Dicke narrowing of the polarized Stokes–Raman <i>Q</i> branch of the <i>v</i>=0→1 transition of D2 in He

Abstract

A b initio calculations were carried out for several Stokes–Raman Q branch line shapes of the v=0→1 transition of D2 in helium in the low pressure regime where Dicke narrowing is observed. Five different approximations were studied: a generalization of Hess’s method, which is based on the equivalent of a Bhatnagar–Gross–Krook model of the Waldmann–Snider kinetic equation; two high pressure asymptotes of Hess’s method; a moment method of the Chapman–Cowling type due to Corey and McCourt; and a collision kernel method that has found extensive use lately in rarefied gas dynamics and other highly nonequilibrium states. Using generalized cross sections incorporating S-matrices calculated in the close coupled approximation with an ab initio potential energy surface, most of the methods agreed moderately well with the most recent line width determinations at most densities. Of the five methods, the generalized Hess and collision kernel methods agreed best with experiment over the whole range. At the lowest densities where the collision kernel method might be expected to have convergence problems none were observed, and at high densities the ab initio generalized Hess method may not be flexible enough.