The optical excitation spectra of impurity atoms in metals and on metal surfaces

Abstract

Electronic excitations of adsorbates on metal surfaces, like those of impurities in the bulk metal, couple the local electronic configurations of the excitation center to the plasmon, phonon and electron-hole pair creation processes of the host metal. In this paper we discuss the basic excitation spectra of metal-adsorbate and metal-impurity complexes. The experimental parallel between adsorbate and impurity spectra is traced for certain especially simple systems, notably halogens, rare gases, alkali impurities and divalent metal atoms, interacting with alkali metal substrates. These experiments focus particular attention on the problem of electron-hole pair creation in the excitation process (the “X-ray edge” problem), which does not appear to he even qualitatively comprehended at present, and on the problem of charge-transfer excitations at adsorbates on metals, for which a semiquantitativc understanding appears to be at hand.