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

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 2 O , NH 3 , CH 3 CN , and CF 3 CH 3 . Auger electron photon coincidence (AEPICO) spectra of condensed H 2 O at the O :1s ionization showed that H + 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 + 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 3 . The H + desorption at the 4a 1 ← O(N ):1s resonance of both condensed H 2 O and condensed NH 3 was found to be greatly enhanced. Based on the AEPICO spectra the following four-step mechanism was proposed: (1) the 4a 1 ←1s transition, (2) extension of the HO–H ( H 2 N–H ) distance within the lifetime of the (1s) −1 (4a 1 ) 1 state, (3) spectator Auger transitions leading to ( valence ) −2 (4a 1 ) 1 states, and (4) H + desorption. The enhancement of the H + desorption yield was attributed to the repulsive potential surface of the (1s) −1 (4a 1 ) 1 state. At the 3p← O :1s resonance of condensed H 2 O , on the other hand, the H + yield was found to be decreased. The AEPICO spectra showed that the H + desorption was stimulated by spectator Auger transitions leading to ( valence ) −2 (3p) 1 states. The decrease in the H + 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 2 O showed that the core level of the surface H 2 O responsible for the H + desorption was shifted by 0.7 eV from that of the bulk H 2 O . The H + desorption from condensed CH 3 CN was also investigated. In a study of condensed CF 3 CH 3 using PEPICO spectroscopy, site-specific ion desorption was directly verified; that is, H + and CH 3 + desorption was predominant for the C :1s photoionization at the - CH 3 site, while C 2 H n + , CFCH m + , and CF 3 + desorption was predominantly induced by the C :1s photoionization at the - CF 3 site. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a pow