometers at University of Tennessee and Goddard Space Flight Center. A calculated spectrum, including mixing coefficients for ν4, ν7, ν10 and ν12, and calculated relative intensities, and the TDL-calibrated FTS line intensities were used as data in a non-linear regression analysis to determine the vibrational band intensities of , , and . These vibrational band intensities combined with the theoretical spectral-line atlas make possible the generation of an ethylene spectrum at an arbitrary temperature. Such spectra prove useful to the planetary-atmosphere modeling-community. A web site is available where an individual can interact with the model and download a custom atlas. The URL is http://aurora.phys.utk.edublass/ethyatl/.
N2-broadening coefficients have been measured for 35 lines of C2H4 in the ν7 fundamental transition, using a tunable diode–laser spectrometer. These lines with 3 ≤ J" ≤ 23, 0 ≤ Ka″ ≤ 4, and 1 ≤ Kc″ ≤ 22 are located in the spectral range 880–1024 cm−1. The collisional half-widths are obtained by modeling each spectral line with a Voigt and a Rautian profile. The measured broadening coefficients belonging to transitions with the same J" generally increase with increasing Ka″ and the coefficients decrease on the whole with increasing J". These behaviors can be predicted by an approximate semiclassical calculation performed by approximating ethylene to a prolate symmetric-top molecule.
We present detailed experimental and theoretical results on population transfer with frequency-swept picosecond laser pulses. Here, we demonstrate that intense frequency-swept pulses, when applied in the adiabatic limit, lead to both more efficient and more selective excitation than do unmodulated laser pulses. The experimental work is performed on quasi-two-level systems (pentacene/p-terphenyl crystal and Na vapor), quasi-three-level systems (Na vapor), and on more complex multilevel systems (I-2 vapor). We discuss the different characteristics of adiabatic population transfer in both few-level, and multilevel cases, and, in particular, present computer calculations to explore the effects of molecular rotations in multilevel adiabatic population transfer.
A multipass absorption cell, based on an astigmatic variant of the off-axis resonator (Herriott) configuration, has been designed to obtain long path lengths in small volumes. Rotation of the mirror axes is used to obtain an effective adjustability in the two mirror radii. This allows one to compensate for errors in mirror radii that are encountered in manufacture, thereby generating the desired reentrant patterns with less-precise mirrors. Acombination of mirror rotation and separation changes can be used to reach a variety of reentrant patterns and path lengths with a fixed set of astigmatic mirrors. The accessible patterns can be determined from trajectories, as a function of rotation and separation, through a general map of reentrant solutions. Desirable patterns for long-path spectroscopy can be chosen on the basis of path length, distance of the closest beam spot from the coupling hole, and tilt insensitivity. We describe the mathematics and analysis methods for the astigmatic cell with mirror rotation and then describe the design and test of prototype cells with this concept. Two cell designs are presented, a cell with 100-m path length in a volume of 3 L and a cell with 36-m path length in a volume of 0.3 L. Tests of low-volume absorption cells that use mirror rotation, designed for fast-flow atmospheric sampling, show the validity and the usefulness of the techniques that we have developed.
We report observation of coherent transient amplification that is due to free-induction decay and optical nutation in inhomogeneously broadened rubidium atoms by a sudden switch of diode-laser frequency. An amplification coefficient of 2.2 x 10(-2) cm(-1) is observed when the diode-laser frequency, initially tuned to the blue side of the Doppler-broadened absorption profile, is switched to be out of resonance by application of a step-function pulsed injection current. The transition from free-induction decay to optical nutation is observed and discussed.
The saturation spectrum of ethylene has been studied between 900 and 1100 cm-1 with the sub-Doppler sideband spectrometer of Lille. Two hundred absorption lines have been recorded with an absolute accuracy of approximately 15 kHz. On the basis of these accurate data, a new global analysis of the GS and of the nu7, nu10, and nu4 upper states was achieved taking into account the Coriolis interactions up to the third order. A statistical agreement is obtained with all experimental data with an estimated standard deviation equal to 0.825. The spectra of ethylene between 900 and 1100 cm-1 become very good wavenumber standards with an absolute accuracy of 10(-6) cm-1. Copyright 1998 Academic Press.
Highly efficient and selective population transfer in NO molecules in the electronic ground state (X2Π1/2) from the vibrational level v’’=0 to the level v’’=6 is demonstrated. It shows, for the first time with pulsed lasers, that a rather generally applicable scheme for complete control over the level population in atoms and molecules is now available. The efficiency relies on a counterintuitive interaction sequence of two lasers with the molecule in a process of stimulated Raman scattering involving adiabatic passage.
