Electronic transitions of Ar, Xe, N 2, CO physisorbed on Ag(111) and Al(111)

Abstract

High Resolution Electron Energy Loss Spectroscopy has been extended to study also the excitonic (low lying electronic) transitions of physisorbed rare gas atoms (Ar, Xe) and diatomic molecules (N 2, CO) on Ag(111) and Al(111) surfaces at ∼20K. Electron Loss Spectra were performed using a pair of hemispherical analyzers mounted at a fixed scattering angle (90°). This spectrometer allowed high transmission in the range of 0–15eV loss energies and incident beam energies up to 2OeV. AES, LEED and UV Photoemission (HeI) were also used in situ to characterize these surfaces and to identify the adsorbed gases and delineate their absolute coverage regimes. In contrast to optical absorption experiments, we observe both, optical (dipole) forbidden and allowed electronic transitions which show vibrational line structure for condensed multilayers. By comparison to gas phase data we find only weak perturbations in the condensed state. The observed electronic excitations show changes in intensity and FWHM depending on the coverage of the adsorbed gases. The FWHM of the electronic excitations of CO and N 2 adsorbed in the monolayer regime is larger than in multilayers. Nitrogen, on both surfaces exhibits an increase from 60meV to 120meV (FWHM) whereas for CO the vibronic features are broadened out leaving peaks with FWHM of ∼1eV. The intensities of the electronic losses for all gases are smaller in the first monolayer than in the second or in multilayers. At submonolayer coverage the loss intensifies due to electronic excitations are strongly reduced and no longer observable although vibrational bands and photoelectron spectra show the presence of physisorbed adsorbates. Our results will be compared to optical absorption experiments (ref.1) on similar systems and to atom-on-jellium calculations (ref.2).