Photoemission study of WC(0001)

Abstract

We report results of photoemission experiments and a new bulk band-structure calculation for monotungsten carbide (WC). Three types of photoemission experiment were performed on a WC(0001) surface, with the use of synchrotron radiation. Normal-emission angle-resolved spectra in the range $10\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}30$ eV cannot be interpreted assuming dominance of direct transitions, rather a one-dimensional density-of-states (1D DOS) sampling of initial states is required. By exploiting the natural polarization of the synchrotron-radiation source, initial-state symmetries of three features are unambiguously assigned, ${\ensuremath{\Lambda}}_{1}$ at -9.4 and -1.0 eV and ${\ensuremath{\Lambda}}_{3}$ at -3.0 eV, and these same features are associated with appropriate-symmetry high-DOS regions along $\ensuremath{\Delta}(\ensuremath{\Gamma}\ensuremath{-}A)$ in the bulk band structure. Two features displaying no clear-cut polarization dependence cannot be associated with high-DOS regions along $\ensuremath{\Delta}$ and are attributed to emission scattered from elsewhere in the Brillouin zone. The strong amplitude dependence with emission angle of a sharp peak near ${E}_{F}$ is attributed to symmetry selection rules related to a mirror-plane collection geometry excited by polarized radiation. Data from a second experiment using a cylindrical mirror analyzer (CMA), $10\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}30$ eV, are also interpreted with the use of the bulk band structure, but with more limited success. Here, however, clear evidence of direct transitions is seen. A total DOS is probed in the third experiment, which again uses a CMA, with $65\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}200$ eV. The main valence band extends 8 eV below ${E}_{F}$ while C $2s$ electrons form a separate band, 3-eV FWHM centered at -12.5 eV. Utilizing the W $5d$ Cooper minimum at $160 \mathrm{eV}=h\ensuremath{\nu}$, we assign the upper half of the main valence band to predominately W $5d$---derived states, while in the lower half, C $2p$---derived states dominate.