HYDROGEN ADSORPTION ON MO(211) - RECONSTRUCTION AND BINDING-SITE CONVERSION

The H/Mo(211) chemisorption system has been investigated over the temp erature range 100-400 K using LEED, HREELS, and work function measurem ents. Above 1 ML, a transition from the (1 x 1) structure of the clean surface to a hydrogen induced (1 x 2) phase is observed and attribute d to changes in the substrate periodicity. Vibrational spectroscopy su ggests that below 1 ML hydrogen is bridge bonded and induces local dis tortions of the substrate. The transition to the (1 x 2) phase involve s the ordering of these displacements and the occupation of three-fold sites partially populated by conversion of the bridge bonded species. This conversion is correlated with a sawtooth-like coverage dependenc e of the work function. In contrast to recent reports of delocalizatio n on W(211), hydrogen is found to occupy well-defined binding sites at all coverages. Results for the present system are also compared with those for hydrogen adsorption on Mo(100), other bcc (211) substrates a s well as the structurally similar fcc (110) surfaces.