ADSORPTION OF HYDROGEN ON STEPPED NI AND PD SURFACES - OBSERVATION OFCHEMISORBED HYDROGEN MOLECULES

We report the observation of chemisorbed hydrogen molecules at the ste p sites of Ni(510) and Pd(510) surfaces which are saturated with atomi c hydrogen. No such states have been observed on the corresponding fla t (100)-surfaces. For Ni four dipole active vibrational modes related to the H-2 molecule are observed. The mode assignments and the intensi ties of the loss peaks indicate that the molecule is side-on bonded at the step edge. The internal stretch mode is shifted down to 398 meV ( gas-phase value: 516 meV). In the case of Pd only the internal stretch mode is observed at 414 meV. Adsorption deeper into the step may expl ain the difficulty of observing any other H-2 modes. We suggest that t hese H-2 chemisorption states can be stably formed at the step sites d ue to residual unfilled d-states and the simultaneous blocking of the dissociative channel by the saturated atomic adsorbate layer.