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Time-Resolved Detection of the CF 3 Photofragment Using Chirped QCL Radiation
Abstract
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.
... This method offers very good signalto-noise capability, however, it suffers from an increasingly limited spectral modulation depth at higher frequencies, thus prohibiting the scan over complete line profiles. An alternative approach is that of the intrapulse technique [10][11][12][13][14][15][16][17]. ...
... If the pulse duration is set to hundreds of nanoseconds, then an ultra-rapid scan with sufficient spectral range can be achieved ($1-2 cm À1 ), rendering the technique both time-resolved and calibration-free. Thus far, applications of this technique to monitor molecular species (e.g., NH 3 , CF 3 , CO, COS) comprise only a scant number of concentration measurements [10][11][12][13]15] and rapid-passage (RP) effect studies [11,14,16,17]. ...
... This is made possible through good SNR because of the near-perfect shot-to-shot stability, 30 mW pulse power, and laser wavelength corresponding to the stronger fundamental vibrational bands in the mid-infrared (near 7.62 lm for H 2 O). Preservation of time resolution is reconciled with good spectral resolution of 0.013 cm À1 ; the latter of which is possible through reasonable chirp rates, and therefore less likely to encounter the RP effect at pressures [13,14] seen in combustion environments. This novel sensor is also easy with regards to implementation and data interpretation, is compact and uses only a single laser. ...
We report on exploiting the down-chirp phenomenon seen in quantum cascade lasers (QCLs), when modulated with long pulses, for the purpose of performing calibration-free and temporally resolved measurements. Intrapulse spectra of a native species (e.g., H₂O), common to combustion environments, were generated near λ = 7.62 μm at repetition rates as high as 3.125 MHz. Two-line absorption spectroscopy was employed to infer calibration-free temperature from the chirp-induced intrapulse spectra. In this study, such temperature measurements were limited to rates of 250 kHz due to spectral distortion at higher repetition rates. We demonstrate the ease at which accurate temperatures and H₂O compositions can be achieved using simple and compact QCLs operated in the intrapulse mode. The sensor is also applicable to other species, and has the potential to be integrated into commercial technologies.
... These S(T) values are individually corrected by a factor which includes all deviations from the linear Beer-Lambert law. In special cases where no line strength data or absorption cross sections are known but molecular constants for the species of interest are available, a spectral simulation can be applied to retrieve S as demonstrated by Hancock et al. for the CF 3 [57]. This approach is particularly useful for transient molecules. ...
... For the majority of relevant polyatomic molecules neither high resolution absorption cross sections nor spectral data for additional calculations are reported in the literature. The absorption spectra are commonly complex in nature and as a result, they lack any appearance of the rapid passage phenomena (see Figure 5 or [32,57,58]). It has been demonstrated that defining effective absorption cross sections σ eff within a spectral micro-window <ν> in both the inter or intra pulse method may also lead to an adequate calibration (case ii) [32,58]. ...
... If a time resolution below the laser repetition period is required (∆t < T rep , Figure 7a), a delayed trigger scheme has to be applied [31,57]. By changing the delay time after igniting different plasma pulses the discharge is probed with ∆t = 2 t i+1 -1 t i and thus ∆t is set by the difference in delay times for different plasma pulses m. ...
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 is particular need for broadband sources of UV light for fluorescence imaging and for diagnostic and therapeutic uses in medicine [8], and the MIR covers the molecular fingerprint region and the atmospheric windows of 3-5 µm and 8-12 µm [9,10]. Covering this MIR range is necessary for OCT imaging, remote sensing, air pollution monitoring, homeland security, and minimally invasive medical surgery [11][12][13][14][15][16][17][18][19]. ...
... SC generation in the MIR regime has been extensively investigated in the past 15 years with more than 400 scientific papers published on this topic. It is still a very active field of research, driven by a wide range of potential applications including molecular spectroscopy, OCT, material processing, and optical sensing [9][10][11][12][13][14][15][16][17][18][19]. ...
The physics and applications of fiber-based supercontinuum (SC) sources have been a subject of intense interest over the last decade, with significant impact on both basic science and industry. New uses for SC sources are also constantly emerging due to their unique properties that combine high brightness, multi-octave frequency bandwidth, fiber delivery, and single-mode output. The last few years have seen significant research efforts focused on extending the wavelength coverage of SC sources towards the 2 to 20 $\mu$m molecular fingerprint mid-infrared (MIR) region and in the ultraviolet (UV) down to 100 nm, while also improving stability, noise and coherence, output power, and polarization properties. Here we review a selection of recent advances in SC generation in a range of specialty optical fibers, including fluoride, chalcogenide, telluride, and silicon-core fibers for the MIR; UV-grade silica fibers and gas-filled hollow-core fibers for the UV range; and all-normal dispersion fibers for ultralow-noise coherent SC generation.
... There is particular need for broadband sources of UV light for fluorescence imaging and for diagnostic and therapeutic uses in medicine [8], and the MIR covers the molecular fingerprint region and the atmospheric windows of 3-5 µm and 8-12 µm [9,10]. Covering this MIR range is necessary for OCT imaging, remote sensing, air pollution monitoring, homeland security, and minimally invasive medical surgery [11][12][13][14][15][16][17][18][19]. ...
... SC generation in the MIR regime has been extensively investigated in the past 15 years with more than 400 scientific papers published on this topic. It is still a very active field of research, driven by a wide range of potential applications including molecular spectroscopy, OCT, material processing, and optical sensing [9][10][11][12][13][14][15][16][17][18][19]. ...
The physics and applications of fiber-based supercontinuum (SC) sources have been a subject of intense interest over the last decade, with significant impact on both basic science and industry. New uses for SC sources are also constantly emerging due to their unique properties that combine high brightness, multi-octave frequency bandwidth, fiber delivery, and single-mode output. The last few years have seen significant research efforts focused on extending the wavelength coverage of SC sources towards the 2 to $20\,\,\unicode{x00B5}{\rm m}$ 20 µ m molecular fingerprint mid-infrared (MIR) region and in the ultraviolet (UV) down to 100 nm, while also improving stability, noise and coherence, output power, and polarization properties. Here we review a selection of recent advances in SC generation in a range of specialty optical fibers, including fluoride, chalcogenide, telluride, and silicon-core fibers for the MIR; UV-grade silica fibers and gas-filled hollow-core fibers for the UV range; and all-normal dispersion fibers for ultralow-noise coherent SC generation.
... With PGOPHER molecular spectra can be simulated from the basic spectroscopic parameters generally available in the literature to give line positions and line strengths. In this way, the lack of line-by-line spectroscopic parameters of radicals can be overcome as it has been reported in literature [56,57]. ...
... Using a multiplexed detection arrangement, N 2 O, NO and NO 2 were measured with a (total) time resolution of better than 1 μs. How a pulsed QCL can provide a simple method for probing the kinetics of transient species has been reported for CF 3 [57]. ...
