Adsorption of Molecular Hydrogen on Cu, Pd and Ni Surfaces

Doktorsavhandling, 1997

This thesis is devoted to spectroscopic studies of molecular hydrogen adsorbed on metal surfaces. Hydrogen is the simplest molecule available and is a very valuable model adsorbate. From a spectroscopic point of view there are also three easily accessible molecular isotopes, H2, HD and D2, with widely different masses. Hydrogen adsorption proceeds along radically different routes on surfaces of noble metals, like copper, and transition metals, like nickel and palladium. A room-temperature gas will only interact weakly with a copper surface and physisorb at a low surface temperature. The attractive interaction is due to van der Waals forces, which bind the molecule to the surface. These forces also polarize the molecule and the vibrational motion in the physisorption potential well becomes dipole active as is demonstrated by electron energy-loss measurements for the H2-Cu(100) system. A good quantitative estimate of the strongly nonlinear dipole moment function can be obtained from a van der Waals model. Photons can also induce transitions among the well states and direct infrared photodesorption becomes feasible. On the stepped Cu(510) surface, the hydrogen molecules are more strongly bound but also exhibit larger dipole activity resulting in a substantial photodesorption rate. This desorption, which is induced by background blackbody radiation, occurs via a direct transition from the vibrational groundstate to continuum translational states. On Cu(100), physisorbed H2 displays rotational excitations weakly perturbed by the orientational dependence of the molecule-surface interaction. The crossection for electron impact excitation of the rotation is close to the gas phase value. In general hydrogen molecules dissociate on nickel and palladium surfaces and end up as chemisorbed atoms. Once the surface is saturated with atomic hydrogen the molecule-surface interaction is in most cases weak. The stepped Ni(510) and Pd(510) surfaces turn out to be more active, however, and hydrogen molecules eventually chemisorb at the step sites. The molecules have a substantially weakened internal bond and these adsorption states may act as precursors for additional dissociative adsorption, in particular for palladium which is known to dissolve hydrogen into the bulk.