Collision-induced velocity changes from molecular dynamic simulations in H 2–Ar: A test of the Keilson–Storer model and of line-broadening/shifting calculations for the Q(1) Raman line

Abstract

Classical Molecular Dynamics Simulations (MDS) are carried for the calculation of the collision kernels f ( v → , v → ′ ) describing the velocity changes of H 2 molecules induced by interactions with Ar atoms. These MDS results are first used for the test of a previously proposed three dimensional bi-parametric extension (KS-3D) of Keilson and Storer velocity-changing model. Semi-classical and close-coupling calculations of the speed-dependent collisional-broadening γ coll( v) and -shifting δ coll( v) coefficients are also made for the Q(1) line, using the best available H 2–Ar potential energy surface. These data are compared with the corresponding values previously extracted from measured Raman spectra. Furthermore, the calculated values of f ( v → , v → ′ ) , γ coll( v) and δ coll( v) are used to calculate the density dependence of the Q(1) line-width from the Doppler to the collisional regime and compare the results with experiments. The conclusions of this exercise are three fold. The first is that the KS-3D approach gives a satisfactory description of the velocity changes. The second is that the procedure used in a previous study in order to extract γ coll( v) and δ coll( v) from measured spectra was correct. The last, consistent with the findings of other independent studies, is that the H 2–Ar interaction potential used needs refinements at short distances.