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

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 H2O (hv = 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 (hv = 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 (hv = 537 eV), the spectator Auger stimulated ion desorption mechanism is concluded to be probable. In the case of H2O 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. (C) 1999 Elsevier Science B.V. All rights reserved.