Absorption spectroscopy (AS) represents a reliable method for the characterization of cold atmospheric pressure plasma jets. The method’s simplicity stands out in comparison to competing diagnostic techniques. AS is an in situ, non-invasive technique giving absolute densities, free of calibration procedures, which other diagnostics, such as laser-induced fluorescence or optical emission spectroscopy, have to rely on. Ground state densities can be determined without the knowledge of the influence of collisional quenching. Therefore, absolute densities determined by absorption spectroscopy can be taken as calibration for other methods. In this paper, fundamentals of absorption spectroscopy are presented as an entrance to the topic. In the second part of the manuscript, a review of AS performed on cold atmospheric pressure plasma jets, as they are used e.g. in the field of plasma medicine, is presented. The focus is set on special techniques overcoming not only the drawback of spectrally overlapping absorbing species, but also the line-of-sight densities that AS usually provides or the necessity of sufficiently long absorption lengths. Where references are not available for measurements on cold atmospheric pressure plasma jets, other plasma sources including low-pressure plasmas are taken as an example to give suggestions for possible approaches. The final part is a table summarizing examples of absorption spectroscopic measurements on cold atmospheric pressure plasma jets. With this, the paper provides a ‘best practice’ guideline and gives a compendium of works by groups performing absorption spectroscopy on cold atmospheric pressure plasma jets.
... Over the last two decades, a few novel mid-IR laser light sources, such as quantum cascade laser (QCL), have been extensively utilized in the investigations of molecular spectroscopy, reaction dynamics, sensing of trace species, chemical analysis, and biomedical diagnostics. [40][41][42][43][44][45][46][47][48][49][50][51][52][53] Currently, the emission regions of QCL (3 to 25 μm) can cover the most mid-IR spectral region, so that they can access the fingerprint spectra of most species of interest. Compared with FTIR spectroscopy, which utilizes thermal light sources, the main advantages of mid-IR laser spectroscopy are its high sensitivity and selectivity due to the following properties of mid-IR laser sources: (a) narrow linewidth, (b) high spectral brightness, (c) long optical path length due to efficient mode matching of mid-IR lasers to multipass cells or high-finesse cavities, and (d) capability to couple with various modulation schemes for rapid acquisition of spectra or achieving high sensitivities, such as intrapulse thermal chirp or frequency modulation. ...
In this mini-review article, we present the recent progress of utilizing three types of infrared (IR) spectroscopic tools to investigate the chemistry of Criegee intermediates (CIs). The first part of this article described a recent work of Wang et al., which utilized rapid-scan FTIR to detect CIs created from ozonolysis. The second part of this article describes the advantages of quantum cascade lasers (QCLs) used as novel IR spectroscopic tools to probe CIs and the key concepts of experimental implementations. We also summarize the works of Chang and co-workers, which utilized distributed feedback QCLs with thermal chirping to detect the simplest CI, CH2OO, and the subsequent kinetics measurements, such as reactions of CH2OO with ozone, CH3SH, and dimethyl sulfoxide. In the last part of this article, we described the key ideas of external-cavity QCLs coupled with off-axis integrated cavity output spectroscopy, which was proposed by Chung et al., and also the latest progress on determination of the band strength of CH2OO and its rate coefficient with O3.
... There are novel methods of tuning QCLs beyond the DFB configuration that invoke temperature tuning of a frequencyselective element in the cavity 5 or taking advantage of the time evolution of the spectral content of a short (200 ns) pulse. 6 But the most effective method for controlling both the linewidth and wavelength while accessing the entire gain bandwidth of the chip is by employing an External Cavity. In its simplest form, the External Cavity consists of the gain chip, collimating lenses and a rotatable diffraction grating ( Figure 4). ...
Broad tunability in the mid-IR (~3-12 µm) is desirable for a number of applications. We have built a number of External Cavity quantum cascade Lasers (ECqcL™) that maximize tuning range from the quantum cascade chip. As much as 525 cm -1 of pulsed and 121 cm -1 of cw, mode hop-free tuning has been obtained. Various mechanisms for the broadening of the gain of the QC chip are considered.
... Although many articles have been written about the use of QCLs for spectroscopic detection there are still considerable developments required to see their deployment in general applications. Many recent studies have utilised these lasers for absorption spectroscopy, see for example [6][7][8][9], and discovered that they can be used in very interesting ways, such as intrapulse mode spectroscopy, due to the complicated scanning behaviour of these lasers. The work presented here concentrates on the cavity ringdown spectroscopy (CRDS) technique, as opposed to complimentary techniques such as integrated cavity output spectroscopy (ICOS), because the CRDS instrument can be reduced to a small size for applications that require portability due to the beam path being on axis. ...
This paper presents experimental results from a pulsed quantum cascade laser based cavity ringdown spectrometer used as a high-throughput detection system. The results were obtained from an optical cavity with 99.8% input and output coupling mirrors that was rapidly swept (0.2s to 7s sweep times) between 1582.25 cm⁻¹ (6.3201μm) and 1697.00 cm⁻¹ (5.8928μm). The spectrometer was able to monitor gas species over the pressure range 585 torr to 1μtorr, and the analysis involves a new digital data processing system that optimises the processing speed and minimises the data storage requirements. In this approach we show that is it not necessary to make direct measurements of the ringdown time of the cavity to obtain the system dynamics. Furthermore, we show that correct data processing is crucial for the ultimate implementation of a wideband IR spectrometer that covers a range similar to that of commercial Fourier transform infrared instruments.
Infrared (IR) spectroscopy is considered to be among the most important analytical tools in laboratories, remote sensing, and industry. In addition to the commonly used Fourier-transform infrared (FTIR) spectrometers, spectral detection using IR laser sources plays important roles in varied applications. The quantum cascade laser (QCL) recently emerged as a novel, compact, tunable, and versatile IR laser source with narrow linewidth, high power, wide spectral coverage, and relatively straightforward operation. The scope of this chapter is limited primarily to mid-IR QCL and mainly their applications to spectroscopy and kinetics of reactive gaseous intermediate species. We introduce briefly the fundamental theories of QCL, their various types, advantages, and limitations. Implementations of several types are discussed, a detailed description of a continuous-wave external-cavity QCL coupled with a Herriott cell is presented. The high-resolution spectra of some stable molecules and reactive reaction intermediates, especially Criegee intermediates and other free radicals, are discussed. An account of kinetic investigations of Criegee intermediates and other free radicals in a flow reactor and of species in a shock tube is provided. Finally, we present some future perspective on research using the QCL.
Infrared laser-absorption spectroscopy (IR-LAS) sensors play an important role in diagnosing and characterizing a wide range of combustion systems. Of all the laser-diagnostic techniques, LAS is arguably the most versatile and quantitative, as it has been used extensively to provide quantitative, species-specific measurements of gas temperature, pressure, composition and velocity in both laboratory- and industrial-scale systems. Historically, most IR-LAS work has been conducted using tunable diode lasers; however, today’s researchers have access to a wide range of light sources that provide unique sensing capabilities and convenient access to nearly the entire IR spectrum (≈ 0.8 to 16 µm). In particular, the advent of room-temperature wavelength-tunable mid-infrared semiconductor lasers (e.g., interband- and quantum-cascade lasers) and hyperspectral light sources (e.g., MEMS VCSELs, Fourier-domain mode-locked lasers, dispersed supercontinuum, and frequency combs) has provided a number of unique capabilities that combustion researchers have exploited. The primary goals of this review paper are: (1) to document the recent development, application, and current capabilities of IR-LAS sensors for laboratory- and industrial-scale combustors and propulsion systems, (2) to elucidate the design and use of IR-LAS sensors for combustion gases through a discussion of the modern sensor-design process and state-of-the-art techniques, and (3) to highlight some of the remaining measurement opportunities, challenges, and needs. A thorough review and description of the fundamental spectroscopy governing the accuracy of such sensors, and recent findings and databases that enable improved modeling of molecular absorption spectra will also be provided.
