Unoccupied electronic structure of Ru(0001)

The unoccupied electronic states of the clean Ru(0001) surface were measured along the Γ0.2em0exK̄ and Γ0.2em0exM̄ high symmetry axes of the surface Brillouin zone (SBZ) using angle-resolved inverse photoemission spectroscopy. The spectra are dominated by strong emission within the first 20.3em0exeV above the Fermi level and, but for a weak image potential state, are essentially featureless at higher energies (up to ~60.3em0exeV). The spectra obtained along the Γ0.2em0exK̄ direction exhibit three well-defined features that are identified as originating from bulk direct transitions. The dispersions with parallel momentum for two of the three features run along the edges of projected band gaps found in ab initio electronic structure calculations. Detailed examination of these spectra reveals the presence of a surface state that appears near k_=0.70.3em0ex and persists to the boundary of the SBZ. The spectra along the Γ0.2em0exM̄ direction exhibit less well-defined features, but comparing spectra from the clean surface to those obtained from a surface exposed to 0.50.3em0exL (10.3em0exL=10^-60.3em0exTorr0.2em0exs) of CO enables identification of four additional peaks arising from bulk direct transitions. One feature may be associated with a surface resonance that is predicted to exist near the M̄ point. Comparison of the observed dispersions with those predicted for states along the high symmetry directions of the bulk gives a good account of the experimental data.