Valence-level photoemission spectroscopy and photon-stimulated ion desorption studies of CH₃Cl adsorbed on Al(111) surface using synchrotron radiation

The interaction of CH₃Cl adsorbed on Al(111) surface at ∼90 K has been investigated by means of valence-level photoemission spectroscopy (PES), work function change and photon-stimulated ion desorption (PSID) using synchrotron radiation. The one-to-one correspondence between the gas-phase and the condensed-phase CH₃Cl PES spectra suggests that the molecules are molecularly adsorbed without decomposition. Adsorption of CH₃Cl leads to a decrease in work function of Al(111) (Δφ∼-0.6eV at a monolayer), indicating a dipole pointing away from the substrate and Cl is in direct contact with the metal. In the H⁺ PSID spectra, three weak thresholds are observed, at ∼9.6, ∼13.9 and ∼16.5 eV, and a major threshold appears at ∼18.9 eV. CH₃⁺ PSID spectra show a weak threshold at ∼13.2 eV and two major thresholds near 9.0 and 16.7 eV. The desorption threshold below 18 eV may be due primarily to the single-hole excitation or Rydberg transition, whereas the ion desorption above 18 eV may result predominantly from excitations of C 2s electron correlation states.