On the mechanism of desorption from surfaces induced by electronic transitions

Abstract

We analyze the mechanism by which positive ions and neutral species desorb from surfaces as a result of photon- or electron-beam induced electronic transitions. The system fluorine on aluminum is used as a prototype. We first present results of ab initio density-functional calculations of the potential energy curves of several charge states of fluorine on aluminum. We find that fluorine adsorbs as F-(2s22p6) and is strongly bound in the ground state. Valence (i.e., 2s22p5) and core (e.g., 2s12p6)-onized states are, however, repulsive. F+(2s22p4) ions form bound states on Al but their adsorption energies are much smaller and their equilibrium distance is further out from the surface than those of F- ions. The difference in the bonding of positive and negative ions is ascribed to differences in the corresponding screening mechanisms. Screening of negative ions proceeds only by an image mechanism, while screening of positive ions can proceed by both image and charge-transfer mechanisms in which charge from the metal occupies the large-radius 3s and 3p orbitals of F. The resulting partially neutralized positive-ion states have reduced image attraction and increased electron kinetic energy (Pauli) repulsion. Franck-Condon transitions from the ground state populate the repulsive part of F+ potential energy curve and lead to efficient F+ desorption. F++ states are strongly bound and do not desorb. The same conclusions are reached by a more general analysis of the desorption of electronegative atoms from any metallic substrate, based on the concepts of effective medium theory. Finally, we discuss the applicability of our conclusions regarding the desorption of neutral and ionic fluorine to desorption of molecular adsorbates and also desorption from nonmetallic substrates. © 1988 American Institute of Physics.