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.
Plasmas in interaction with surfaces are of key importance in a wide range of applications, which include materials technology, microelectronics, environmental issues, and biomedicine. The intense use of plasma technological processes demands proper plasma diagnostic techniques for monitoring, controlling and optimization purposes in industrial environments. In particular, for the improvement of the efficiency of a production process, in situ diagnostic techniques with online capabilities are favorable. From the middle of the last decade a variety of phenomena in molecular non-equilibrium plasmas in which many short-lived and stable species are produced have been successfully studied with quantum cascade laser absorption spectroscopy (QCLAS) in the mid-infrared spectral range. It has been possible to determine absolute concentrations of species, temperatures, degrees of dissociation, dynamics of reaction processes and phenomena involving plasma-surface interactions using spectroscopy, thereby providing a link with chemical and kinetic modelling of the plasma. Since quantum cascade lasers (QCLs) emit near room temperature, i.e., without the need of cryogenic cooling, very compact and robust spectroscopic instruments can be designed. This has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. Recent applications of infrared absorption spectroscopy using QCLs for in situ monitoring of plasma processes in industrial environments are reviewed. Examples which emphasize the capabilities of QCLAS as plasma diagnostic technique in industrial plasma processes will be given. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Pulsed middle infrared distributed feedback (DFB) quantum cascade laser (QCL) used in this work was tunable between 1248-1257 cm -1, and the temperature tuning characterization of the QCL housing system had been presented by using hydrogen sulfide spectroscopy in the spectral range of 2.6 cm -1. Different spectral range were obtained by scanning the laser in corresponding temperatures, from a relatively better range that some interest gas absorption spectral lines were chosen to be studied. Etalon fringes were used to calibrate the frequency and also gave the chirp rate of the sensing system. The QCL based gas sensing system was set up to measure methane transmissions of which high resolution spectrum using direct absorption spectroscopy based on multi-pass White cell was shown, and it shows good agreement with the simulation from HITRAN. It could also present a good idea to detect the poisonous gas using QCL detection system because of its high resolution, high sensitivity and selectivity.
Low-pressure rf plasmas have been applied for etching of ultra-low-k SiCOH wafers using an Oxford Plasmalab System 100. In pure CF4 plasmas, SiCOH layers have been etched for different power values. Using quantum cascade laser absorption spectroscopy in the mid-infrared spectral range, the correlation of online and in situ measured concentrations of two etching products, CO and SiF4, with the ex situ determined etching rates has been studied. The concentration of SiF4 was found to range between 0.6 and 1.4 × 1013 molecules cm−3. In contrast the concentrations of CO were measured to be only about 50 % of the SiF4 density with 7 × 1012 molecules cm−3 in maximum. The production rate of SiF4, determined from the time behavior of its concentration after plasma ignition, was found to be between 1 and 5 × 1012 cm−3 s−1. The etching rates varied between 2 and 7 nm s−1. Both parameters increase nearly linearly with the applied rf power. It was found that for power values of up to 1.1 kW, the etching rate depends nearly linearly on the in situ monitored concentrations of both etching products. Therefore, the concentration of the etching products can be directly used as a measure of the etching rate.
The photochemistry and photophysics of metal carbonyl compounds (W(CO)6, Cp*Rh(CO)2 (Cp* = η5-C5Me5), and fac-[Re(CO)3(4,4′-bpy)2Br] [bpy = bipyridine]) have been examined on the nanosecond
timescale using a time-resolved infrared spectrometer with an external cavity quantum cascade laser (QCL) as the infrared source. We show the photochemistry of W(CO)6 in alkane solution is easily monitored, and very sensitive measurements are possible with this approach, meaning
it can monitor small transients with absorbance changes less than 10−6 ΔOD. The C‐H activation of Cp*Rh(CO)(C6H12) to form Cp*Rh(CO)(C6H11)H occurs within the first few tens of nanoseconds following photolysis, and we
demonstrate that kinetics obtained following deconvolution are in excellent agreement with those measured using an ultrafast laser-based spectrometer. We also show that the high flux and tunability of QCLs makes them suited for solid-state and time-resolved measurements.
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.
Intrapulse quantum cascade (QC) laser spectrometers are able to produce both saturation and molecular alignment of a gas sample owing to the rapid sweep of the radiation through the absorption features. In the QC lasers used to study the 14N and 15N isotopologues of the ν4 band of ammonia centred near 1625 cm-1, the variation of the chirp rate during the scan is very large, from ca. 85 to ca. 15 MHz ns-1. In the rapid chirp zone the collisional interaction time of the laser radiation with the gas molecules is short, and large rapid passage effects are seen, whereas at the slow chirp end the line shape resembles that of a Doppler broadened line. The total scan range of the QC laser of ca. 10 cm-1 is sufficient to allow the spectra of both isotopologues to be recorded and the rapid and slow interactions with the laser radiation to be seen. The rapid passage effects are enhanced by the use of an off axis Herriott cell with an effective path length of 62m, which allows a build up of polarization to occur. The effective resolution of the chirped QC laser is ca. 0.012 cm-1 full width at half maximum in the 1625 cm-1 region. The results of these experiments are compared with those of other studies of the ν4 band of ammonia carried out using Fourier transform and Laser Stark spectroscopy. They also demonstrate the versatility of the down chirped QC laser for investigating collisional effects in low pressure gases using long absorbing path lengths.
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.
In this Letter, we investigate, for the first time to our knowledge, the spectral properties of a quantum-cascade laser (QCL) from a point of view of a new application as a laser seeder for a nanosecond-pulse high-repetition frequency CO 2 laser operating at 10.6 μm wavelength. The motivation for this work is a renewed interest in such a pulse format and wavelength driven by a development of extreme UV (EUV) laser-produced-plasma (LPP) sources. These sources use pulsed multikilowatt CO 2 lasers to drive the EUV-emitting plasmas. Basic spectral performance characteristics of a custom-made QCL chip are measured, such as tuning range and chirp rate. The QCL is shown to have all essential qualities of a robust seed source for a high-repetition nanosecond-pulsed CO 2 laser required by EUV LPP sources.
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.
