Unoccupied surface states on Ta(100) observed with inverse photoemission

In an attempt to understand the interrelation of surface electronic structure and surface geometry, we have used inverse photoemission to measure the unoccupied surface states on the (100) face of tantalum. Tantalum has one fewer electron than tungsten and the Ta(100) surface is bulk terminated at room temperature, while the W(100) surface is reconstructed. We observe several well-defined features within 2 eV of the Fermi level in the Ta(100) spectra that show strong sensitivity to surface contamination and are therefore associated with surface states and/or resonances. It is difficult to relate the observed features to the W surface states in an empirical way. The two-dimensional dispersion of these states is rather flat, as expected for d-derived states. The results are compared in detail to a recent slab calculation. The spectral intensity of the surface states is significantly weaker than for states on Cu and Ag, a fact that we relate to the higher density of unoccupied states immediately above the Fermi level for the 5d metals as compared to the 3d metals. A peak seen near the Fermi level for the normal-incidence spectrum is related to a strong surface state or resonance below the Fermi level broadened so that it spans the Fermi level. The most likely origin of this broadening is coupling to bulk bands.