The Dynamics of O+ Desorption from TiO2

Abstract

Electrons and photons can stimulate positive ion desorption from surfaces provided that: (1) the positive ion is formed by electronic transitions initiated by the incident electron or photon, (2) repulsive forces act to expel the particle, and (3) the positive ion escapes without neutralization or re-capture by the solid. A microscopic mechanism has been proposed by Knotek and Feibelman [1] to explain positive ion desorption from ionic insulators. The prototypical system for this mechanism is O+ desorption from TiO2. Since the Ti cation is maximal valent, i.e. a filled 3p shell and no 3d electrons, a Ti core hole that is produced by electron or photon impact can only be filled by an inter-atomic Auger decay process that involves O valence electrons. There is a finite branching ratio for two Auger electrons to be ejected from a single oxygen center, resulting in the formation of O+. This O+ ion can then be expelled from the surface by repulsive Coulomb forces. Since the repulsive forces are relatively well known, the nuclear motion can be predicted. In this article, we discuss classical trajectory calculations of O+ desorption from TiO2. These calculations establish the time scale for desorption, and they provide predictions of ion kinetic energy and angular distributions which can be directly compared with experimental results [2,3]. The calculations also provide qualitative insight into the effects of lattice recoil and the origin of site-dependent desorption probabilities.KeywordsAngular DistributionRepulsive ForceBridge SiteNuclear MotionTrajectory CalculationThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.