Photoelectron diffraction determination of the geometry of a clean metal surface: Ta(100)

The ratio of photoemission intensities from the bulk- and surface-shifted 4{ital f} core levels of the clean Ta(100) surface was measured in two different modes: as a function of photon energy at normal emission and as a function of polar angle near the (011) azimuth at {h bar}{omega}=65 eV. We have also 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{plus minus}5)% contraction of the first interlayer spacing.