Photofield emission from clean and titanium covered (111) plane of tungsten

Optical transitions are observed as connected with the occurrence of shoulders in the photofield emission characteristics. For the photon energy hv = 1.96 eV, two optical transitions in the [111] direction of clean tungsten and three maxima of the surface density of states on the (111) plane are observed. One transition, in the bulk energy band, is connected with the Fermi level and the second one with a minimum in the empty band. They are in good agreement with the calculated electronic structure. Thin titanium films (0.5-1.5 monolayer) cause 'smoothing up' of the photofield emission characteristics and shoulders (due to optical transitions) are scarcely noticeable, if at all. Noticeable changes in the photofield characteristics are observed after annealing the emitter at a temperature of about 1600 K. At a temperature of approximate to 2100 K titanium is removed almost completely Similar results have been obtained for the quantum energy hv = 2.4 eV. Electron states connected with the titanium layer adsorbed on tungsten (111) face have not been observed. (C) 1997 Elsevier Science Ltd.