Ion desorption from molecules condensed at low temperature: a study with electron-ion condense spectroscopy combined with synchrotron radiation (Review)

摘要

This article reviews our recent work on photostimulated ion desorption (PSID) from molecules condensed at low temperature. We have used electron-ion coincidence (EICO) spectroscopy combined with synchrotron radiation. The history and present status of the EICO apparatus is described, as well as our recent investigations of condensed H{sub}2O, NH{sub}3, CH{sub}3CN, and CF{sub}3CH{sub}3. Auger electron photon coincidence (AEPICO) spectra of condensed H{sub}2O at the O:1s ionization showed that H{sup}+ desorption was stimulated by O:KVV Auger processes leading to two-hole states (normal-Auger stimulated ion desorption (ASID) mechanism). The driving forces for H{sup}+ desorption were attributed to the electron missing in the O-H bonding orbitals and the effective hole-hole Coulomb repulsion. The normal ASID mechanism was also demonstrated for condensed NH{sub}3. The H{sup}+ desorption at the 4a{sub}1← O(N):1s resonance of both condensed H{sub}2O and condensed NH{sub}3 was found to be greatly enhanced. Based on the AEPICO spectra the following four-step mechanism was proposed: (1) the 4a{sub}1←1s transition, (2) extension of the HO-H (H{sub}2N-H) distance within the lifetime of the (1s){sup}(-1)(4a{sub}1){sub}1 state, (3) spectator Auger transitions leading to (valence){sup}(-2)(4a{sub}1){sup}1 states, and (4)H{sup}+ desorption. The enhancement of the H{sup}+ desorption yield was attributed to the repulsive potential surface of the (1s){sup}(-1)(4a{sub}1){sup}1 state. At the 3p←O: 1s resonance of condensed H{sub}20, on the other hand, the H{sup}+ yield was found to be decreased. The AEPICO spectra showed that the H{sup}+ desorption was stimulated by spectator Auger transitions leading to (valence){sup}(-2)(3p){sup}1 states. The decrease in the H{sup}+ yield was attributed to a reduction in the effective hole-hole Coulomb repulsion due to shielding by the 3p electron. Photoelectron photon coincidence (PEPICO) spectra of condensed H{sub}2O showed that the core level of the surface H{sub}2O responsible for the H{sup}+ desorption was shifted by 0.7 eV from that of the bulk H{sub}2O. The H{sup}+ desorption from condensed CH{sub}3CN was also investigated. In a study of condensed CF{sub}3CH{sub}3 using PEPICO spectroscopy, site-specific ion desorption was directly verified; that is, H{sup}+ and (CH{sub}3){sup}+ desorption was predominant for the C: 1s photoionization at the -CH{sub}3 site, while (C{sub}2H{sub}n){sup}+, (CFCH{sub}m){sup}+, and (CF{sub}3){sup}+ desorption was predominantly induced by the C: is photoionization at the -CF{sub}3 site. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a powerful tool for studying PSID of molecules condensed at low temperature.