Abstract
A recent paper by Golser and Semrad [Phys. Rev. Lett. 66, 1831 (1991)] showed a departure from velocity proportionality in their energy-loss measurement of a hydrogen projectile in the gas below 20 keV. We point out that their result is a consequence of complex collision dynamics that is common in the molecular regime and has been known for some time among atomic physicists. The observation is well reproduced by a compilation of published stopping cross sections.