AVS 46th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS3-TuA

Paper SS3-TuA7
Study of Subsurface Hydrogen in Pd(111) by STM

Tuesday, October 26, 1999, 4:00 pm, Room 607

Session: Diffusion on Surfaces
Presenter: M.K. Rose, Lawrence Berkeley National Laboratory
Authors: M.K. Rose, Lawrence Berkeley National Laboratory
A. Borg, Norwegian University of Science and Technology
D.F. Ogletree, Lawrence Berkeley National Laboratory
M. Salmeron, Lawrence Berkeley National Laboratory
Correspondent: Click to Email
Using Variable Temperature Scanning Tunneling Microscopy we have studied hydrogen atoms embedded in the topmost layers of Pd(111) and the influence of these impurities on surface adsorbates. Hydrogen is imaged beneath surface three-fold hollow sites at 20K. Subsurface hydrogen atoms exhibit attractive interactions leading to aggregation into clusters. We observe diffusion of two distinct hydrogen species within the subsurface layer above 200K, with activation energies differing by approximately 10%. Significant variation in the diffusion barriers is found in the presence of surface adsorbates. Interlayer diffusion is not observed below 220K, which sets lower bounds on the surfacesubsurface and subsurface-bulk diffusion barriers and provides a benchmark for ab-initio total energy calculations. Scanning Tunneling Spectroscopy of subsurface hydrogen has been performed and the results are compared with tight-binding calculations. Adsorbed molecules interact strongly with subsurface H. In the case of CO, binding occurs preferentially at sites separated by @sr@3 lattice vectors from surface hollow sites above embedded hydrogen. For O@sub 2@, thermal dissociation proceeds adjacent to H sites. Subsurface H clusters provide nucleation sites for ordered adsorbate structures, pin adsorbate domain boundaries, and hinder adsorbate diffusion.