Following recent observations of molecular iodine (I2) in the coastal marine boundary layer (MBL) (Saiz-Lopez and Plane, 2004), it has become important to determine the absolute absorption cross-section of I2 at reasonably high resolution, and also to evaluate the rate of photolysis of the molecule in the lower atmosphere. The absolute absorption cross-section (σ) of gaseous I2 at room temperature and pressure (295K, 760Torr) was therefore measured between 182 and 750nm using a Fourier Transform spectrometer at a resolution of 4cm-1 (0.1nm at λ=500nm). The maximum absorption cross-section in the visible region was observed at λ=533.0nm to be σ=(4.24±0.50)x10-18cm2molecule-1. The spectrum is available as supplementary material accompanying this paper. The photo-dissociation rate constant (J) of gaseous I2 was also measured directly in a solar simulator, yielding J(I2)=0.12±0.03s-1 for the lower troposphere. This is in excellent agreement with the value of 0.12±0.015s-1 calculated using the measured absorption cross-section, terrestrial solar flux for clear sky conditions and assuming a photo-dissociation yield of unity. A two-stream radiation transfer model was then used to determine the variation in photolysis rate with solar zenith angle (SZA), from which an analytic expression is derived for use in atmospheric models. Photolysis appears to be the dominant loss process for I2 during daytime, and hence an important source of iodine atoms in the lower atmosphere.
The need to substitute SiO2 with low dielectric constant (κ) materials increases with each complementary metal–oxide–semiconductor process generation as interconnect RC delay, crosstalk, and power dissipation play an ever larger role in high-performance integrated circuits. Fluorinated amorphous carbon films (a-C:F,H) with low-κ properties (κ∼2.0–2.4) deposited by plasma-assisted chemical vapor deposition (CVD) techniques provide several advantages including low temperature processing, good gap fill capabilities, minimal moisture absorption, and simple implementation. Several deposition techniques have been examined, including high-density plasma and parallel-plate plasma-assisted CVD. In each case, it is possible to deposit a-C:F,H films with widely varying properties, such as κ and thermal stability. This has led to a good deal of confusion as to what is required to produce useful material. Results from many different sources are examined to develop a coherent picture of the relationships between deposition techniques, microstructural features, and macroscopic properties, and to summarize the scientific and technical challenges that remain for a-C:F,H implementation. The relationships between film deposition parameters such as applied substrate bias and film properties are presented in the discussion. In addition, x-ray photoelectron spectroscopy and network constraint theory are used to develop connections between microstructural and macroscopic properties, as well as to show how deposition parameters can be used to create a predictive model. This will demonstrate what process parameters are important in film formation. Finally, efforts to incorporate this material into integrated circuits, as well as measurements of the reliability and performance will be reviewed. © 1999 American Vacuum Society.
The surface reaction of CF_2 radicals on Si and fluorocarbon films was investigated in electron cyclotron resonance (ECR) Ar and H_2 /Ar downstream plasmas employing CF_2 radical injection technique. The effects of Ar^+ ions, Ar^* metastable species and radiation from plasmas on the fluorocarbon film formation were evaluated in ECR Ar downstream plasma with CF_2 radical injection. As a result, CF_2 radicals with assistance of Ar+^ ion bombardment were found to play an important role in the fluorocarbon film formation. The adsorptive reactions of CF_2 radicals on the fluorocarbon film surface with and without Ar and H_2 /Ar plasma exposures were successfully investigated by in situ Fourier transform infrared reflection absorption spectroscopy and in situ x-ray photoelectron spectroscopy. It was found that the formation of fluorocarbon film in the plasma proceeded through the adsorptive reaction of CF_2 radicals at a high probability on the active sites formed by the bombardment of Ar^+ ions on the fluorocarbon film surface. Journal of Vacuum Science & Technology A 16(1), 1998, p.233-238
The quantum yields of I∗(2P1/2) production from iodobenzene and pentafluoroiodobenzene at five different dissociation wavelengths of 222, 236, 266, 280, and ∼305 nm are presented and compared with those obtained from nonaromatic cyclic iodides (i.e., cyclohexyl iodide and adamantyl iodide). The I(2P3/2) and I∗(2P1/2) atoms generated in the photolysis of the above iodides were monitored using a two-photon laser-induced fluorescence technique. From the measured I∗ quantum yields, two general observations are made for aryl iodides. They are that (i) the I∗ yield is influenced by the σ∗←n as well as π∗←π transitions at all photolysis wavelengths within the A band and (ii) there is a clear indication of a fluorine substitution effect on the dynamics of I∗ production. The contribution from the benzene type π∗←π transition varies with excitation wavelength. Fluorine substitution in aryl iodides is found to increase the I∗ quantum yield similar to what is reported in alkyl iodides. The effect of fluorine substitution is more pronounced at the red edge of the A-band excitation than at any other wavelengths. This is explained by invoking the presence of a charge-transfer band arising due to the transition of a 5pπ nonbonding iodine electron to the π∗ molecular orbital near the red edge of the A band. This charge-transfer state is coupled more strongly to the 3Q1 state of the σ∗←n transition in pentafluoroiodobenzene than in iodobenzene. The dynamics of I∗ formation is found to be unaltered by ring strain in cyclic iodides except at the blue wing excitation. At the blue wing, B-band transitions affect the dynamics of I∗ production in cyclic iodides, leading to the formation of more I∗ from adamantyl iodide. © 2002 American Institute of Physics.
The methyl iodide A-band photodissociation process CH3I+hν→CH3(v,N,K)+I(2P3/2), I∗(2P1/2) has been studied in a cold molecular beam. Full three-dimensional state-specific speed and angular distributions of the nascent fragments were recorded using (2+1) resonance-enhanced multi-photon ionization (REMPI) and velocity imaging, a new variant of ion imaging. By combining the I∗ quantum yield and anisotropy parameters for both I and I∗ channels, the relative absorption strength to the contributing electronic states (3Q0, 3Q1 and 1Q1) as well as the probability for curve crossing (3Q0→1Q1) are determined for excitation wavelengths across the full A band (240–334 nm). Parallel excitation to the 3Q0 state turns out to dominate the A band even more than previously thought. © 1998 American Institute of Physics.
Knowledge of the absolute densities of small radicals like CF, CF 2 and CF 3 in fluorocarbon plasmas is essential for a fundamental understanding of plasma chemical processes and plasma surface interaction. Infrared absorption spectroscopy by means of tunable diode lasers (IR-TDLAS) was established and widely used for density measurements in the last decade. The often unknown parameter in the calculation of absolute radical densities from a measured absorption of a single line is the rotational temperature. In particular, a strong dependence of the line strength on rotational temperature has a significant influence on density calculation. In this paper we report on measurements of the CF 2 rotational temperature in capacitively coupled CF 4 /H 2 plasmas (CCP) with rf (13.56 MHz) powers up to 200 W. Rotational temperatures in continuous and pulsed modes of the discharge were found to be between 300 and 450 K. Furthermore, first measurements of the time dependence of the rotational temperature in pulsed rf plasma are presented. The rotational temperature rises in the plasma phase within 0.1 s and goes down again to the temperature of the background gas in the plasma pause within 0.5 s. It is also shown that accurate density measurements of the radicals by means of single line absorption need correct information about the rotational temperature and careful selection of a suitable absorption line.
