Surface-Atom Core Level Shift in Gold

Abstract

The line shape of the 4f7/2 level of gold obtained by high-resolution X-ray photoemission spectroscopy has been measured and analysed. The low binding energy side of this line shows a quite exceptional shape at variance with the Doniach-Sunjic (SD) analytical expression accounting for most core level line shape in metals. The surface sensitivity was enhanced by varying the take-off angle θ of the electrons. A shoulder then appears, revealing the presence of a line shifted by 0.4 eV and corresponding to the 4f excitation in surface atoms. The decrease of the binding energy can be interpreted as a surface-atom core level shift (SCS). The bulk and surface atom components can be accurately described by two identical DS functions; their energy position is remarkably constant, while their intensity ratio varies exceptedly with θ. A narrowing and a shift to lower binding energy is also found for the surface contribution to the valence band photoemission, indicating that the origin of the SCS is an increased localization of the valence band states.