Identification of underlayer components in the surface core-level spectra of W(111)

Core-level photoemission spectra from a W(111) surface using synchrotron radiation are presented. These core-level spectra are shown to be composed of four components with the same peak shape: one from bulk atoms and three from different crystallographic sites in the selvedge. The selvedge peaks comprise a peak from the topmost surface atoms (shifted by -0.45 +- 0.01 eV) and two different underlayer sites (shifted by -0.30 +- 0.02 eV and -0.11 +- 0.02 eV from the bulk peak). This contrasts with a previous model, proposed by Wertheim, Citrin and Van der Veen, which used only two surface components: a broadened surface atom peak and one underlayer peak. The data presented can be fitted with three peaks, yielding good agreement with the peak energies obtained by these authors; however, we show that the four-peak model gives a better fit to the experimental spectra. The use of two underlayer peaks is in good agreement both with intuitive arguments based on the coordination number of atoms in the selvedge and with two previous theoretical calculations of the surface core-level shift. The values determined from the fitting parameters for the core-level line shape are also in good agreement with values calculated by other workers.