3
Radiative Processes and Diagnostics

Introduction

This chapter discusses gas phase and surface spectroscopic diagnostic techniques that can be used in plasma processing tools. As noted previously, techniques for measuring gas phase and surface quantities (species and energies) are essential for identifying the key chemical species in the plasma and for characterizing the chemical mechanisms that link these key species. Diagnostics are often best used in a complementary fashion with modeling: measurements help to test and validate models, and in turn, validated models help to augment the limited information provided by individual diagnostic techniques. For proper understanding and quantification, each diagnostic technique requires data on one or more physical quantities. Examples are discussed of sources of information on the physical principles of the various techniques, of sources on applications to specific systems, and of critically reviewed compilations of data on physical properties. It is shown that much can be done even when these sources do not provide all the information needed to plan or interpret a diagnostic experiment.

Techniques for Measurements of Gas Phase Species

There are various well-established optical techniques for the measurement of gas phase species in plasmas. Table 3.1 is a list derived from several recent review articles.1 These techniques are usually easier to apply at low pressure since lines and bands overlap less and so are easier to identify and analyze. Species identities are provided by spectral signatures (positions and shapes of spectral features), while absolute intensities of the spectral features can provide absolute concentrations. Often, linewidths (for atoms and small molecules) or band structure (for molecular species) can be related to translational and rotational energies or temperatures.

TABLE 3.1 Optical Diagnostic Techniques for Plasma Processing Systems

Gas Phase

Surface

Infrared absorption

Reflection/absorption

Ultraviolet/visible absorption

Multiple internal reflection

Electronic emission

Emission

Actinometry

Ellipsometry

Laser-induced fluorescence

Reflectance difference

Multiphoton ionization

Photoluminescence

Optogalvanic spectroscopy

Optogalvanic spectroscopy

Raman scattering

Surface electromagnetic waves

Stimulated Raman scattering

Second harmonic generation

Stimulated emission

Photoacoustic absorption

Laser-induced photofragment emission

Photothermal displacement

Third harmonic generation

Photothermal deflection

Particle scattering

Laser desorption

Mass spectrometry is also important, though not included in this chapter's discussion of optical spectroscopic techniques. The database needs for this diagnostic are associated primarily with interpretation of mass-resolved ion spectra that result from electron-impact dissociative ionization. The need for data regarding cross sections for electron-impact dissociative ionization, and especially the sensitivity of those cross sections to molecular internal energy near threshold, is addressed in Chapter 5, "Electron Collision Processes."



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--> 3 Radiative Processes and Diagnostics Introduction This chapter discusses gas phase and surface spectroscopic diagnostic techniques that can be used in plasma processing tools. As noted previously, techniques for measuring gas phase and surface quantities (species and energies) are essential for identifying the key chemical species in the plasma and for characterizing the chemical mechanisms that link these key species. Diagnostics are often best used in a complementary fashion with modeling: measurements help to test and validate models, and in turn, validated models help to augment the limited information provided by individual diagnostic techniques. For proper understanding and quantification, each diagnostic technique requires data on one or more physical quantities. Examples are discussed of sources of information on the physical principles of the various techniques, of sources on applications to specific systems, and of critically reviewed compilations of data on physical properties. It is shown that much can be done even when these sources do not provide all the information needed to plan or interpret a diagnostic experiment. Techniques for Measurements of Gas Phase Species There are various well-established optical techniques for the measurement of gas phase species in plasmas. Table 3.1 is a list derived from several recent review articles.1 These techniques are usually easier to apply at low pressure since lines and bands overlap less and so are easier to identify and analyze. Species identities are provided by spectral signatures (positions and shapes of spectral features), while absolute intensities of the spectral features can provide absolute concentrations. Often, linewidths (for atoms and small molecules) or band structure (for molecular species) can be related to translational and rotational energies or temperatures. TABLE 3.1 Optical Diagnostic Techniques for Plasma Processing Systems Gas Phase Surface Infrared absorption Reflection/absorption Ultraviolet/visible absorption Multiple internal reflection Electronic emission Emission Actinometry Ellipsometry Laser-induced fluorescence Reflectance difference Multiphoton ionization Photoluminescence Optogalvanic spectroscopy Optogalvanic spectroscopy Raman scattering Surface electromagnetic waves Stimulated Raman scattering Second harmonic generation Stimulated emission Photoacoustic absorption Laser-induced photofragment emission Photothermal displacement Third harmonic generation Photothermal deflection Particle scattering Laser desorption Mass spectrometry is also important, though not included in this chapter's discussion of optical spectroscopic techniques. The database needs for this diagnostic are associated primarily with interpretation of mass-resolved ion spectra that result from electron-impact dissociative ionization. The need for data regarding cross sections for electron-impact dissociative ionization, and especially the sensitivity of those cross sections to molecular internal energy near threshold, is addressed in Chapter 5, "Electron Collision Processes."