Resonant two-photon ionization R2PI has been applied to the measurements of the quantum yields φ of iodine atoms in the fine structure states 2P1/2 and 2P3/2 arising in the UV-photodissociation process (λ=266 and 355 nm) of the perfluoroalkyliodides C2F5I, n-C3F7I and i-C3F7I as well as of methyl iodide CH3I (λ=266 nm). The measured values of the quantum yields of the excited state (2P1/2) of the I atoms are found to be φ266*=0.921±0.010 and φ355*=0.178±0.025 for C2F5I, φ266*=0.919±0.011 and φ355*=0.296±0.029 for n-C3F7I, φ266*=0.900±0.015 and φ355*=0.396±0.034 for i-C3F7I, and φ266*=0.71±0.03 for CH3I. The results obtained show that the quantum yield φ266* of the excited state atoms I(2P1/2) in the photodissociation process of the perfluoroalkyliodides differs noticeably from unity which has previously been assumed or measured for these molecules.
IR band intensities have been measured for the species: CF4, C2F6, C3F8, C4F10, C5F12, and C6F14 via Fourier transform spectroscopy and compared to previous literature values if available. Relative radiative forcing calculations have been performed using these data in order to determine the global warming potential of the particular species. The relative forcing (compared to CFC11, per volume) increases with molecular weight in the above series from 0.47 to 2.1, the GWP for a time horizon of 100 yrs from 1.1 to 4.7. This corresponds to a GWP on CO2 basis per mass of about 5000.
Resonant two-photon ionization R2PI has been applied to the measurements of the quantum yields ϕ of iodine atoms in the fine structure states 2P1/2 and 2P3/2 arising in the UV-photodissociation process (λ=266 and 355 nm) of the perfluoroalkyliodides C2F5I, n-C3F7I and i-C3F7I as well as of methyl iodide CH3I (λ=266 nm). The measured values of the quantum yields of the excited state (2P1/2) of the I atoms are found to be and for C2F5I, and for n-C3F7I, and for i-C3F7I, and for CH3I. The results obtained show that the quantum yield of the excited state atoms I(2P1/2) in the photodissociation process of the perfluoroalkyliodides differs noticeably from unity which has previously been assumed or measured for these molecules.
The intensity of the low fundamental of C2F6 at 219 cm—1 was measured using a CsI prism. This completed earlier studies on the other fundamentals, and permits extension and revision of the interpretation. Effective bond moments are compared with those of other fluorocarbons.
IR band intensities have been measured for the species: CF4, C2F6, C3F8, C4F10, C5F12, and C6F14 via Fourier transform spectroscopy and compared to previous literature values if available. Relative radiative forcing calculations have been performed using these data in order to determine the global warming potential of the particular species. The relative forcing (compared to CFC11, per volume) increases with molecular weight in the above series from 0.47 to 2.1, the GWP for a time horizon of 100 yrs from 1.1 to 4.7. This corresponds to a GWP on CO2 basis per mass of about 5000.
Laboratory measurements of the infrared and near-ultraviolet absorption characteristics of CF3I (a potentially useful substitute for halons) are presented. Using these data together with a detailed photochemical model, it is shown that the lifetime of this gas in the sunlit atmosphere is less than a day. The chemistry of iodine in the stratosphere is evaluated, and it is shown that any iodine that reaches the stratosphere will be very effective for ozone destruction there. However, the extremely short lifetime of CF3I greatly limits its transport to the stratosphere when released at the surface, especially at midlatitudes, and the total anthropogenic surface release of CF3I is likely to be far less than that of natural iodocarbons such as CH3I on a global basis. It is highly probable that the steady-state ozone depletion potential (ODP) of CF3I for surface releases is less than 0.008 and more likely below 0.0001. Measured infrared absorption data are also combined with the lifetime to show that the 20-year global warming potential (GWP) of this gas is likely to be very small, less than 5. Therefore this study suggests that neither the ODP nor the GWP of this gas represent significant obstacles to its use as a replacement for halons.
Plasma processing has been a key technology for large-volume integrated circuit manufacturing for more than 30 years. In particular, various configurations of plasma reactors, along with a range of plasma chemistries, have enabled high-throughput anisotropic and selective etching of materials with attendant precision transfer of resist patterns for feature sizes from 1 mum down to 100 nm and below. This article surveys the historical developments in oxide, metal, gate, and crystalline silicon etching, along with future challenges.
Ozone depletions in the lower stratosphere outside of polar regions are difficult to explain using only local chlorine and bromine chemistry. We speculate that iodine chemistry in combination with trends in anthropogenic chlorine and bromine may also be a factor in determining the widespread current depletion of lower stratospheric ozone. We also speculate on a related role for iodine in the sudden springtime surface ozone loss observed in the Arctic.
The absolute intensities of the strong infrared absorption bands of several chlorofluorocarbons (CFCs) located in the atmospheric window have been measured. The CFCs studied in the present experiments are, in addition to CFC11 and CFC12, CFCs 13, 14, 13B1, 22, 113, 114, and 116.
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.
Coherent multiphoton excitation of polyatomic molecules with pulsed CO2 lasers leads to unimolecular reactions induced by monochromatic infrared radiation (URIMIR). We report a detailed study of the dynamics of dissociation of trifluoroiodomethane (CF3I), 1,1,1,2-tetrafluoro-2-iodoethane (CF3CHFI) and pentafluoroiodobenzene (C6F5I). The primary dissociation after ro-vibrational excitation in the electronic ground state results in iodine atoms I(2P3/2), which are detected by diode laser IR absorption on the (2P3/2–2P1/2) magnetic dipole transition with about 1 MHz frequency resolution and up to 1 ns time resolution, essentially bounded by the uncertainty principle. This allows us to detect the product-state distribution over nuclear hyperfine levels in I atoms, and product translational-energy distributions from Doppler lineshapes combined with quantitative, time-resolved kinetic analysis under conditions of irradiation with shape-controlled CO2 laser pulses of well defined fluence and intensity. The kinetic results for absolute rates are analysed in terms of the laser chemical rate coefficient kI(st) and compared to theoretical calculations based on the case B/C master equation including non-linear intensity effects, which are found to be important only for CF3I. The results for relative rates are analysed in terms of a simple theoretical model for the centre-of-mass product translational-energy distribution P(Et). The results are discussed in relation to the foundations of IR laser chemistry.
The 248 nm photodissociation of trifluoromethyl iodide CF3I→CF3 + I(2P, 2P) has been studied by high-resolution photofragment translational spectroscopy. Time-of-flight (TOF) distributions of the CF3 and I photofragments were measured by means of an apparatus equipped with a rotatable pulsed molecular beam source. The dissociation energy D0 (CF3-I) ≤ 19300±350cm−1 (231 ± 4 kJ/mool) is obtained from the high-energy thresholds of the TOF spectra. The average internal energy of the CF3 fragments produced in coincidence with ground state I atoms is ≈ 4900 cm−1. In the case of the dominant reaction, which yields CF3 radicals and spin-orbit excited I (2P) atoms, the average internal energy amounts to 3100 cm−1. Vibrational structure with a spacing of ≈ 700 cm−1 is resolved in some of the TOF spectra, indicating excitation of the ν2 “umbrella” mode of the CF3 fragment. The vibrational distribution has a width of ≈ 2400 cm−1 (fwhm) and is thus considerably narrower than reported in a previous TOF study. Our results support the mechanism assumed for CF3I photodissociation and provide information on the importance of translation-vibration coupling on the repulsive excited state potential energy surfaces.
