Semiclassical theory of alignment effects in near-resonant energy-transfer collisions of rare-gas atoms with aligned Rydberg atoms

Recent experiments [E. M. Spain et al., J. Chem. Phys 102. 24 (1995)] discovered alignment effects in cross sections for near-resonant energy-transfer collisions of Xe atoms with Ca Rydberg atoms at a single mean relative velocity. A collaborative quantum-mechanical stud; [W. Issacs and M. A. Morrison. Phys. Rev. A, 57, R9 (1993)] confirmed these findings and discovered pronounced oscillations in the velocity dependence of state-to-state cross sections. Collisions corresponding to the 17d(m0)--> 18p(m) transitions in the Ca-He system are here analyzed semiclassically. This analysis shows that the origin of these oscillations is a phase interference process unique to Rydberg target states. We further demonstrate the importance of retaining the energy defect and of using quantum-defect phase-shifted radial functions in calculations of alignment cross sections for Rydberg states.