Direct Observation of4fSplitting between (110) Surface and Bulk Atoms of W

With large surface-sensitivity enhancement achieved by use of p-polarized light, soft-x-ray photoemission spectra of $4f$ core levels from the outermost atoms of the (110) face of tungsten have been observed. High-resolution spectra show two well-separated $4f$ lines split by 0.30 eV, with the low-binding-energy peak being assigned to surface atoms. The ratio of the intensity of the surface peak to the bulk peak as a function of photon energy exhibits striking features which are tentatively explained.