AVS 45th International Symposium
    Surface Science Division Monday Sessions
       Session SS-MoP

Paper SS-MoP4
Vibrational Spectra and Structure of Hydrogen Adsorbed on Ni(111)

Monday, November 2, 1998, 5:30 pm, Room Hall A

Session: Surface Science Division Poster Session
Presenter: M. Gostein, Massachusetts Institute of Technology
Authors: M. Gostein, Massachusetts Institute of Technology
Q.Y. Yang, Massachusetts Institute of Technology
S.T. Ceyer, Massachusetts Institute of Technology
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The structure and adsorption site of hydrogen on Ni(111) were investigated using high-resolution electron energy loss spectroscopy. At 0.5 ML coverage where the adsorbed hydrogen forms a (2x2)2H unit cell, the vibrational spectra show a pair of fundamentals at 733 and 791 cm@super -1@ and a pair at 1077 and 1109 cm@super -1@, as well as three overtones at 1260, 1396, and 2180 cm@super -1@. The pair of fundamentals at the lower frequency and the two lower frequency overtones are assigned to H-Ni modes parallel to the surface, while the higher frequency pair and highest frequency overtone are assigned to modes perpendicular to the surface. These assignments are based on the expected anharmonicity of the modes and on the angular distributions of the loss features. The features in each fundamental pair, which are closely-spaced in frequency, are resolved by exploiting their different electron impact energy dependence. Preliminary results indicate that the fundamentals are split into pairs because of different potential energy surfaces at the hcp versus fcc three-fold hollow binding sites of hydrogen in a structure previously proposed from the observation of a (2x2)2H unit cell.