C. Boulet

Line coupling in the temperature and frequency dependences of absorption in the microwindows of the 4.3 μm CO2 band

Abstract The shapes of the self- and N2-broadened ν3CO2 fundamental vibration-rotation band in the microwindows (troughs between the lines) have been measured at various temperatures. Important deviations with respect to the superposition of Lorentzian profiles are observed. These deviations are interpreted in terms of line coupling, which redistributes the intensity in the whole band. In order to take into account this line coupling, two models are considered within the frame of the impact theory. The first model uses the strong-collision approximation to describe the rotational energy transferred by collisions. It leads to a simple analytical expression for the band profile. The second model is based on the exponential-gap law. These two models account well for the frequency dependence of the measured absorption in the microwindows and for the temperature dependence in the case of the N2-broadened CO2 band but not in the self-broadened case. The influence of the line-coupling rotational distribution, which differs significantly in the two models, is discussed. The possible role of the finite duration of collision in rotational energy transfer is examined.

Molecular dynamics simulations for CO2 spectra. III. Permanent and collision-induced tensors contributions to light absorption and scattering.

Classical molecular dynamics simulations have been performed for gaseous CO(2) starting from an accurate anisotropic intermolecular potential. Through calculations of the evolutions of the positions and orientations of a large number of molecules, the time evolutions of the permanent and collision-induced electric dipole vector and polarizability tensor are obtained. These are computed from knowledge of static molecular parameters taking only the leading induction terms into account. The Laplace transforms of the auto-correlation functions of these tensors then directly yield the light absorption and scattering spectra. These predictions are, to our knowledge, the first in which the contributions of permanent and collision-induced tensors are simultaneously taken into account for gaseous CO(2), without any adjusted parameter. Comparisons between computations and measurements are made for absorption in the region of the ν(3) infrared band and for depolarized Rayleigh scattering in the roto-translational band. They demonstrate the quality of the model over spectral ranges from the band center to the far wings where the spectrum varies by several orders of magnitude. The contributions of the permanent and interaction-induced (dipole and polarizability) tensors are analyzed for the first time, through the purely permanent (allowed), purely induced, and cross permanent∕induced components of the spectra. It is shown that, while the purely induced contribution is negligible when compared to the collision-broadened allowed component, the cross term due to interferences between permanent and induced tensors significantly participates to the wings of the bands. This successfully clarifies the long lasting, confusing situation for the mechanisms governing the wings of the CO(2) spectra considered in this work.

Infrared line collisional parameters of HCl in argon, beyond the impact approximation: Measurements and classical path calculations

Measurement of room temperature absorption by HCl-Ar mixtures in the 1-0 and 2-0 bands have been made for pressures between 10 and 50 atm. Fits of these spectra are made for the determination of the width, spectral shift, asymmetry, and intensity of individual lines. The broadening and shifting parameters are in satisfactory agreement with previous determinations but provide the first complete and self-consistent sets covering P(15)-R(14) and P(7)-R(8) in the 1-0 and 2-0 bands, respectively. The asymmetries of the profiles, which have been studied for the first time, are smaller than typically 10(-3) atm(-1) and cannot be determined experimentally. On the other hand, the intensities of the low j lines show a significant linear decrease with increasing Ar pressure. Calculations of all measured quantities are made with a classical path approach and an accurate vibrational-dependent HCl-Ar potential energy surface (PES). Comparisons with experimental values show that widths and shifts are well predicted, confirming the quality of the PES and of the theoretical model, and the calculations confirm that asymmetries are small. The damping factors of the intensities are analyzed by considering three contributions: The first is due to the formation of van der Waals complexes, the second results from the finite duration of collisions, and the last comes from initial correlations. Calculations indicate that the last process has negligible consequences but that the first two processes lead to effects of the same order and explain most of the observed decrease of the intensities, even if some discrepancies persist for the mid R:mmid R:=1 rotational components.

The frequency detuning and band‐average approximations in a far‐wing line shape theory satisfying detailed balance

We develop the basic formalism of a far‐wing line shape theory that satisfies the detailed balance principle. For molecular systems of interest, e.g., CO2–Ar at room temperature or higher, there are many individual vibration–rotational lines in a given band and many bands in the spectrum. In such cases, one must make additional approximations in order to carry out accurate calculations of the absorption coefficient using a reasonable amount of computer time. In the present paper, we discuss two such simplifications: the frequency detuning approximation of the line‐coupling functions and the band‐average approximation. We then apply the theory to a calculation of the far‐wing absorption of the ν3 band of CO2 perturbed by Ar, successively including the effects of more lines in the calculations by increasing Jmax from 40 to 108. From the results of this work, we find that the frequency detuning approximation is good only for frequencies of interest far from the band center. In addition, we find that contrary...

Molecular dynamics simulations for CO2 absorption spectra. I. Line broadening and the far wing of the ν3 infrared band

Classical molecular dynamics simulations (CMDS) have been carried out for gaseous CO(2) starting from the intermolecular potential energy surface. Through calculations for a large number of molecules treated as rigid rotors, various autocorrelation functions (ACFs) are obtained together with probabilities of rotational changes. Those used in the present paper are the ACFs of the center of mass velocity and of the molecular orientation, and the conditional probability of a change of the angular speed. They enable calculations, respectively, of the mass diffusion coefficient, of the infrared (dipolar) band shape including the wings, and of individual line-broadening coefficients. It is shown that these calculations, free of any adjustable parameter, lead to good agreement with measured values. This is expected from previous studies for the mass diffusion coefficient and line-broadening coefficients, but it is, to our knowledge, the first demonstration of the interest of CMDS for the prediction of band wings. The present results thus open promising perspectives for the theoretical treatment of the difficult problem of far wings profiles.

