Adsorption and photon-stimulated desorption of CCl ₄ on an Al(111) surface investigated with synchrotron radiation

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 Cl2p→7a₁ ^* excitation, compared to the Cl2p→8t₂ ^* and Cl2p→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 Cl2p→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.