Interference oscillations in the excitation cross section for slow ion - hydrogen collisions

We have determined the minimum set of molecular states and couplings that can explain the origin of ubiquitous oscillations in the excitation cross section for low-energy ion - atom collisions which have recently drawn significant theoretical and experimental attention. To this end, the model of non-adiabatic radial couplings between quasimolecular states in slow heavy particle collisions is reformulated using the hidden-crossings theory to circumvent certain inherent shortcomings of the conventional molecular orbital close-coupling approach. We find that four diabatic states, two of which are formed by appropriate S superseries of branch points of the complex adiabatic quasimolecular energy surface, are required to represent the physical mechanism responsible for the oscillations. In particular, the oscillations originate from the interference of two paths along these terms through which the initial state is promoted to the same final quasimolecular state.