H2-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 ${\mathrm{H}}_2$ perturbed by He. Measurements have been made for the $Q$-branch transitions ($J=0\mathrm{} \mathrm{to} 5$) 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 coefficients 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 scaling law. The line shift and broadening coefficients exhibit a square root and a linear dependence on temperature, respectively, and a significant $J$ dependence. Semiclassical calculations based on an accurate ab initio potential lead to line-shape parameters consistent with experiment. They allow a clear understanding of their observed temperature dependence.