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The generalized Hess method (GHM) gives a line shape expression which is formally equivalent to the Rautian–Sobel’man hard collision model of Dicke narrowing, but differs radically in the definition of one of the relaxation terms. The relaxation term leading to pressure broadening is the same, but the term leading to Dicke narrowing and ultimately to Doppler line shapes at zero density differs in certain important respects: (1) in GHM it is a weighted sum of the pressure broadening coefficient and an optical diffusion coefficient and (2) there is no sharp distinction between ‘‘velocity changing’’ and ‘‘phase changing’’ collisions. The Dicke narrowing term should thus be understood as including both collision types irretrievably intermixed, with GHM providing a prescription for both relaxation terms. Applied to HF v=0→1, j→j±1 absorption spectra in a bath of Ar and using an accurate interaction potential obtained from spectra of the van der Waals complex and essentially exact close coupling scattering S matrices, GHM provides a rather good description of recently measured line shapes. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An interaction potential for Ar–HF, which was recently determined from extensive spectroscopic data for the van der Waals dimer and which includes dependence on the HF vibrational state, has been used with accurate close-coupling molecular scattering calculations to predict line shape parameters for the pure rotational and the fundamental and first overtone vibrational bands of HF in a bath of Ar. Agreement with experiment is good; in fact, considering inconsistencies among the experimental values, the theoretical values may be the more reliable. This confirms the accuracy of the Ar–HF interaction potential, including the dependence on the HF vibrational level which is sensitively probed by the line shift cross sections. [ABSTRACT FROM AUTHOR]

Using an intermolecular potential determined from spectra of van der Waals complexes [R. J. Le Roy and J. M. Hutson, J. Chem. Phys. 86, 837 (1987)] and accurate close coupling scattering matrices, line-shape cross sections were calculated for vibrational Raman Q-branch spectra of H2 and D2 in Ar to compare with recent experimental values. While predicted room temperature linewidths are in moderate (≊25%) agreement with experiment, predicted line shifts are small by nearly a factor of 2. The calculations show a strong collision speed dependence in the shift cross sections, in qualitative if not quantitative accord with the model developed by R. L. Farrow et al. [Phys. Rev. Lett. 63, 746 (1989)] to explain their H2 data. However, the calculations predict a strong temperature dependence in the linewidth cross sections as well, whereas the data show little variation between room temperature and 1000 K. It is suggested that these discrepancies point to inadequacies in the vibrational dependence of the intermolecular potential at close intermolecular separations (approximately 2.8 to 3.1 Å). [ABSTRACT FROM AUTHOR]

An interaction potential for H2O–Ar that was recently determined from spectra of the van der Waals complex [R. C. Cohen and R. J. Saykally, J. Chem. Phys. 94, 7991 (1990)] is used with accurate molecular scattering calculations to predict line broadening parameters for the 183 and 380 GHz transitions of water vapor. Theoretical values are about 15% higher than experiment. [ABSTRACT FROM AUTHOR]