Determination of the W(001) and W(110) surface potential barriers

Very low-energy electron diffraction fine structure profiles measured on W(001) and W(110) are presented. These data are analysed using a new surface potential barrier and by taking into account a possible variation of the inner potential inside the atomic top layer. From a thorough comparison of several measured and calculated spectra it is found that a good overall fit of fine structure data can be obtained on both surfaces using a unique one-dimensional barrier model. Optimal values of the model parameters (the top layer inner potential Vs and the centre of mass of the screening charge z0) are however quite different on the two surfaces. On W(001), Vs is nearly 4 eV less attractive than the bulk inner potential. On W(110), this value is only about 0.4 eV. Also, z0 is about 0.5 AA farther from the centre of the topmost layer on W(001) than on W(110). These results are explained by the difference in the atomic density on these two tungsten surfaces. This work shows that, especially for loosely packed surfaces, possible variation of the ion-core potentials near the surface has not to be overlooked in low-energy scattering on surfaces.

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