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--> Information Resources The spectroscopic database for the properties and interactions of many of the gases used in plasma processing is quite extensive. Ultraviolet or infrared absorption strengths and fluorescence quenching or pressure broadening cross sections are the parameters needed to predict or interpret the results of absorption, emission, and fluorescence diagnostic experiments, and while not every parameter is available for every molecule, information on related species can often provide a basis for estimation. Data may be sparser for more specialized techniques, but the coverage for systems of the greatest interest may be adequate. On the other hand, much of this information is dispersed in many journals. Although some reviews have been written, as diagnostic fields mature there is less interest in continuing to produce comprehensive review articles. In addition, it is rare that either reviews or molecular property measurements are focused on the needs of plasma processing diagnostics. Areas receiving focused attention, such as combustion or atmospheric chemistry, sometimes produce results that also serve the plasma processing community. However, rapidly growing areas involving, for example, heavy metal halogens and metal-organics must be supported entirely within the community. Typically, this means that the research group developing the diagnostic application must also carry out any basic property measurements, the result being that those data may not even appear as the subject of that group's own publication in the primary literature, let alone in one of the centralized repositories briefly reviewed below. Workers at the National Institute of Standards and Technology (NIST) have produced a number of compilations of atomic line positions and strengths,2 some of which are now available as electronic databases.3 However, as was pointed out recently, this is still an active area.4 The compilation of Huber and Herzberg5 occupies a similar position for diatomic molecules, and has also been compiled into a NIST database.6 In general the classic volumes of Herzberg7 still provide an excellent entry point to the older spectroscopic literature, while some newer spectroscopy texts provide some useful examples.8 Books edited by Suchard9 and Boyko,10 the book by Pearse and Gaydon,11 two reviews in the 1992 Royal Society annual reports volume12 and one in the 1990 Annual Review of Physical Chemistry,13 several reviews in the book series edited by Rao,14 15 and the ongoing compilation of Jacox, again available in both print16 and electronic17 versions, are other useful sources. The review article by Smith and co-workers18 deserves special mention as a source of collision broadening parameters as well as of infrared band intensities. The HITRAN absorption line listing19 and an associated table of unresolved band absorption coefficients,20 together with software allowing prediction of spectra, form a valuable resource for the set of atmospheric molecules. A NIST electronic database21 provides moderate-resolution infrared spectra for a selection of 5244 molecules, while a number of commercial databases22 provide higher resolution and additional species, in both print and electronic forms. One drawback of almost all these infrared spectral compilations, however, is that they are not quantified (path length or concentration are unavailable, or were initially unknown). For this reason, much earlier compilations of quantitative infrared spectra23 are still useful. A number of sources provide compiled data or useful keys to the primary literature even though their focus is not on plasma processing. The book by Okabe24 lists a variety of ultraviolet absorption cross section spectra. Several review articles by Hirota and co-workers25 provide good introductions to the work of this prolific group and supplement early comprehensive review articles on high-resolution infrared spectroscopy.26 Volumes whose purpose is to list infrared band positions27 also provide primary literature references, often leading to full, quantitative spectra. Finally, the JANAF thermochemical tables28 can perform the same function as part of their documentation of entropy calculations, thus centralizing references to infrared spectra for a large number of molecular species. Examples of what is involved in the collection and application of primary data to estimate the feasibility of particular diagnostic experiments are given in a 1983 SPIE paper by Wormhoudt, Stanton, and Silver.29 Focusing on the techniques of laser-induced fluorescence and infrared absorption, it attempted to tabulate the parameters needed to estimate minimum detectable densities of a wide variety of stable and