The photodissociation of CF3I at 248 nm was performed on our high-resolution photofragment translational spectrometer. For further improvement of the resolution, we have taken the following procedures: (1) Kr was used as carrier gas, (2) optimum molecular beam angle θopt (when VMB⊥VCM) was selected, (3) the longitudinal molecular beam angle spread was reduced, and (4) the ionization region for photofragments was diminished. The photofragment translational spectra of I* (2P1/2) reveal eight well-resolved vibrational peaks. The highest peak was assigned to be v2=5 of the ν2 vibrational mode, and then the dissociation energy would be D0(C–I)=18810±200cm−1=53.8±0.6 kcal/mol.
The homogeneous rate of recombination of CF3 radicals at 300 K was studied in a very-low-pressure photolysis flow reactor. The radicals were generated from CF3I by either CO2 laser (1076 cm−1 or excimer laser (248 nm) photolysis. The precursor and product concentrations were followed by mass spectrometry. The recombination rate constant,Kr, was measured as (3.0±0.4)×10−12 cm3 s−1. A hindered Gorin transition-state model predicts that kr/k∞r is only 0.74 at the effective pressures employed. The model was used to fit these experiments at 300 K, as well as earlier experiments at ≈ 1000 K.
The high-pressure rate constant of the CF3 + CF3 → C2F6 reaction at T = 296 K was measured in the pulse photolysis (λ = 694.3 nm, ruby laser) of CF3NO in the presence of NO by means of the time-resolved detection of CF3NO by the intracavity absorption of He(SINGLE BOND)Ne laser radiation (λ = 632.8 nm). The obtained value is k2∞ = (3.9 ± 1.3) × 10−12 cm3/s. © 1996 John Wiley & Sons, Inc.
The three lowest energy states of molecules H2, M2, and MH are here deemed more or less analogous and labeled N, T, V; these are respectively of the types Σ+1, Σ+3, Σ+1. States N, T and V must exist also in all other uni-univalent molecules, e.g., X2, XY, HX, AgX, MX (M = alkali metal atom, X or Y = halogen atom). The characteristics of the states N, T, V (polarity, ionicness, etc.) are surveyed. Only in H2 is state T yet known empirically; N is the normal state always; V is to be identified with a state known from band spectra in the case of most molecules of the types above named. In molecules containing one halogen atom, there should be an additional low energy group of states (types II3, II1), here labeled the Q group. Various points connected with the approximation of polar diatomic molecule wave functions using atomic orbitals, and using molecular orbitals, are discussed. Difficulties and arbitrariness inherent in the definition of polarity and ionicness are examined.
Resonance-enhanced multiphoton ionization (REMPI) and mass spectrometry have been used to measure the kinetics of CFâ radicals inside a very low pressure photolysis (VLP/Phi/) cell. Infrared multiphoton dissociation (IRMPD) of hexafluoroacetone (HFA) is used to generate the radicals according to CFâCOOFâ ..-->.. CFâ + CFâCO. REMPI spectra of vibrationally hot and thermalized CFâ radicals are presented. The absolute density of CFâ in the reactor is determined from the REMPI signal by using mass spectral data. This puts the calibration on the absolute basis necessary to treat competing unimolecular and bimolecular reactions of CFâ free radical. Measuring the CTâ density as a function of time between pulses of the IR laser and of HFA flow rate allows direct determination of the first- and second-order loss rates for CFâ. The CFâCO radical is stable under the authors conditions and engages in recombination back to HFA at higher radical densities.
Fluorinated amorphous carbon thin films (a-C:F) for interlayer dielectrics are grown by helicon plasma enhanced chemical vapor deposition. The source gases are CH4, CF4, C2F 6 and their H2mixtures. a-C:F films can be fabricated without adding hydrogen using the helicon reactor, while in the previously reported parallel-plate reactor, no film grows unless a hydrogen source is added. The films grown in the helicon reactor have no hydrogen content. The growth rate of the films reaches 0.3 μm/min (C2F 6) and 0.15 μm/min (CF4). The thickness of the films deposited with C2F6 does not decrease on heating to 300 °C, while the films with CF4 shrink. The dielectric constants of the films deposited from C2F6 and CF4 are 2.4 and 2.3 respectively at 1 MHz. The dielectric loss tangent of these films is 0.01 at 1 MHz.
A theory is described for calculating photodissociation spectra for polyatomic molecules larger than triatomics. The general method combines the vibrational close‐coupling, rotational infinite‐order‐sudden approximation with the technique of Kulander and Light for calculating photodissociation integrals. The three‐dimensional theory enables several vibrational states in the polyatomic photofragments to be coupled together and also allows for initial vibrational and rotational excitation in the parent molecule. The method has been applied to the CF3I→CF3+I (2P1/2) photodissociation process for the radiation frequency range 32 000–42 000 cm−1. Cross sections are reported for CF3I in the ground vibrational state, and also with C–I stretching and bending modes excited initially. Considerable vibrational excitation in the CF3 photofragments is obtained in the calculations at higher frequencies, a finding that is in agreement with experimental measurements. There is a marked preferential population of CF3 combination bands involving simultaneous excitation of both the v1 and v2 vibrational levels.
In SiO2/Si selective etching processes using fluorocarbon plasmas, surface reactions of fluorocarbon radicals can affect the etching selectivity considerably. Therefore, information on radicals in plasmas and their surface reactions must be obtained. We developed an in-situ method of measuring various radicals in plasmas using infrared diode laser absorption spectroscopy (IRLAS) and have clarified the behaviors of the CFx (x = 1-3) radicals in fluorocarbon plasmas for the first time. Moreover, ive recently developed techniques of radical injection into plasma (RIT) and clarified the important radical in the plasma etching process. It is expected that these advances will contribute to the further developments in the semiconductor process field.
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.
CF3I is photodissociated in a molecular beam with 248 nm laser lighl. Fragmentation takes place into CF3 and I(2P3/2) or I(2P1/2). The quantum yield for excited I (I* = I(2P1/2)) formation is determined to be 0.92. The CF3 fragments are found to be considerably vibrationally excited. Of the available energy in the CF3 + I* channel 39% (0.69 eV) appears on the average as vibrational energy in the CF3 radical. The angular distributions in both channels show predominantly parallel character. It can be concluded that at this wavelength 97% of the total absorption is due to the 3Q0
Using a very weak electric field to accelerate the photofragment ions, we have constructed a very simple photofragment translational spectrometer with only a 50 mm total flight path, but we can get photofragment translational spectra with high resolution. On this apparatus, we have performed the photodissociation of CF3I at 281.73 nm, and the same laser will ionize I*(2P1/2) via REMPI. Seven well-resolved vibrational peaks were obtained. Six of them are assigned to the umbrella vibrational mode v2=0–5 states of photofragment CF3. The average internal energy E¯int=1928cm-1 is about 21% of the available energy.
