Auger electron photoion coincidence technique combined with synchrotron radiation for the study of the ion desorption mechanism in the region of resonant transitions of condensed H2O

The Auger electron photoion coincidence (AEPICO) technique has been applied for the study of H+ desorption induced by resonant excitations of O 1s of condensed H2O. The peak positions of the AEPICO yield spectrum at the 4 alpha(1) <-- O 1s resonance (hv = 533.4 eV) are found to correspond to spectator-Auger transitions leaving (O 2s)(-2)(4 alpha(1))(1), (O 2s)(-1)(O 2p)(-1)(4 alpha(1))(1), and (O 2p)(-2)(4 alpha(1))(1) states. The H+ AEPICO yield is greatly enhanced at 4 alpha(1) <-- O 1s while it is suppressed at 3p <-- O 1s (hv = 537 eV) as compared with that at the O 1s ionization (hv = 560 eV). On the basis of these results, the ultrafast ion desorption mechanism is suggested to be favorable for the H+ desorption at 4 alpha(1) <-- O 1s, that is, the repulsive potential energy surface of the (O 1s)(-1)(4 alpha(1))(1) state is responsible for the H+ desorption. For H+ desorption at 3p <-- O 1s, a spectator-Auger stimulated ion desorption mechanism is concluded to be probable. The suppression of the H+ AEPICO yield is ascribed to the reduction of the hole-hole repulsion due to the shield effect of the 3p electron. These results demonstrate the pou er of the AEPICO technique to clarify the mechanism of ion desorption induced by core-electron excitations. (C) 1998 American Institute of Physics.