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--> radical species then of interest in the electronic materials area. The fact that most of the 144 references in the paper are to sources of these molecular parameters reinforces the above remarks on the lack of centralized data sources. When molecular parameters were unknown, they were estimated. More than 10 years later, a number of these parameters have been measured, but no systematic review has been performed to assess the reliability of such estimation attempts. The minimum detectability estimates for absorption include a 100 cm path length. In a practical reactor, this path length would require a multipass geometry, a potential barrier to implementation. An alternative approach to achieving the same predicted sensitivity would be to assume smaller minimum detectable absorbances than the 10-3 assumed in the paper. Lower values are routinely achieved in the laboratory but would have to be demonstrated for a particular reactor. Roles of the Database In Motivating Diagnostic Experiments The SPIE paper discussed above was intended to promote work in two areas: diagnostics of semiconductor processing systems and laboratory measurements of supporting data. It is of interest to examine a few of the experiments of both types carried out following publication of the paper, the idea being to exemplify some general thoughts about the relationship between diagnostics and their supporting databases. The general points are as follows: Quantitative diagnostic measurements, and comparison with the predictions of quantitative models, are necessary for the quantitative understanding of any phenomenon. The level of quantitative accuracy required for a measurement to be useful is determined by the uncertainty limits expected from current or projected predictive models. This in turn determines the uncertainty levels that are acceptable in the supporting database. Data that are enabling, in the sense that diagnostic experiments will not be attempted in their absence, are often different from the accurately quantified molecular parameters that allow derivation of absolute molecular concentrations and other system quantities. It is important to realize that although at least one molecular parameter must be known accurately for a diagnostic experiment to yield quantitative results, typically at least one system parameter must also be known, and its uncertainties may determine the overall accuracy. To summarize, database needs should be evaluated using at least three criteria: (a) What unknowns prevent first diagnostic experiments from being attempted? (b) What unknowns prevent meaningful comparisons with theoretical predictions? (c) Would reduction in database uncertainties for either reason have no effect because of continuing difficulties in quantifying system parameters? Examples of the interaction between the fundamental database and the work of diagnostics developers can be obtained from an extensive series of studies using tunable infrared diode laser spectroscopy.30 Another method that has great promise in studies of processing plasmas is the two-photon allowed laser-induced fluorescence (TALIF) method. This method measures atomic species' densities using the high peak power of a commercially available 10 ns pulsed laser. Since the atomic species are very reactive, it is relatively easy to devise titration reactions that allow absolute calibration of the atom densities. TALIF has been applied mostly to hydrogen dissociation studies. A listing of other candidates for the TALIF approach and their titration reactions is given in Table 3.2. The work required to ensure linearity of the detector response over a wide dynamic range is quite demanding. The point to be made here in the context of database needs is that the titration calibration, necessary to quantify the fluorescence collection efficiency, also has the effect of removing the TALIF determination of absolute concentrations from any dependence on atomic or molecular parameters. This independence of the spectroscopic and kinetic database would not apply, however, in cases in which the quenching environment of the experiment was very different from that in which the titration was done. Some data useful in plasma

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--> applications can be extrapolated from flame studies, but apart from the data of Piper and co-workers31 on nitrogen-silane reactions, the database for quenching excited species in other etching and deposition environments is sparse. TABLE 3.2 Candidate Species for Application of TALIF   Excitation Fluorescence Species λ (nm) Transition λ (nm) Transition Titration Reaction H 205 1s2S -3d2D 656 3d2D -2p2P   C 280 2p23p -2p3p3P 910 3p3P -3p3P0   N 207 2 p3 4S0 -2p23p S0 747 3p4S0 -3p4p   O 225 2p43p -2p33p3P 844 3p3P -3s3S   Cl 210 3p52p -3p44p2F 904 4p2F -4s2D   Cl 233 3p2p0 -4p4S0 725-775 4p4S0 -4s4P     SOURCE: Data from A.D. Tserepi, J.R. Dunlop, B.L. Preppernau, and T.A. Miller, J. Appl. Phys. 72:2638 (1992); Alden et al., Optics Communications 71(5):263 (1989); Das et al., J. Chem. Phys. 79:725 (1983). Another technique reported in a number of very encouraging recent studies32 is degenerate four-wave mixing (DFWM). This technique has several advantages: first, only two ports are needed, and they can be in line, as opposed to one large-aperture port at right angles to the entrance probe beam port; second, because of the fully resonant nature of DFWM, it is significantly more sensitive than other four-wave mixing techniques such as coherent anti-Stokes Raman scattering (CARS). Examples of the sensitivity include OH in a flame, about 4 × 1011 cm-3; and NO, about 8 × 1011 cm-3. The molecules C3 and SiC2 have been measured using DFWM and stimulated emission pumping (SEP) at sensitivities of 1012 cm-3 per rotational state. The main advantage of DFWM in the present context, however, is an insensitivity to quenching, allowing measurements with laser-induced fluorescence (LIF) levels of sensitivity without the corrections for molecular interactions required for LIF quantification. Surface Reaction Database and Diagnostics Surface reactions are often paramount in controlling the concentrations of atoms and other reactive radicals produced in etching and deposition plasmas. Many measurements have been made of surface reactions, generally using ex situ techniques. Some progress is being made with in situ sensors and ellipsometry (including infrared ellipsometry). Other recent studies have either used measurements of the steady-state (time-averaged) species concentration profiles adjacent to surfaces to extract reaction rates with surfaces, or made time-gated measurements of species decay after switch-off. The first method is more applicable at higher pressures in the reactor. The dynamic range of this approach depends on the accuracy and spatial resolution of the probing method. Generally the limited depth resolution will make extraction of data from finely structured surfaces quite difficult. Recent measurements on defined large areas of different materials adjacent to each other have revealed that surface reactivity is very sensitive to very small concentrations of sputtered material. An alternative diagnostic method is to make time-resolved measurements of the concentration decay, after the radio-frequency supply has been turned off or reduced to a significantly lower power level. Modulated plasma experiments can be used also to determine the gas phase kinetics, the negative ion formation rates, and the formation of clusters. At lower pressures, surface recombination and reaction often dominate the losses, and coefficients can be extracted from the species decay constants and time-dependent profiles adjacent to the surface. There are many indications that the surface reactivity often depends not only on the material and its history, but also on the fluxes and synergisms of other species (ions and neutrals) arriving at the surface. Except for a few cases, relatively little is known about these synergisms.

