Transfer excitation processes in ion-atom collisions at high energies

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Abstract
A theory of ion-atom and ion-molecular collisions at high energies is formulated in which the projectile ions undergo simultaneous electron transfer and excitation (TE). A unified description of both resonant and nonresonant processes is presented. The resonant mode (RTE) is mediated by one of the target electrons, which excites the projectile and at the same time is captured by the projectile ion, thus forming a doubly excited intermediate state of the projectile ion. In the impulse approximation, this is related to the dielectronic recombination process in electron-ion collisions. The same doubly excited states can also be formed by one of the target electrons and the target core, interacting with the projectile ion in which the target core does the exciting and the electron is simply transferred. This is a nonresonant process (NTE), although the projectile is again left in a doubly excited state. We propose here still another mechanism for the formation of doubly excited states, UTE. The new process involves two target electrons in a nonresonant way, just as in the NTE, such that the electron that excites the projectile is left in the continuum, as viewed in the projectile rest frame. Some of the experimental evidence for these reactions is discussed.