Surface-mediated formation of vanadium hydrides

Surface phenomena which occur in the process of vanadium hydride VHy formation were studied through simultaneous measurements of surface potential and hydrogen pressure during H-2 interaction "in situ" with thin vanadium films deposited under UHV conditions. The measurements were 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 reaction were found, namely, (i) a negatively polarized beta(-) form, which appears at the beginning of adsorption, and (ii) an induced, positively polarized beta(+) form, which is a precursor state for VHy formation. The kinetics of the beta(+) adspecies incorporation from the surface into the bulk was examined. Their equilibrium population on the surface with dependence on the hydrogen concentration in the bulk of VHy was determined.