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--> Information Resources The database for surface reactions relevant to plasma processing of silicon is also highly dispersed. Useful information is included in several well-known texts,33 refereed journals, symposia proceedings, trade journals, and course notes. The complete characterization of a modem process is often proprietary to the manufacturer or to the equipment vendor. Many surface reactions have been studied using clean surfaces under ultrahigh-vacuum (UHV) conditions, and the different types of reactions and possibilities are reasonably well characterized. The difficulty arises when the surface is not that of a freshly cleaned crystal. The coefficients of the present database are not expected to be applicable to the surfaces found in low-pressure plasma reactors. Further, these surfaces are usually exposed simultaneously to fluxes of radicals, ions, and photons. Research is therefore required to bridge the gap between the clean surface science techniques that lead to fundamental understanding, and the etching/deposition/plasma-induced synergisms and the effective etching/deposition rates and selectivities that are important to the process engineer. It is by now well documented34 that infrared spectroscopy is one of the most powerful and broadly applicable of surface-sensitive diagnostic techniques. A key factor in its emergence has been the development of generations of capable, reliable, and relatively inexpensive FTIR spectrometers. Experimental configurations include reflection-absorption at grazing or oblique angles, transmission (through a thin film at the Brewster angle), emission, and multiple internal reflection using specially prepared substrates. Some of these techniques are suitable not only for laboratory experiments but also for monitoring and process control applications. In addition to identifying chemical species and their environments, infrared spectroscopy can be the method of choice for measuring surface temperature. Although many features in surface spectra are readily identifiable based on gas phase band positions, a great deal of the information contained in these spectra is in the properties of these bands (such as details of their shape), or in new bands, which result from environments specific to the surface. Unfortunately, there is a lack of controlled laboratory experiments to conclude that a band assignment is correct beyond all doubt. However, most of the experimental and theoretical techniques exist to create the needed database for infrared surface spectroscopy. For example, isotopic substitution can often supply key evidence for both molecular identities and orientations. Coupling of infrared studies with other surface spectroscopies that do not have in situ potential but that can be applied in UHV systems has already formed the basis for some very fruitful studies, but much remains to be done. A particularly important avenue of investigation, now that atomic force microscopy is becoming a widely available technique, is the correlation of surface roughness with the results of optical techniques such as reflection-absorption. New Diagnostic Techniques Other new methods that are assisting the understanding of surface processes include the following: Imaging of radicals interacting with surfaces (IRIS) combines a molecular beam source of radicals in a vacuum chamber with laser-induced fluorescence detection of both incident and reflected species.35 This technique is described in Chapter 4, "Heterogeneous Processes." Photoluminescence can be used to monitor process-induced damage. Calibration is needed to allow for the effects of temperature, surface condition, and other variables. There is widespread agreement that future ultralarge-scale integration (ULSI) and other applications such as diamond films will be strongly influenced by initial surface properties and nucleation phenomena on surfaces. Recent studies have used not only transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM), but also synchrotron radiation x-ray photoelectron spectroscopy to determine actual surface bonding and oxidation mechanisms. Calibration of the signals from these surfaces and from the much more complex photoresist surface is a formidable

