Temperature-dependent broadening of the surface core-level spectra of W(110)

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Abstract
The authors present the first measurement of phonon broadening on a tungsten surface using surface-sensitive high-resolution photo-electron spectroscopy. They find that the core-level spectra, obtained at four different photon energies, are well fitted by two peaks: from the surface and from the bulk atoms. The surface peak is shifted to lower binding energy by 0.29+or-0.1 eV. They have determined the lineshape parameters of these peaks at 290 and 100 K by a nonlinear least-squares fitting procedure in which the core levels are represented by peaks having a Doniach-Sunjic lineshape (FWHM, 50 meV; alpha =0.06) convoluted with a Gaussian, representing the instrument response function and the photon broadening. They find the Gaussian width (FWHM) to be 0.175+or-0.014 eV at 100 K and 0.185+or-0.011 eV at 290 K. These values for the Gaussian broadening are in excess of the instrumental broadening of 0.162+or-0.005 eV and correspond to convolution with an additional Gaussian of width 0.07+or-0.03 eV at 100 K and 0.09+or-0.02 eV at 290 K. Because these values change with temperature, they ascribe them to phonon broadening of the photo-emission line. These latter values are in good agreement with the calculation of the expected phonon broadening of bulk tungsten by Sebilleau and co-workers (1987). They also describe the relevance of this measurement to the decomposition of surface core-level spectra from the room temperature results for W(100) and W(111) surfaces, and to the analysis of surface core-level spectra from stepped tungsten surfaces.