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

The recent investigations of ion desorption induced by core-electron transitions using electron–ion coincidence (EICO) spectroscopy are described. In a study of CF3CH(OH)CH3 chemisorbed on a Si(100) surface using photoelectron 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 that H+ desorption is stimulated by O KVV Auger processes leaving two-hole states. The H+ desorption probability is found to depend on the bonding character of the orbitals where holes are created and on the effective hole–hole Coulomb repulsion. AEPICO investigations of H+ desorption induced by resonant core-electron excitations of condensed H2O clearly showed that one-electron–one-core hole or one-electron–two-valence hole states are responsible for the H+ desorption mechanism. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a novel and powerful tool for the study of ion desorption induced by core-electron excitations. Furthermore, 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 electronic structure of the specific sites responsible for ion desorption.