Semiclassical theory of molecular collisions in a laser field

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Abstract
A semiclassical theory of radiative transitions for molecular collisions in an intense laser field is developed. The resulting expressions for the S‐matrix involve electronic‐field potential surfaces which are constructed from field‐free adiabatic surfaces and transition moments. The electronic‐field surfaces, which exhibit avoided crossings due to radiative coupling, are analytically continued to complex intersection points (branch points). Transitions between two such surfaces are effected by classical trajectories propagating on one surface and passing smoothly to the other surface through a branch point. Each of the two surfaces is seen to be a branch of a quadruple‐valued potential function, and the branch‐point structure of this function is related to field‐free nonadiabatic coupling and dynamic radiative coupling. Interference effects between these two kinds of couplings are discussed and illustrated for model potentials. Numerical results show enhanced transition probabilities for the collinear process Br(2P3/2)+H2(v=0)+h/ω →Br(2P1/2)+H2(v=0). Experimental investigations of laser enhanced energy transfer, interference effects, and the possible formation of long‐lived complexes, which are not necessarily present in the field‐free case, are suggested for a molecular beam experiment carried out in the presence of a laser beam.