Angular-resolved secondary-electron-emission spectroscopy of clean and adsorbate covered tungsten single-crystal surfaces

Energy-distribution measurements are reported for secondary electrons back-scattered into a narrow angle about the normal direction to three low-index tungsten single-crystal surfaces, viz. (100), (110) and (111). Improved spectral resolution provides unambiguous evidence for scattering out of excited “final” states located above the vacuum level; the results for all three faces correlate closely with high-energy states of a calculated energy band structure, the intensity of emission being directly related to features in the one-dimensional density of unfilled states along the corresponding low-index symmetry directions. In the presence of ordered adsorbate monolayers, additional SEE spectral fine-structure is observed at energies which lie within finalstate band gaps of the crystal. Results are presented for the specific case of CO adsorption on W(110), which shows a distinct disorder-order structural transition after exposure of the clean surface to 10 L of gas at 300 K and subsequent annealing to temperatures ?1000 K. We interpret these adsorbate surface resonances to be due to two-dimensional Bloch-like surface states produced by the periodicity of the adsorbate layer, which manifest themselves as a direct consequence of the special circumstances associated with “band-gap emission”.