N(2)-broadening coefficients have been measured for 35 lines of C(2)H(4) in the nu(7) fundamental transition, using a tunable diode-laser spectrometer. These lines with 3 </= J" </= 23, 0 </= K(a)(") </= 4, and 1 </= K(c)(") </= 22 are located in the spectral range 880-1024 cm(-1). The collisional half-widths are obtained by modeling each spectral line with a Voigt and a Rautian profile. The measured broadening coefficients belonging to transitions with the same J" generally increase with increasing K(a)(") and the coefficients decrease on the whole with increasing J". These behaviors can be predicted by an approximate semiclassical calculation performed by approximating ethylene to a prolate symmetric-top molecule. Copyright 2000 Academic Press.
We review some basic techniques for laser-induced adiabatic population transfer between discrete quantum states in atoms and molecules.
We describe a mid-infrared spectrometer that is based on the combination of a multiple-pass absorption cell and a submicrosecond pulsed quantum-cascade laser. The spectrometer is capable of both making sensitive measurements and providing a real-time display of the spectral fingerprint of molecular vapors. For a cell with a path length of 9.6 m, dilution measurements made of the nu9 band transitions of 1,1-difluoroethylene indicate a sensitivity of 500 parts in 10(9), corresponding to a fractional absorbance of 4 x 10(-4).
EPR spectra at 250 MHz for a single crystal of lithium phthalocyanine (LiPc) in the absence of oxygen and for a deoxygenated aqueous solution of a Nycomed triarylmethyl (trityl-CD3) radical were obtained at scan rates between 1.3 x 10(3) and 3.4 x 10(5)G/s. These scan rates are rapid relative to the reciprocals of the electron spin relaxation times (LiPc: T1 = 3.5 micros and T2 = 2.5 micros; trityl: T1 = 12 micros and T2 = 11.5 micros) and cause characteristic oscillations in the direct-detected absorption spectra. For a given scan rate, shorter values of T2 and increased inhomogeneous broadening cause less deep oscillations that damp out more quickly than for longer T2. There is excellent agreement between experimental and calculated lineshapes and signal amplitudes as a function of radiofrequency magnetic field (B1) and scan rate. When B1 is adjusted for maximum signal amplitude as a function of scan rate, signal intensity for constant number of scans is enhanced by up to a factor of three relative to slow scans. The number of scans that can be averaged in a defined period of time is proportional to the scan rate, which further enhances signal amplitude per unit time. Longer relaxation times cause the maximum signal intensity to occur at slower scan rates. These experiments provide the first systematic characterization of direct-detected rapid-scan EPR signals.
A direct-absorption spectrometer, based on a pulsed, distributed feedback, quantum cascade laser with a 10.26-microm wavelength and an astigmatic Herriott cell with a 66-m path length, has been developed for high-resolution IR spectroscopy. This spectrometer utilizes the intrapulse method, an example of sweep integration, in which the almost linear wavelength up-chirp obtained from a distributed feedback, quantum cascade laser yields a spectral microwindow of as many as 2.5 wave numbers/cm(-1). Within this spectral microwindow, molecular fingerprints can be monitored and recorded in real time. This system allows both the detection of carbon dioxide and ethylene and the real-time observation of the evolution of these gases in the exhaust by-products from several cars.
The Quantum cascade (QC) laser is an entirely new type of semiconductor device in which the laser wavelength depends on the band-gap engineering. It can be made to operate over a much larger range than lead salt lasers, covering significant parts of both the infrared and submillimetre regions, and with higher output power. In this tutorial review we survey some of the applications of these new lasers, which range from trace gas detection for atmospheric or medical purposes to sub-Doppler and time dependent non-linear spectroscopy.
We have observed optical free induction decay on the (3)H(6)(1) ? (3)H(4)(1) transition of Tm(3+) ions in LaF(3), using an external-cavity diode laser. The diode-laser frequency was switched by the application of a current step, and the resulting transient was detected by heterodyne beating with the laser at its new frequency. This provides a simple new way to measure coherent transients. The beat note shows a frequency chirp that gives a real-time measure of the diode-laser frequency. This result is attributed to changes in the laser lattice temperature on the microsecond time scale.
The free-induction decay (FID) technique combined with broadly tunable lasers offers a convenient way to survey molecular dephasing times. We measured FID's in molecular iodine by frequency switching an extended-cavity diode laser with a small current modulation. These decay times provide valuable information on the Doppler-free linewidths for frequency-standard applications.
Advances in Magnetic Resonance
- Rr Ernst