Effect of speed-changing collisions on spectral line shape

High-resolution inverse Raman experiments for ${\mathrm{H}}_2$ dilute in Ar [R. L. Farrow et al., Phys. Rev. Lett. 63, 746 (1989)] have revealed remarkable deviations from the usual linear mixing rule for the Q-line broadening. They have been interpreted using a line-profile model that takes into account the spectral narrowing induced by speed-changing collisions. The present approach includes the role of collisions changing both speed and phase, leading to an alternative interpretation of the above-noted experiments. This interpretation generalizes the earlier study and points out the various mechanisms of line formation.