Strong competition between velocity-changing and phase- or state-changing collisions in H2 spectra perturbed by Ar

A collisional inhomogeneous broadening of the H-2 Q(1) line perturbed by Ar was observed for the first time 25 years ago. Several attempts were made to explain the line broadening from ab initio calculations, which, however, resulted in fundamental discrepancies between the theory and experiment. To resolve this problem we investigate two possible sources of these differences. First, we repeat the ab initio calculations of the broadening and shifting speed dependence using in the scattering calculations a highly accurate ab initio H-2-Ar interaction potential. Second, we replace the previous phenomenological models of velocity-changing collisions with a more physical one based on the interaction potential. This allows us to properly reproduce the experimental broadening over a wide range of temperatures and pressures. We show that this abnormal broadening is affected by strong competition between the velocity-changing collisions and speed-dependent collisional shifting, especially at high pressures.