Mechanism of ion desorption induced by core-electron transitions of condensed molecules and adsorbates studied by electron ion coincidence spectroscopy

Abstract

Abstract Mechanism of ion desorption induced by core-electron transitions of condensed molecules and adsorbates on surfaces is studied by Auger electron photoion coincidence (AEPICO) spectroscopy combined with synchrotron radiation. In the case of O:1s ionization of condensed H 2 O ( hν =564 eV), H + desorption is attributed to the normal Auger stimulated ion desorption (ASID) mechanism, that is normal Auger final states are concluded to be responsible for H + desorption. One of the driving forces for the H + desorption is concluded to be the electron missing in the orbitals with O–H bonding character. At the 4a 1 ←O:1s resonance ( hν =533.4 eV), on the other hand, the ultrafast ion desorption mechanism is suggested to be favorable, that is, the repulsive potential energy surface of the (O:1s) −1 (4a 1 ) 1 state is responsible for the H + desorption. For H + desorption at 3p←O:1s ( hν =537 eV), the spectator Auger stimulated ion desorption mechanism is concluded to be probable. In the case of H 2 O chemisorbed on a Si(100) surface, the lifetime of the excited electron is found to be short in comparison with the time scale of H + desorption. The dominant H + desorption channels are attributed to multi-(more than two) hole states created by minor Auger processes, such as shake-up/off-like or cascade Auger decays. These investigations demonstrate the power of AEPICO spectroscopy to clarify the mechanism of ion desorption induced by core-electron excitations.