Origin of the structures in the excitation cross sections in ion-atom collisions

It is demonstrated that the structures in the low-energy cross sections for excitation to certain levels of atoms by ions originate from a shift from direct excitation at high and intermediate energies to pronounced two-center molecular mechanisms at lower energies. The molecular mechanisms dominate at small internuclear separations, whereas the direct mechanism is driven by the familiar dipole long-range coupling. The demonstration is based on maps of the excitation probabilities over the collision velocity and impact parameter, which display two typical regions. System specific details determine the separation between the two regions and give rise to a plateau, an oscillation or a hidden shoulder in the excitation cross sections. Maps of classical-trajectory Monte Carlo method results show similar qualitative features, but the details of these maps differ considerably from those of the quantal calculations.