Adsorption and photon-stimulated desorption of CCl4 on an Al(111) surface investigated with synchrotron radiation

Abstract

Adsorption and desorption of CCl4 molecules on an Al(111) surface at 90 K are characterized with photoemission spectroscopy (PES) and photon-stimulated ion desorption (PSID) techniques following valence-level and core-level excitations. Results of valence-level and Cl(2p) core-level PES spectra indicate that CCl4 dissociates partially upon adsorption on an Al(111) surface at submonolayer coverage and that molecular CCl4 adsorbs to form multilayers at large exposures. The dissociation upon adsorption of CCl4 on an Al surface at 90 K is likely mediated by the charge-transfer process. The Cl+ desorption threshold at ∼18.5 eV in valence-level PSID spectra may originate from the 5t2→7a1* (C–Cl antibonding orbital) transition consistent with the Menzel–Gomer–Redhead (MGR) mechanism. The total-electron yield (TEY) spectrum and the Cl+ PSID spectrum of solid CCl4 following the Cl L-edge excitation are clearly dissimilar. The enhanced desorption yield of Cl+ ions is detected at the Cl 2p→7a1* excitation, compared to the Cl 2p→8t2* and Cl 2p→Rydberg state excitations. Cl(2p) core-level excitations yield much greater desorption of ions compared with direct valence-band excitation. Based on resonant photoemission spectra, core resonant excitations decay predominantly via spectator Auger transitions, whereas shape resonance excitation is followed by normal Auger decay. Enhanced Cl+ ion desorption from solid CCl4 following Cl 2p→7a1* excitation is interpreted in terms of the rapid desorption via a repulsive surface which is directly related to spectator electrons localized in antibonding orbitals.