Synchrotron radiation photoemission studies of core level excitation effects

Abstract

The recent availability of intense polarized radiation tuneable over the range 20-150 eV enables several core levels to be excited. Several remarkable effects have recently been reported when core excitation thresholds have been reached. For example, an increase in yield of 50% was observed in GaSe at the Se 3d threshold ( h {combining short stroke overlay}ω = 57 eV). A considerable increase in photoemission intensity at 6 eV binding energy has been reported by Guillot et al. for Ni at the 3p threshold. We have made a very careful study using the CIS, CFS and EDC models of photoemission spectroscopy of core threshold effects in Ni, Pr and GaSe. We find a modulation of the Ni Fermi edge emission as a function of photon energy which we attribute to a configuration interaction between 3p5 3d10 4s and 3p6 3d8 4s ef. Similar effects occur in Pt at the 4f threshold involving the configurations 4f13 5d10 6s and 4f14 5d8 6s. Further, the lineshapes of the Pt and Ni upper valence band CIS's resemble those obtained by electron loss (ELS) and photoabsorption, all of which can be fitted theoretically to a Fano model. Emission from levels lying well below the Fermi level often has superimposed contributions due to incoherent effects which complicate the interpretation. In cases in which core levels are excited well above threshold, the CIS technique may be used to examine final states. Of particular interest is the possible consequent determination of adsorbate site geometry. We present data 2p emission from a c(2 × 2) Na overlayer on Ni(001). © 1979.