An isolated line-shape model based on the Keilson-Storer function for velocity changes. II. Molecular dynamics simulations and the Q(1) lines for pure H2.

This paper presents comparisons between molecular dynamics simulations (MDSs) and the Keilson and Storer (KS) model for collision-induced translational velocity changes in pure H(2) at room temperature from four different points of view. The first considers various autocorrelation functions associated with the velocity. The second and third comparisons are made for the collision kernels and for the time evolutions of some conditional probabilities for changes in the velocity modulus and orientation. Finally, the evolutions, with density, of the half widths of the Q(1) lines of the isotropic Raman (1-0) fundamental band and of the (2-0) overtone quadrupole band are investigated. The results demonstrate that, while the KS approach gives a poor description of detailed velocity-to-velocity changes, it leads to accurate results for the correlation functions and spectral shapes, quantities resulting from large averages over the velocity. On the opposite, collision kernels derived from MDS lead to accurate predictions of all considered quantities. The results open promising perspectives for modeling of the spectral shapes of other systems. They also stress the value of direct calculations of speed-dependent broadening and shifting parameters from the intermolecular potential to avoid their determination from measured spectra and permit fully meaningful tests of the models.