A MICROSCOPIC THEORY OF DESORPTION INDUCED BY ELECTRONIC-TRANSITIONS

Desorption induced by electronic transitions (DIET) in two cases is in vestigated from a microscopic point of view. In case A, where a single electron in a state of the localized kind (localized around adsorbate s) is excited into a state of the extended kind (extended into the bos om of the substrate), the shape of the excited-slate potential-energy surface (PES) may differ markedly from that of the ground-state PES fo r adsorbate motion. The Franck-Condon factor then takes a finite value , giving rise to a finite desorption probability. In case B, where a s ingle electron in a state of the extended kind is excited into another state of the extended kind, the shape of the excited-state PES is pra ctically the same as that of the ground-state PES. The Franck-Condon f actor is then zero. In such a case, one should take DIET as a single-s tep (coherent) process and take into account the adsorbate-position de pendence of the matrix element for stare transitions of the electron s ystem in order to obtain a finite desorption probability.