Study of ion desorption induced by a resonant core-level excitation of condensed H2O using Auger electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation

Ion desorption induced by a resonant excitation of O Is of condensed amorphous H2O has been studied by total ion and total electron yield spectroscopy, nonderivative Auger electron spectroscopy (AES) and Auger electron photo-ion coincidence (AEPICO) spectroscopy. The spectrum of total ion yield divided by total electron yield exhibits a characteristic threshold peak at hv=533.4 eV, which is assigned to the 4a(1)<--O Is resonant transition. The AES at the 4a(1)<--O Is resonance is interpreted as being composed of the spectator-AES of the surface H2O, and the normal-AES of the bulk H2O, where the 4a(1) electron is delocalized before Auger transitions. H+ is found to be the only ion species in AEPICO spectra measured at the 4a(1)<--O Is resonance and at the O Is ionization (hv= 560 eV). The electron kinetic energy dependence of the AEPICO yield (AEPICO yield spectrum) at the 4a,t0 Is resonance is found to be greatly different from that at the O Is ionization. The peak positions of the AEPICO yield spectrum at the 4a(1)<--O Is resonance are found to correspond to those of the spectator-AES of the surface H2O, which is extracted from the AES at the 4a(1)<--O Is resonance. Furthermore, the AEPICO yield is greatly enhanced at the 4a(1)<--O Is resonance as compared with that at the O Is ionization. On the basis of these results, a spectator Anger-stimulated ion desorption mechanism and/or ultra-fast ion desorption mechanism are concluded to be responsible for the H+ desorption at the 4a(1)<--O Is resonance. The enhancement of the H+ yield is ascribed to the O-H anti-bonding character of the 4a(1) orbital. (C) 1997 Elsevier Science B.V.