Electronic mechanism for alkali-metal promoted dissociation of oxygen on semiconductors

Abstract

As an oxygen molecule approaches a semiconductor surface, charge transfer from the surface dangling bond bands to the partly occupied 2π* orbital of the oxygen molecule takes place. Preadsorbed alkali atoms supply empty surface states with electrons, which lead to a work function decrease. This will in turn promote the charge transfer and increase the probability for an electronic excitation of the molecule by hot electron-hole pairs when resonant tunneling to the 2π* state takes place. If this excitation occurs, additional charge will presumably be transferred to the molecule which leads to dissociation. The dissociation probability is estimated by the probability for the electronic excitation by hot electron-hole pairs, weighted by the amount of electron charge transferred on to the molecule. It is found that dissociation is substantially increased for an oxygen molecule approaching the surface when an alkali overlayer is preadsorbed on the surface.