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--> BOX 3.1 Radiative Surface Diagnostic Methods Reflection/absorption Multiple internal reflection Emission Ellipsometry Reflectance difference Photoluminescence Optogalvanic spectroscopy Surface electromagnetic waves Second harmonic generation Photoacoustic absorption Photothermal deflection Photothermal displacement Laser desorption and product analysis by mass spectrometry or spectroscopy challenge and even raises the question of whether this methodology is practicable. In situ fiber-coupled ellipsometry monitoring has been accomplished by Tachibana et al.36 A recent technique that might be applied on-line is surface desorption spectrometry, where the spectrometry may be mass spectrometry or laser-reduced fluorescence spectrometry. The former technique has been used with reflected molecular beams to characterize surface reactions at different surface temperatures and materials in molecular beam epitaxy; recently the output of the mass spectrometer has been applied in the control loop replacing the signal from the oven temperature. Remarkable improvements have been obtained in the quality of multiple quantum wells manufactured in this way. A recent study37 employed a laser to desorb SiClx from the surface of a silicon wafer being etched in a high-plasma-density, low-pressure Cl2 helical resonator plasma. Then laser-induced fluorescence of SiCl in the gas phase was used to monitor the desorbed species. The results that were obtained include estimates of the chlorine content of the SiClx adsorbed layer under plasma etching conditions, and measurements of the thickness of this layer with the plasma reactor parameters. Several other approaches have the potential to contribute to model development and validation in the longer term. Several laboratories are examining the use of in-surface ion energy analyzers, which directly provide the transfer function of the plasma sheath on the ions. Additionally it has been proposed that ion beam techniques such as elastic recoil detection and Rutherford backscattering might be applied in situ to determine surface atom coverage. Careful deconvolution of the results to allow for gas scattering would be required. Use of the new array of methods for surface characterization is expanding rapidly. The atomic force microscope and its derivatives offer supreme precision in surface definition. These techniques will be very useful off-line when combined with surface scattering or radiative measurements. It is unlikely, however, that such methods will soon see on-line or plasma reactor control applications. A summary of the potential surface characterization techniques is given in Box 3.1. Findings There now exists a wealth of sensitive radiative and laser-based techniques that permit species concentration and temperature measurements in processing plasmas. Some have high spatial and temporal resolution. However, in most cases considerable effort is required to apply these methods to real reactors, and to thereby realize the full potential of the data they can provide by interactive comparisons with model predictions. All spectroscopic diagnostic techniques depend on a database of atomic and molecular parameters. No technique can begin without a clear understanding of the spectral features observed, and a quantitative answer can sometimes be derived only with the help of one or more system parameters (such as absorption path length or fluorescence collection efficiency) as well. Some classes of data are more likely to motivate new diagnostic experiments than others. While the preexistence of the basic data needed to quantify an experiment can often be an incentive to carry out that observation, it is necessary to know the best spectral region in which to apply a given spectroscopic diagnostic. Thus, spectral databases are suggested as having particular value in initiating new investigations.

