Search

The surface chemical properties of Y, Ti, Zr and Hf relevant to exoelectron and ion emission have been characterised by a range of techniques (XPS, UPS, AES, LEED, TDS, area-average and threshold φ). The chemisorptive properties of chlorine, bromine, sodium and rubidium are described, and (alkali + halogen) coadsorption has been studied also. Evolution of the trihalide corrosion layer is examined as are its decomposition properties; these features are rationalised in thermochemical and kinetic terms. In certain cases (Y/Na, Y/Rb, Hf/Rb) alkali adsorption results in a system work function equal to that of the bulk alkali at one-half monolayer coverage. Coadsorption of alkali and halogen leads to some particularly interesting observations. Substantial discrepancies (~ 1 eV) are found between the area-average work function \gf and the photoelectric threshold work function (φ∗); over certain coverage regimes, φ∗ actually decreases as \gf increases. This implies the formation of very low work function (< 2 eV) “exopatches”. Desorption from such systems does not occur via the alkali halide — in marked contrast with the behaviour of less electropositive substrates; this can be accounted for. The presence of halogens substantially increases the adsorption strength of the alkali. Models for the various surface phases are advanced and discussed.