SURFACE-MEDIATED FORMATION OF VANADIUM HYDRIDES

Surface phenomena which occur in the process of vanadium hydride VHy f ormation were studied through simultaneous measurements of surface pot ential and hydrogen pressure during H-2 interaction ''in situ'' with t hin vanadium films deposited under UHV conditions. The measurements we re performed over large intervals of pressure and temperature from 10( -10) to 10(-2) Torr and 78-298 K, respectively. Two distinct forms of hydrogen adsorbate that occur on the surface in the course of the reac tion were found, namely, (i) a negatively polarized beta(-) form, whic h appears at the beginning of adsorption, and (ii) an induced, positiv ely polarized beta(+) form, which is a precursor state for VHy formati on. The kinetics of the beta(+) adspecies incorporation from the surfa ce into the bulk was examined. Their equilibrium population on the sur face with dependence on the hydrogen concentration in the bulk of VHy was determined.