The Role of Single and Multi-Electron Excitations in Electron Stimulated Desorption
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THE ROLE OF SINGLE AND MULTI-ELECTRON EXCITATIONS IN ELECTRON STIMULATED DESORPTION
PH. AVOURIS, F. BOZSO AND A. R. ROSSI IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598
ABSTRACT We investigate the nature of the electronic transitions which lead to the desorption of ions from adsorbate-covered metal and semiconductor surfaces. F+ desorption from F/Si occurs via a Knotek-Feibelman mechanism, while H+ desorption from H/Si and 0+ from CO/metals involve multi-electron transitions. The desorption of CO+ from CO/metals and NO+ from NO/Si apparently occurs via a simple Menzel-Gomer-Redhead mechanism.
INTRODUCTION There has been an increasing interest towards the understanding and utilization of electronand photon-stimulated desorption (ESD and PSD) from solid surfaces [1]. In ESD and PSD, the incident electron or photon induces electronic transitions which lead to states that have repulsive character. The repulsive potential energy is converted into kinetic energy, resulting in the ejection of particles from the surface. The study of ESD/PSD provides important information regarding the local character of bonding at the surface, and can provide insight into the nature of the electronically excited states of the system. In addition to the fundamental scientific interest in ESD/PSD, there are also strong practical motivations for studying such processes. Desorption/fragmentation processes determine the stability of solid surfaces under irradiation by energetic photon or electron beams, or in more complex environments such as plasmas where there is a substantial flux of photons or electrons to the surface. Also, stimulated desorption enters into a variety of surface analytical techniques such as Auger spectroscopy, photoemission, and electron microscopy. Furthermore, the localized nature of the electronic excitations leading to desorption provides a possibility for the control of surface chemistry with unique chemical specificity. Here we present several examples of ESD from adsorbates on metal and semiconductor surfaces. Our emphasis is on deciphering the desorption mechanisms and in particular the character of the electronic excitation leading to desorption. We find that although in some cases simple single-particle excitations can lead to desorption, in most cases more complex multi-electron excitations are required.
DESORPTION MECHANISMS In the first theoretical model of ESD/PSD proposed by Menzel, Gomer 12] and Redhead 13] (MGR) the deposition process was described as a Franck-Condon transition to a repulsive neutral or ionic state. The model is very general, and the nature of the excitation process or the character of the repulsive state responsible for desorption was not specified. In the ESD/PSD literature however, the MGR mechanism is often identified with single valence electron excitations giving 1h 1e states or single electron ionizations to give lh states. Such simple excitations are in general easily quenched by the substrate. Another general desorption mechanism was proposed by Kn
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