Interference Structure in Ion-Atom Collisions

In low-energy ion-atom collisions, electronic excitation occurs dominantly via non-adiabatic transitions between quantum states of the quasimolecule (collision complex) formed momentarily during the collision. A growing list of cases have been found by several groups in which collision-induced optical emission of particular target or projectile spectral lines oscillates as a function of bombarding energy. These "Rosenthal oscillations" result from multiple excitation paths where there is long range mixing. In the light emitted from certain states of helium atoms excited in He+-He collisions, we have seen these oscillations emerge dramatically in the velocity-dependence of the polarization fraction of the light, even though they were not obvious in the total emission cross-section. In particular, from the sign of the polarization of light from the 3, D states, one can infer that rotational coupling in the excitation is nearly maximal, leading to strong collisional alignment. The interpretation of these oscillation patterns is discussed with particular reference to polarization effects. Possible applications to spectroscopy of collision complexes and to the analysis of the first few monolayers of solid surfaces will be described.