Speed-Dependent inhomogeneity in H2 vibrational line profiles

We have observed line profiles in Raman Q‐branch transitions of H2 perturbed by Ar that are asymmetric about the line center using high resolution inverse Raman spectroscopy. These effects are observed at densities normally considered to be in the impact regime, i.e., well above the Dicke minimum for H2‐Ar(∼1 amagat) and below those densities for which finite collision‐duration or collision‐induced scattering become important. We have successfully modeled the observed line profiles by incorporating inhomogeneous broadening arising from a strong dependence of the collisional line shift on radiator speed. The model is more complex than the speed‐dependent Voigt line profile1,2 since the effects of speed‐changing collisions, which coherently narrow the inhomogeneous part of the line profile, must be included. In H2 the asymmetry is more pronounced for heavy perturbers and essentially disappears in H2/He mixtures and in pure H2.