A recent quantum mechanical study [W. Isaacs and M. A. Morrison, Phys. Rev. A 57, R9 (1998)] discovered pronounced oscillations in cross sections for near-resonant energy transfer collisions of rare-gas atoms with initially aligned Rydberg atoms. We analyze such collisions for 17d m ! 18p m 0 transitions in the Ca-He system semiclassically and show that the oscillations arise from a phase interference process unique to Rydberg target states. In addition to explaining the origin of these

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... es, this analysis explains their disappearance when the relative Ca-He velocity goes to infinity and /or the energy defect vanishes and their dependence on the initial and final magnetic quantum numbers of the transition.