Raman Q-branch line shapes as a test of the H2–Ar intermolecular potential.

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]