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--> Database development and review provide common reference points and save individual researchers the task of reviewing the primary literature. In addition, substantial benefits will accrue from new, electronic database technologies: searchability and compatibility with plotting and analysis software, not to mention compact storage of previously unwieldy amounts of data. There is little doubt that the electronic availability of a wide range of spectral data would stimulate the development of new diagnostic techniques and the wider application of existing methods. References 1. G. Hancock, L. Lanyi, J.P. Sucksmith, and B.K. Woodcock, ''Atoms, Radicals and Ions Observed in Plasmas— Their Gas Phase and Surface Chemistry,'' Pure Appl. Chem. 66:1207 (1994); P.B. Davies and P.M. Martineau, "Diagnostics and Modeling of Silane and Methane Plasma CVD Processes," Adv. Mater. 4:729 (1992); M. Konuma, "Plasma Diagnostics," ch. 4 of Film Deposition by Plasma Techniques (Springer-Verlag, New York, 1992); B.L. Preppernau and T.A. Miller, "Laser-Based Diagnostics of Reactive Plasmas," in Glow Discharge Spectroscopies, ed. R.K. Marcus (Plenum Press, New York, 1993), pp. 483-508; O. Auciello and D.L. Flamm, eds., Plasma Diagnostics (Academic Press, Boston, 1989); R.W. Dreyfus, J.M. Jasinski, R.E. Walkup, and G.S. Selwyn, "Optical Diagnostics of Low Pressure Plasmas," Pure Appl. Chem. 57:1265 (1985); R.F. Karlicek, Jr., V.M. Donnelly, and W.D. Johnston, Jr., "Laser Spectroscopic Investigation of Gas-Phase Processes Relevant to Semiconductor Device Fabrication," Mat. Res. Soc. Symp. Proc. 17:151 (1983); I.P. Herman, Optical Diagnostics of Thin Film Processing (Academic Press, Boston, 1996). 2. J.R. Fuhr, G.A. Martin, and W.L. Wiese, "Atomic Transition Probabilities," J. Phys. Chem. Ref. Data 17, suppl. 4 (1988); R.L. Kelly, "Atomic and Ionic Spectrum Lines Below 2000 Angstroms," J. Phys. Chem. Ref. Data 16, suppl. 1 (1987); J. Reader, C.H. Corliss, W.L. Wiese, and G.A. Martin, Wavelengths and Transition Probabilities for Atoms and Atomic Ions, NSRDS-NBS 66 (U.S. Department of Commerce, December 1980). 3. J. Fuhr, National Institute of Standards and Technology Atomic and Molecular Physics Database 24, NIST Atomic Transition Probabilities Data Files (Scandium Through Nickel), available from Standard Reference Data, NIST, Bldg. 221/Room A320, Gaithersburg, MD 20899 (1994); J.W. Gallagher, National Institute of Standards and Technology Atomic and Molecular Physics Database 38, NIST Spectroscopic Properties of Atoms and Atomic Ions Database, available from Standard Reference Data, NIST, Bldg. 22 I/Room A320, Gaithersburg, MD 20899 (1995). 4. P.S. Doidge, "A Compendium and Critical Review of Neutral Atom Resonance Line Oscillator Strengths for Atomic Absorption Analysis," Spectrochim. Acta B 50:156 (1995). 5. K.-P. Huber and G. Herzberg, Constants of Diatomic Molecules (Van Nostrand, New York, 1979). 6. J.W. Gallagher, National Institute of Standards and Technology Atomic and Molecular Physics Database 48, NIST Spectroscopic Properties of Diatomic Molecules Database, available from Standard Reference Data, NIST, Bldg. 221/Room A320, Gaithersburg, MD 20899 (1995). 7. G. Herzberg, Spectra of Diatomic Molecules (Van Nostrand, New York, 1950, reprinted by Krigger, Malabar, Fla., 1985); G. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand, New York, 1945); G. Herzberg, Electronic Spectra of Polyatomic Molecules (Van Nostrand, New York, 1966). 8. J.M. Hollas, Modern Spectroscopy (Wiley, New York, 1986); J.I. Steinfeld, Molecules and Radiation: An Introduction to Modern Molecular Spectroscopy (MIT Press, Cambridge, Mass., 1981). 9. S.N. Suchard, ed., Spectroscopic Data (IFI/Plenum, New York, 1975). 10. V.A. Boyko, Spectroscopic Constants of Atoms and Ions (CRC Press, Boca Raton, Fla., 1994). 11. R.W.B. Pearse and A.G. Gaydon, The Identification of Molecular Spectra, 4th edn. (John Wiley and Sons, New York, 1976). 12. R.F. Barrow and P. Crozet, "Gas Phase Molecular Spectroscopy," Annu. Rep. Prog. Chem. Roy. Soc. Chem. 89C:353 471 (1992); P.B. Davies, "High Resolution Tunable Infrared Laser Spectroscopy of Transient Molecules," Annu. Rep. Prog. Chem. Roy. Soc. Chem. 89C:89-110 (1992). 13. P.F. Bernath, "High Resolution Infrared Spectroscopy of Transient Molecules," Ann. Rev. Phys. Chem. 41:91-122 (1990). 14. M.A.H. Smith, C.P. Rinsland, B. Fridovich, and K.N. Rao, "Intensities and Collision Broadening Parameters from Infrared Spectra," in Molecular Spectroscopy: Modern Research, vol. III, ed. K.N. Rao (Academic Press, New York, 1985), pp. 112-248.

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