Interference effects in rotational state distributions: Propensity and inverse propensity

Semiclassical scattering theory has been used to investigate interference effects in rotational state distributions for inelastic atom–diatom collisions. The Δj=even selection rule for homonuclear molecules is seen semiclassically to be an interference effect, and when this symmetry is weakly broken—i.e., an ’’almost homonuclear’’ molecule, one for which the odd anisotropy is much smaller than the even anisotropy—the interference persists in the form of a propensity rule, σ (odd Δj) ≪σ (even Δj). If the odd anisotropy is sufficiently large, however, one can see an inversion of the normal propensity, i.e., it can happen that σ (odd Δj) ≳σ (even Δj). It is suggested that rotationally state-selected experiments which resolve this interference structure would be an extremely sensitive measure of the anisotropy in the interaction potential.