ADSORPTION AND PHOTON-STIMULATED DESORPTION OF CCL4 ON AN AL(111) SURFACE INVESTIGATED WITH SYNCHROTRON-RADIATION

Adsorption and desorption of CCl4 molecules on an Al(111) surface at 9 0 K are characterized with photoemission spectroscopy (PES) and photon -stimulated ion desorption (PSLD) techniques following valence-level a nd core-level excitations. Results of valence-level and Cl(2p) core-le vel PES spectra indicate that CCl4 dissociates partially upon adsorpti on on an Al(111) surface at submonolayer coverage and that molecular C Cl4 adsorbs to form multilayers at large exposures. The dissociation u pon adsorption of CCl4 on an Al surface at 90 K is likely mediated by the charge-transfer process. The Cl+ desorption threshold at similar t o 18.5 eV in valence-level PSID spectra may originate from the 5t(2)-- >7a(1) (C-Cl antibonding orbital) transition consistent with the Menz el-Gomer-Redhead (MGR) mechanism. The total-electron yield (TEY) spect rum and the Cl+ PSID spectrum of solid CCl4 following the Cl L-edge ex citation are clearly dissimilar. The enhanced desorption yield of Clions is detected at the Cl 2p-->7a(1) excitation, compared to the Cl 2p -->8t(2) 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, cor e resonant excitations decay predominantly via spectator Auger transit ions, whereas shape resonance excitation is followed by normal Auger d ecay. Enhanced Cl+ ion desorption from solid CCl4 following Cl 2p-->7a (1) excitation is interpreted in terms of the rapid desorption via a repulsive surface which is directly related to spectator electrons loc alized in antibonding orbitals. (C) 1998 American Institute of Physics . [S0021-9606(98)71242-5]