Adsorption of molecular hydrogen on an ultrathin layer of Ni(1 1 1) hydride

We have used high resolution electron energy loss spectroscopy and temperature-programmed desorption to study the interaction of atomic hydrogen with Ni(1 1 1). Our results agree mostly with previous reports. We find that exposing Ni(1 1 1) to atomic hydrogen below 90 K leads to a 125 K TPD feature and two additional HREELS losses. Isotopic exchange studies lead us to attribute these features to molecular hydrogen bound to an ultrathin nickel hydride layer formed on the surface. We suggest that such binding is induced by reversible surface roughening that accompanies the phase change between the metallic nickel and the ultrathin nickel hydride layer.