H-2-HE VIBRATIONAL LINE-SHAPE PARAMETERS - MEASUREMENT AND SEMICLASSICAL CALCULATION

High-resolution inverse Raman spectroscopy has been used to obtain the line shifting and line broadening coefficients of H-2 perturbed by He . Measurements have been made for the Q-branch transitions (J=0 to 5) in a density range of 10 to 20 amagat and from 296 to 995 K. Up to 795 K we have directly deduced from the experimental broadening coefficie nts the inelastic rotational state-to-state and vibrational dephasing rates. At higher temperatures, owing to the larger number of channels of relaxation which occur, the results have been analyzed using a scal ing law. The line shift and broadening coefficients exhibit a square r oot and a linear dependence on temperature, respectively, and a signif icant J dependence. Semiclassical calculations based on an accurate ab initio potential lead to line-shape parameters consistent with experi ment. They allow a clear understanding of their observed temperature d ependence.