A pulsed photolysis, laser-induced fluorescence method has been used to study the kinetics of OH(v= 1) and OD(v= 1). The following rate constants (cm3 molecule–1 s–1) for relaxation of these vibrationally excited radicals have been measured at 298 ± 4 K: (a) for OH(v= 1) with HNO3, H2O, NO and NO2: (2.54 ± 0.11)× 10–11, (1.36 ± 0.1)× 10–11, (3.8 ± 0.6)× 10–11 and (4.8 ± 0.8)× 10–11; (b) for OD(v= 1) with DNO3, NO and NO2: (1.53± 0.2)× 10–11, (2.7 ± 0.3)× 10–11 and (4.3 ± 0.3)× 10–11. Rate constants are also reported for recombination of OH(v= 0) and, for the first time, OD(v= 0) with NO and NO2 in the presence of 18 Torr Ar. The ratios of rate constants (kOH/kOD) under these conditions are (1.0 ± 0.16) for association with NO and (0.99 ± 0.17) for combination with NO2. The rate data for all processes involving radical–radical collisions are discussed in terms of models developed by Quack and Troe for treating processes proceeding via strongly bound collision complexes.
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.
Surface reactivities for CF2 radicals formed in a CHF3 plasma molecular beam are measured during film deposition on a variety of substrates. The imaging of radicals interacting with surfaces (IRIS) technique was used to collect spatially resolved laser-induced fluorescence (LIF) images of CF2 radicals interacting with SiO2, Si3N4, Si, 304 stainless steel, and system 8 photoresist substrates. Films deposited during IRIS experiments were characterized using x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy and were found to be nearly identical in composition on all substrates. Simulation of LIF cross-sectional data shows high scattering coefficients for CF2 radicals on all substrates. These extremely large scattering coefficients (>1.0) indicate that CF2 molecules are generated through plasma interactions with the substrate. Possible CF2 surface generation mechanisms are discussed, with consideration of CF and ion bombardment contributions to the generation of CF2. © 1998 American Institute of Physics.
The ground (2P03/2) and first excited (2P01/2) states of iodine atoms can absorb two photons at 304.7 and 306.7 nm, respectively, to reach 2D05/2 and 2D03/2 states. The excited atoms fluoresce twice, emitting an IR and then a VUV quantum. This is the basis of a new method for measuring the relative quantum yields of the two fine structure states at very short times after the atoms are formed. Quantum yields for I∗ production are reported for a number of alkyl halides and HI upon photodissociation.
The photodissociation of jet-cooled CF3I into CF3+I(2P3/2) and CF3+I∗(2P1/2) has been investigated between 304 and 277 nm by using velocity map ion imaging. The two-dimensional images provide detailed information on the partition of available energy into kinetic and internal energy of the photofragments. Vibrational structure with spacing of 695±100 cm−1 is resolved in both I and I∗ images, indicating excitation of the umbrella mode ν2 of the CF3 photofragment. The fragment recoil anisotropies β(I) and β(I∗) are determined as a function of the excitation wavelength and their variations are interpreted in terms of the crossing between the 3Q0 and 1Q1 dissociative electronic states. The high-resolution images allow the determination of the variation of the anisotropy parameter β as a function of the vibrational level of CF3 fragment, and provide a complementary method for the determination of the C–I bond energy. The vibrational dependence of the anisotropy values is discussed in terms of final-state interactions between the CF3 umbrella motion and the C–I dissociation coordinate, as discussed previously by Hennig et al. [J. Chem. Phys. 84, 544 (1986)]. © 2003 American Institute of Physics.
Vibrational state distributions following the direct photodissociation of a collinear, triatomic molecule is investigated with particular emphasis on the so‐called final state interaction, i.e., the translational–vibrational coupling due to the excited state interaction potential. In order to separate the various effects which determine the state distribution we performed calculations on three levels of accuracy: The energy sudden (ES) approximation, the modified sudden (MS) approximation, and the exact close‐coupling (CC) formulation. The pure ES distributions peak at high states and are very broad. They are explained within the semiclassical limit as a mapping of an amplitude onto the quantum number axis. We call this effect vibrational reflection principle in analogy to the equivalent effect in rotational excitation processes. It is a direct and sensitive probe of the parameters of the system, most importantly the potential energy surface. Energy conservation strongly modifies the ES distributions. The MS and CC distributions are much narrower and peak at considerably lower states. A detailed analysis is given within the MS approximation. Based on these general conclusions we suggest a particular excited state potential for the dissociation of CF3I which qualitatively reproduces the recently reported experimental CF3 distribution. The essential feature of this potential is a distance dependent local frequency which we find necessary to obtain distributions as broad as in the experiment. Because of the inherent difficulties with the time‐of‐flight technique if several energy transfer channels are involved we certainly do not know how realistic our final potential energy surface is. However, the general trends found in this study should be valid for a larger class of systems.
The photodissociation of CF3I cooled in a supersonic molecular beam has been investigated at 277 nm by state‐selective photofragment imaging. Fragmented iodine atoms of two spin–orbit states are state‐selectively ionized and projected onto a two‐dimensional position‐sensitive detector, to obtain their speed and angular distribution. The anisotropy parameter for an excited iodine atom I∗(2P1/2), β(I∗), is found to be 1.83 and is consistent with a dissociation lifetime in the order of 150–350 fs from rotational correlation function. Contrary to earlier reports, the parallel‐like distribution for the ground state iodine atom I(2P3/2) at this wavelength, shows a more favorable curve‐crossing dissociation path (68%) from 3Q0 to 1Q1 and a less favorable direct dissociation path (32%) from 3Q1. The recoil energy distribution of I is found to be broader than that of I∗ and is correlated with a variety of energy disposal channels by an e symmetry vibration at the crossing point. The results are compared with previous works, and the strong photon energy dependence of the energy partitioning in CF3+I∗ channel and curve crossing are interpreted in terms of the final state interaction and curve crossing probability, respectively. © 1996 American Institute of Physics.
The ν3 degenerate C–F stretching vibration–rotation band of the trifluoromethyl radical was observed in the gas phase by infrared diode laser spectroscopy and was analyzed to determine precisely the rotational constants, centrifugal distortion constants, and the band origin. The spectrum was found to be perturbed by the so‐called Δl,ΔK=2,2 and 2,‐1 interactions, and an analysis of the perturbation yielded the C0 rotational constant which was otherwise difficult to determine. The two r0 structure parameters, the C–F distance and the F–C–F angle, were calculated from the observed B0 and C0 rotational constants to be 1.318±0.002 Å and 110.76±0.4°, respectively, where the errors were entirely due to the uncertainties of B0 and C0. A general quadratic force field was derived using the fundamental frequencies of 12CF3 and 13CF3 and also the ζ3 Coriolis coupling constant obtained by the present work. The observed vibration–rotation constants α3B and α3C were used to estimate two cubic potential constants ϕ133 and ϕ233, and two more constants ϕ333 and ϕ334 were obtained from the observed q(2,2 interaction) and r(2,‐1 interaction) constants.
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.
