Ion desorption induced by core-electron transitions studied with electron-ion coincidence spectroscopy

The recent investigations of ion desorption induced by core-electron transi tions using electron-ion coincidence (EICO) spectroscopy are described. In a study of CF3CH(OH)CH3 chemisorbed on a Si(100) surface using photoelectro n photoion coincidence (PEPICO) spectroscopy, excitation site-specific ion desorption is directly verified, that is, F+ desorption is predominant for C 1s photoionization at the -CF3 site, while H+ desorption is predominantly induced by C 1s photoionization at the -CH3 site. A study of condensed H2O using Auger electron photoion coincidence (AEPICO) spectroscopy showed tha t H+ desorption is stimulated by O KW Auger processes leaving two-hole slat es. The H+ desorption probability is found to depend on the bonding charact er of the orbitals where holes are created and on the effective hole-hole C oulomb repulsion. AEPICO investigations of H+ desorption induced by resonan t core-electron excitations of condensed H2O clearly showed that one-electr on-one-core hole or one-electron-two-valence hole states are responsible fo r the H+ desorption mechanism. These investigations demonstrate that EICO s pectroscopy combined with synchrotron radiation is a novel and powerful too l for the study of ion desorption induced by core-electron excitations. Fur thermore, a comparison of PEPICO and photoelectron spectra showed that the surface core-level shift of condensed H2O is 0.7 eV. This study shows that PEPICO spectroscopy is also promising as a method to investigate the electr onic structure of the specific sites responsible for ion desorption. (C) 20 00 Elsevier Science B.V. All rights reserved.