First interlayer spacing of Ta(100) determined by photoelectron diffraction

A photoelectron diffraction study of Ta(100), employing the surface shifted 4 f core levels, has been performed to determine its first interplanar spacing. Photoemission intensities of the bulk and surface shifted 4 f core levels were measured as a function of photon energy at normal emission and near the <111> symmetry direction of the crystal. The binding energy of 21.7 eV (23.6 eV) for the bulk 4 f 7/2 (5/2) core level, and the large surface core level shift of 0.75 eV, result in well-resolved peaks in the photoemission spectra with areas that are easily measured independently. We find that the ratio of the surface to bulk core level intensities at normal emission exhibits large oscillations as a function of photon energy due to diffraction effects. Intensity modulation is also seen in the off-normal intensities, but the magnitude is much smaller. We have performed a multiple-scattering photoemission calculation for several first interlayer spacings ranging from the bulk value to a 15% contraction. The best agreement with experiment was obtained for a 10% +- 3% contraction of the first interlayer spacing, which is consistent with an earlier low-energy electron diffraction study