Showing papers by "Robert E. Walkup published in 1988"

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.

The Origin of Positive Ions and Excited Neutrals in Electron Stimulated Desorption from Alkali Halides

Abstract: The interaction of electron and photon beams with insulating materials is of substantial current interest. Alkali halides are the most ionic of the ionic insulators, and they have comparatively simple geometric and electronic structures, thus they are useful prototypes for basic studies. When energetic electrons or photons are incident on alkali halides, particles are desorbed with high efficiency. The flux of particles leaving the bombarded region includes ground-state neutrals, excited-state neutrals, and positive ions. The vast majority of the desorbed particles are ground-state neutral atoms and molecules, which are ejected via bulk-defect mediated processes [l]. The ground-state yields can be remarkably large - for 1 keV electrons incident on NaCl at T=300 C, roughly 5 Na atoms and 5 CI atoms are desorbed per incident electron [2,3]. The desorption mechanisms for the ground-state particles have been qualitatively understood for some time. The initial excitation caused by the incident radiation is electron-hole generation in the bulk. The holes (neutral halogen atoms) are incorporated into stable defects called H centers which can diffuse to the surface, where neutral halogen atom desorption can occur. Neutral alkali atoms can be formed on the surface by re-combination of electrons and alkali ions, and these neutral atoms can then thermally desorb. This process appears to be limited by the diffusion of electrons (as F centers) to the surface [4]. The very large yields of ground state neutrals are related to the fact that the initial excitation results in a stable defect in the bulk, and desorption can occur much later after defect diffusion to the surface. The time delay due to the diffusion of defects to the surface has been directly observed in experiments using pulsed electron beams [4,5].