ION DESORPTION FROM H2O CHEMISORBED ON SI(100) BY O-1S ELECTRON-EXCITATION AT ROOM-TEMPERATURE

The photon stimulated ion desorption (PSID) of H+ and O + from monolayer H2O adsorbed on the Si(100) has been studied using pulsed synchrotron radiation in the 510-620 eV range. Ions were detected and mass analyzed by means of time-of-flight spectroscopy. Relative yield spectra of these ions indicated characteristic behavior near the O K edge. The predominant H+ fragment ion shows a sharp rise at about 530 eV and two broad peaks (535 eV, 558 eV), while O+ shows a delayed threshold at about 570 eV and gradual increase up to 700 eV. Above the O K edge, these ion yields were found to deviate significantly from the O(KVV) Auger electron yield. This was interpreted by the fact that ion desorption was enhanced by multiple ionization and is strongly affected by the decay process with the surface. In order to elucidate the mechanism of the PSID, the technique of photoion-photoion coincidence between H+ and O+ ions was also applied. The results suggested the formation of multiply charged intermediates, i.e., OH4+ ions, which were incompletely reneutralized despite of strong interaction with substrate and desorbed as singly charged H+ and O+ ions. © 1995 American Institute of Physics.