Dynamical thresholds and quantum undulations in the energy dependence of rotationally inelastic integral cross sections

We report the observation of distinct structures in the energy dependence of rotationally inelastic integral cross sections for the systems He–Na₂ and Ne–Na₂. The calculations are performed within the infinite‐order‐sudden and the coupled states approximation using accurate ab initio potential energy surfaces. The integral cross section for a 0→j′ transition typically rises steeply at low energies, reaches a maximum, and declines slowly to the high energy region showing more or less pronounced quantum undulations. The onset of the cross section is determined by dynamical constraints rather than energy conservation. All structures in the integral cross sections are explained in terms of the energy dependence of rotational rainbow features in the corresponding differential cross sections. In particular, the undulations at higher energies stem from the supernumerary rotational rainbows. In this sense they are interpreted as analogous to the glory maxima in the elastic integral cross section for scattering from isotropic potentials. The observed structures are sensitive to the anisotropy of the repulsive branch of the potential energy surface.