Irreducible correlation functions of the S matrix in the coordinate representation : Application in calculating Lorentzian half-widths and shifts

By introducing the coordinate representation, the derivation of the perturbation expansion of the Liouville S matrix is formulated in terms of classically behaved autocorrelation functions. Because these functions are characterized by a pair of irreducible tensors, their number is limited to a few. They represent how the overlaps of the potential components change with a time displacement, and under normal conditions, their magnitudes decrease by several orders of magnitude when the displacement reaches several picoseconds. The correlation functions contain all dynamical information of the collision processes necessary in calculating half-widths and shifts and can be easily derived with high accuracy. Their well-behaved profiles, especially the rapid decrease of the magnitude, enables one to transform easily the dynamical information contained in them from the time domain to the frequency domain. More specifically, because these correlation functions are well time limited, their continuous Fourier transforms should be band limited. Then, the latter can be accurately replaced by discrete Fourier transforms and calculated with a standard fast Fourier transform method. Besides, one can easily calculate their Cauchy principal integrations and derive all functions necessary in calculating half-widths and shifts. A great advantage resulting from introducing the coordinate representation and choosing the correlation functions as the starting point is that one is able to calculate the half-widths and shifts with high accuracy, no matter how complicated the potential models are and no matter what kind of trajectories are chosen. In any case, the convergence of the calculated results is always guaranteed. As a result, with this new method, one can remove some uncertainties incorporated in the current width and shift studies. As a test, we present calculated Raman Q linewidths for the N2-N2 pair based on several trajectories, including the more accurate exact ones. Finally, by using this new method as a benchmark, we have carried out convergence checks for calculated values based on usual methods and have found that some results in the literature are not converged.

Broadening and line mixing in the 20 00←01 10, 11 10←00 00 and 12 20←01 10 Q branches of carbon dioxide: Experimental results and energy-corrected sudden modeling

Using both a difference frequency spectrometer and a Fourier transform spectrometer, we have measured transitions in the 12 (2)0<--01 (1)0 band of carbon dioxide at room temperature and pressures up to 19 atm. The low-pressure spectra were analyzed using a variety of standard spectral profiles, all with an asymmetric component to account for weak line mixing. For this band, we have been able to retrieve experimental line strengths and the broadening and weak mixing parameters. In this paper we also compare the suitability of the energy-corrected sudden model to predict mixing in the two previously measured Q branches 20 (0)0<--01 (1)0, the 11 (1)0<--00 (0)0, and the present Q branch of pure CO(2), all at room temperature.

Temperature, pressure, and perturber dependencies of line-mixing effects in CO2 infrared spectra. II. Rotational angular momentum relaxation and spectral shift in Σ←Σ bands

The Energy Corrected Sudden approach is used in order to deduce collisional parameters and to model infrared quantities in Σ-Σ bands of CO2-He and CO2-Ar mixtures at room temperature. Measurements are first used for the determination (from a fit) of the rotational angular momentum relaxation time and of some parameters representative of the imaginary part of the relaxation operator. It is shown that line-broadening data as well as absorption in both the wing and central part of the ν3 and 3ν3 bands lead to consistent determinations. The model is then used for detailed analysis of line-mixing effects. The influences of pressure, of the band spectral structure, and of the collision partner are studied. Differences between the effects of collisions with He and Ar are pointed out and explained.

Temperature, pressure, and perturber dependencies of line-mixing effects in CO2 infrared spectra. III. Second order rotational angular momentum relaxation and Coriolis effects in Π←Σ bands

The energy corrected sudden approach is used in order to deduce collisional parameters and to model infrared quantities in Π←Σ bands of CO2–He and CO2–Ar mixtures in the 200–300 K temperature range. Measured line-broadening coefficients and absorption in the Q-branch of the ν2 band at moderate pressure are first used for the determination (from a fit) of the time constant associated with the relaxation of the second order traceless tensor of the rotational angular momentum (all other collisional quantities have been determined previously). The results obtained are consistent with previous (calculated) temperature dependent values of the depolarized Rayleigh cross sections. The model is then successfully tested through computations of absorption in the ν2 and (ν1+ν2)I bands at elevated densities. Analysis of line-mixing effects is made, including study of the influence of interbranch transfers and of Coriolis coupling. Differences between the effects of collisions with He and Ar are pointed out and explained.

Metastable dimer contributions to the collision-induced fundamental absorption spectra of N2 and O2 pairs

Abstract The contributions of dimer transitions to the low-temperature collision-induced fundamental spectra of N2 and O2 have been observed in many earlier studies. However, recent measurements made at high resolution have shown the existence of periodic structures superposed on the smooth collision-induced envelope which persist to room temperatures. In a series of recent papers, we have compared theoretical calculations with experimental data for N2–N2, O2–O2, N2–O2 and O2–N2 pairs and have concluded that the observed structure was not due to either collisional line mixing or intercollisional interference. In the present paper, we show that one can get both qualitative and quantitative agreement between theory and experiment by including the contributions due to metastable dimers that are expected to be present at the few percent level.