The possibility that the stratospheric ozone layer could be depleted by half at certain latitudes and seasons would have been deemed a preposterous and alarmist suggestion in the early 1980s. A decade later, the statement is acknowledged as proved beyond reasonable scientific doubt. Observations of the composition of the Antarctic stratosphere have established that the chemistry of this region is highly unusual because of its extreme cold temperatures, leading to a greatly enhanced susceptibility to chlorine-catalysed ozone depletion.
The infrared absorption intensities of the chlorofluorocarbons C2ClxFy, (x + y = 6); the hydrofluorocarbons C2HxFyH (x + y = 6); and a number of hydrochlorofluorocarbons, including some members of the propane series, have been measured. Absorption intensities have been obtained by integration over specified ranges of frequencies. The ranges used include the atmospheric window (1250t-833 cm−1), 3500-450 cm−1, 1300-700 cm−1, and those for selected individual absorption bands. Comparisons of the results have been made with published work where available, and attention is drawn to possible sources of error in the measurement of band areas. The spectra of the halopropanes have been included for the range 3500-150 cm−1. A preliminary study has been made of the relation between the number of fluorine atoms in the molecule and the intensity of absorption of the CF stretching vibrations.
Polarized photofragment translational spectroscopy was used to investigate the photodissociation process CF3I → CF3 + I(2P3/2) and CF3I → CF3 + I*(2P1/2) in the A absorption band between 275 and 303 nm. We determined the fragment recoil anisotropies β(I) and β(I*) and the branching ratio of the products I and I* as a function of the excitation wavelength λexc which covers most of the overlapping transitions 3Q1 ← X and 3Q0 ← X. The results are interpreted within the mechanistic scheme that involves three potential energy surfaces 3Q1, 3Q0, and 1Q1. The variation in the anisotropy β(I*) and the product-yield ratio I/I* with λexc is shown to reflect the variation of mixed 3Q0 and 1Q1 character of the adiabatic states when excited near the avoided curve crossing.
The rate coefficient, k1, for the reaction OH + CF3I → products was measured under pseudo-first-order conditions in hydroxyl radical, OH. OH temporal profiles were monitored by laser-induced fluorescence (LIF), and CF3I concentrations were determined by UV/Visible absorption. We determined k1 (T) to be (2.10 ± 0.80) × 10-11 exp[−(2000 ± 140)/T] cm3 molecule-1 s-1, over the temperature range 271 to 370 K. The quoted uncertainties are 2σ (95% confidence limits, σA = AσlnA). Previous measurements of k1(T) are compared with our values, and possible reasons for the discrepancies are discussed. The heat of formation of HOI is deduced to be less than −16 kcal mole-1, if the products of reaction 1 are mostly HOI and CF3. These measurements support the earlier conclusion that the reaction of OH with CF3I plays a negligibly small role in the atmospheric removal of CF3I.
The high-temperature UV absorption and the recombination of CF3 radicals have been investigated in shock-wave experiments. CF3 radicals were produced near 1300 K in the fast dissociation of (CF3)2N2 and of CF3NO. At pressures of the bath gas Ar between 0.3 and 25 atm, the recombination reaction was found to be in the unimolecular falloff range closer to the high than to the low pressure limit. Extrapolation to the high pressure limit gives krec,∞(CF3) = (1.9 ± 0.9) × 1013 cm3 mol-1 s-1 at T = 1300 K. Earlier low-temperature results are compared with the present data by using unimolecular rate theory.
This paper describes the time-resolved detection of CF3, C2F6, and CO following the infrared multiphoton dissociation of hexafluoroacetone. The primary photolysis mechanism has been established as follows: (CF3)2CO → 2CF3 + CO; 2CF3 → C2F6. Determination of the CO and C2F6 formed in a single photolysis pulse leads to a measure of an infrared line strength and v3 band strength for CF3. Quantification of the CF3 in this manner allows a study of its reaction kinetics. The reactions of CF3 with added O2 and NO were found to have third-body rate constants of (2.1 ± 0.5) × 10-29 and (2.8 ± 0.7) × 10-29 cm6 molecule-2 s-1, respectively, at room temperature in the presence of 600 mTorr of hexafluoroacetone.
In semiconductor processes, reactive plasma is the most important technology for etching, deposition and surface modification of thin films. Radicals have played key roles in plasma processing. In order to realize the high performance of semiconductor processing, important molecular radicals have been measured and their behaviour has been clarified using laser spectroscopic methods such as infrared diode laser absorption spectroscopy, laser induced fluorescence spectroscopy, cavity ring down spectroscopy and recently atomic radicals have also been measured using compact vacuum ultraviolet absorption spectroscopy. Quantitative understanding of kinetics of radicals in plasma will be necessary for nano-scaled semiconductor processing. Their progress is reviewed and the future prospects are presented.
The unimolecular homogeneous decomposition of hexafluoroazomethane was studied in a VLPP apparatus in the temperature range 720–1050 K and is consistent with the following Arrhenius parameters:
at 900 K, where the A factor was assumed to be the same as for 2,2′-azoisobutane. The homogeneous rate of recombination of ·CF3 radicals at temperatures around 1000 K was also studied under VLPP conditions and was found to be in the fall-off region, corresponding to k/k∞ = 8.5 × 10−3 when a rotational transition-state model was used. This model predicts an essentially constant value of kr∞ of 109.7 over the temperature range 300–1000 K.
The time-resolved tunable diode laser detection of CF3 radicals formed by infrared multiphoton dissociation of (CF3)2CO is reported. Knowledge of the linestrength of the rR16(20) CF3 absorption linc at 1264.739 cm−1 allows the calculation of a rate constant for recombination of CF3 radicals following (CF3) 2CO photolysis. This rate constant has a value of (2.2± 0. 5) × 10−12 cm3 molecule−1 st̄1 in (CH3)2CO at 600 mTorr and room temperature.
Polarized laser photodissociation of N2O at 193 nm, investigated by the time-of-light, crossed laser-molecular-beam method, generates the fragments N2(1Σ+g) and O(1D2) with an average translational energy of 26.8 kcal/mol (42%Eavl), corresponding to an average internal energy of N2 of 37.6 kcal/mol (58%Eavl), where Eavl is the available energy. A substantial part of the latter consists of rotational energy, most likely due to the bent excited electronic state of N2O. The recoil anisotropy parameter was measured to be β=0.48±0.2.
An electronic absorption spectrum in the region 165 nm to 146 nm has been assigned to the CF3 radical. The rate constant for the mutual combination of CF3 radicals at ≈ 300°K and a total pressure of 100 torr argon is ≈ 3×1012 cm3 mole−1 sec−1.
quantum yield has been measured for a series of fluorinated alkyl iodides at two different wavelengths in the blue wing of their A-band. The yields are much higher for the fluorinated iodides than their corresponding alkyl iodide analogs at 222 as well as 236 nm. The I* quantum yield at 222 nm is, in general, larger than that at 236 nm which is opposite to what has been found in normal alkyl iodide dissociation in the A-band. This difference in fluorinated iodides has been explained by invoking the participation of the B-band states in the dissociation dynamics.