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Abstract: Theoretical and experimental values have been determined for the pressure broadening of the ν1 +ν3 band of acetylene by hydrogen and deuterium at 195K, and experimental values of the pressure shifts have been determined. Theoretical values have been calculated on the basis of a recent potential energy surface using the close coupling scheme. We discuss the detailed contribution of the various rotational angular momenta of the perturbing gas and the ortho and para contribution to the total pressure broadening cross-sections. We give routes to circumvent the computational cost of such calculations. Experimental values have been measured using a tunable diode laser spectrometer assuming a Voigt line shape. These pressure broadening parameters are compared with measurements performed recently at room temperature and with present measurements performed at 195K in the ν1 +ν3 band of acetylene. A satisfactory agreement is obtained with the present results and available ones at 295K. [Copyright &y& Elsevier]

Abstract: We present measurements of He-broadening parameters for the R(0) and P(2) lines in the fundamental band of 13CO at different temperatures between 12K and room temperature. The broadening parameters are determined, taking into account confinement narrowing, by simultaneous least-squares fitting of spectra recorded using a frequency stabilized diode laser spectrometer. The pressure broadening cross sections are deduced and compared to close-coupling calculations and earlier results obtained for rotational transitions of 12CO. [Copyright &y& Elsevier]

Using a high resolution Raman spectrometer, we have measured Ar-broadening coefficients in the ν2 Q branch of C2H2 for 22 lines at 295 K, 20 lines at 174 K, and 16 lines at 134 K. These lines with J values ranging from 1 to 23 are located in the spectral range 1970.9–1974.3 cm−1. The collisional widths are obtained by fitting each spectral line with a Rautian profile. The resulting broadening coefficients are compared with theoretical values arising from close coupling and coupled states calculations. A satisfactory agreement is obtained at room as well as at low temperatures, especially for odd J lines. By comparing broadening coefficients at 295, 174, and 134 K from a simple power law, the temperature dependence of these broadenings has been determined both experimentally, and theoretically. [Copyright &y& Elsevier]

We present measurements of Ar-broadening parameters for the R(0) and R(7) lines in the fundamental band of 13CO at eight temperatures from 80 to 297 K. The broadening parameters are determined by simultaneous least-squares fitting of spectra recorded using a frequency stabilized diode laser spectrometer. The comparison of the broadening parameter values for R(7) derived at room temperature and different pressures from different line profiles shows that an empirical line profile, which takes into account narrowing effects (Dicke narrowing and absorber speed dependence) but neglects any correlation between collisions, is able to describe the observed lines with constant values of the narrowing and broadening parameters over a 10–500 Torr pressure range. Starting from a recent ab initio potential energy surface, theoretical thermally averaged close coupling values of the Ar broadening parameter are calculated for the same temperatures. The comparison between experimental and calculated values shows an overall agreement of 1.5%. [Copyright &y& Elsevier]

Abstract: The collisional shifts and widths of several P-branch spectral lines in the fundamental band of CO–Ar have been measured at temperatures between 214 and 324K and pressures between 0.025 and 1atm. The widths have been determined using a line shape model based on the solution of the transport/relaxation equation for the appropriate off-diagonal element of the density matrix. The model uses a realistic molecular potential energy surface to calculate the speed dependence of the collisional broadening, and a rigid sphere potential to calculate the translational motion. It is found that both the shifting and broadening coefficients follow a power law dependence on the temperature. Additionally, it is demonstrated that studies have tended to overestimate the accuracy of collisional widths when the line shape model used to obtain the widths involves multiple fitted line shape parameters or fails to fit the measured spectra within the experimental noise. [Copyright &y& Elsevier]

Abstract: We show that the expression for S 1 (i.e., the first term in the expansion of the S matrix) that is essential in calculating vibration-rotation pressure-broadened shifts is not properly given in the usual Robert–Bonamy (RB) formalism. The problem resulted when they considered effects of the vibrational dephasing on S 1; they tacitly made an assumption that the trajectories of interest are vibrationally independent. As a result, the current RB expression is an approximate one. Based on a vibration-dependent trajectory model, which is physically sound, we derive the correct expression for S 1. We present numerical calculations of the broadening and shifting cross sections and of the line shape parameters for the H2–He pair and find for this molecular system, new results differ significantly from those calculated using the existing formalism. In addition, by comparing with those derived from close coupling calculations, we find that the new results are better than the old ones. Finally, we discuss how important effects from this modification could be for other molecular systems. We conclude that for certain molecular systems where the short-range collisions are the dominant source responsible for the line widths and shifts, and in addition, the isotropic potential has small depth, the present modification is a worthwhile step to be taken in order to refine the current RB formalism. Meanwhile, for other systems it may not be necessary. [Copyright &y& Elsevier]

Abstract: The shapes of Dicke-narrowed spectral lines in the fundamental P-branch of CO in Ar are studied by comparing high-resolution measurements and theoretical calculations. The measured spectra were recorded at temperatures between 214 and 324K, and at pressures between 0.025 and 1atm. The calculations are based on solving a transport/relaxation equation for the appropriate off-diagonal element of the density matrix; they use a realistic intermolecular potential to determine the speed-dependent collisional broadening, and a rigid sphere potential to determine the Dicke narrowing. It is found that the calculations can reproduce the measured spectra within the experimental noise under all conditions, but that the magnitude of the Dicke narrowing in the measured spectra is 70–90% less than predicted from the mass diffusion constant. A revised view of the collision operator resolves the discrepancy in principle, and leads to a better understanding of the line shape problem in general. [Copyright &y& Elsevier]