Excitation Spectra of Optically Excited Rare-Gas Atoms Physisorbed on Metal Surfaces

Abstract

In the last decade, Flynn and coworkers (1–4) have extensively examined the optical excitation spectra of rare-gas atoms physisorbed on various metal surfaces as a function of coverage. The data were obtained by the use of differential reflectance methods with synchrotron radiation. The primary spectroscopic effort has focused on the way the substrate modifies levels of the free adsorbate, and on any newly formed excited configurations, which arise through the interaction of the adsorbate atom with the metal surface. The striking changes in the optical excitation spectra have been interpreted as due to the formation of a neutral excited adsorbate configuration on some surfaces and an ionized one on others (1–4). At high coverage (>1 monolayer), the spectra are clearly visible and resemble comparable excitations observed in absorption for both vapor phase and condensed rare gases. However, in the low coverage limit, the peak in the spectrum remains strong for some adatom-substrate combinations, while it completely vanishes for others. For instance, at low coverage the adsorption peaks of Xe on Al, Ti and Au, and Kr on Au are virtually eliminated while those of Xe and Ar on Mg, Kr and Ar on Al and Ar on Au persist (1–4). Arguments against the foregoing interpretation of the optical data have been recently made (5).