Surface resonances on Ta(001)

We present a joint experimental and theoretical study of the electronic structure and surface resonances of the Ta(001) surface. Angle-resolved photoemission experiments were performed with use of synchrotron radiation in the photon-energy range 10 to 100 eV. Three surface resonances have been identified in the angle-resolved photoemission spectra. Their symmetry, dispersion, and orbital character are determined. The experimental data are compared to self-consistent local-density-functional calculations for 9- and 21-layer Ta(001) slabs that are either bulk terminated or have a 14% contraction of the surface interlayer spacing. Since the degenerate d states of bulk Ta (e.g., the ${\mathrm{\Gamma }}_{25\prime }$ and ${\mathrm{\Gamma }}_{12}$ irreducible representations) are above the Fermi level, the spin-orbit interaction—which can lift orbital degeneracies—has little effect on the occupied bands and is not required in order to explain the behavior of the resonances. The calculations for the 14% contracted surface interlayer separation is in better agreement with the experimental result than are the bulk-terminated bands. The role of these surface resonances on the stability of the (001) face of Ta will be discussed.