The Nature of Repulsive States and the Role of Nuclear Dynamics in Desorption Induced by Electronic Transitions

Abstract

Electrons and photons cause desorption by exciting transitions to electronic states that are repulsive, i.e. by breaking bonds. Thus, understanding desorption means understanding the behavior of excited electronic states at surfaces and the coupling of repulsive potential energy into atomic motion. In the first attempt to provide a microscopic description of the ESD/PSD process, Menzel, Gomer[l] and Redhead[2] (MGR) discussed the role of Franck-Condon transitions to repulsive excited states and the role of the substrate in the quenching of these excitations. The MGR model has provided the conceptual framework to discuss DIET processes in the valence region of the spectrum. Unfortunately, however, there has been little progress towards identifying the specific nature of these repulsive states (particularly in the valence region), and understanding the subsequent electron and nuclear dynamics leading to desorption. In the case of ionic solids, a conceptually simple positive-ion desorption mechanism has been identified based on the reversal of the Madelung potential, following multiple ionization of a negative ion in the crystal[3,4]. Despite the fact that the repulsive character of such a state is beyond any doubt, both theoretical [5] and experimental [6] evidence does not always support a desorption mechanism based on Coulombic repulsion.