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Mixing of rotational levels and intracollisional interference in the pure rotational R0(J) transitions of gaseous HD.

A detailed theoretical analysis of the intensities of the pure rotational R/sub 0/(J) transitions in gaseous HD is presented. These transitions all manifest small intracollisional interference effects arising from the cross product of the allowed-- and the induced--dipole-moment matrix elements. Using recent ab initio calculations of the induced dipole moment in an HD-HD pair, this interference is found to be constructive for most transitions, thus implying a density-dependent increase in intensity. For the R/sub 0/(0) transition, a small but significant additional contribution to the constructive intracollisional interference results from the mixing of rotational levels in a single molecule during collisions. Additionally, because of a near-resonance condition, simultaneous mixing of internal rotational levels in both molecules of a colliding pair leads to a large destructive interference affecting primarily the R/sub 0/(2) transition. The present theoretical values of the interference parameter a/sub theor/ are compared with experimental data and it is concluded that while the low-temperature data are in reasonable accord with the present theoretical results, significant differences still remain both between different experimental determinations, and between theory and experiment for the room-temperature data. Possible theoretical refinements to explain the observed large temperature dependence of the intracollisional interference are discussed briefly.