AVS 47th International Symposium
    Surface Science Wednesday Sessions
       Session SS-WeP

Paper SS-WeP27
HREELS on CO/Rh(111): Lateral Interactions in Adsorbate Layers and Vibrational Frequency Shifts

Wednesday, October 4, 2000, 11:00 am, Room Exhibit Hall C & D

Session: Poster Session
Presenter: R. Linke, Eindhoven University of Technology, The Netherlands
Authors: R. Linke, Eindhoven University of Technology, The Netherlands
J.W. Niemantsverdriet, Eindhoven University of Technology, The Netherlands
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Lateral interactions in CO adlayers on Rh(111) were studied by HREELS and TDS. At low coverage CO adsorbs only on-top. From about 0.5 ML also the 3-fold hollow site becomes populated. The occupation of the second adsorption site is accompanied by the development of a low temperature shoulder in the TDS peaks indicating increasingly repulsive interactions in the adsorbate layer. A continuous shift to higher frequencies of the internal C-O vibration is observed over the whole coverage range. At low coverage the on-top Rh-CO frequency hardly shifts but undergoes a shift to lower frequencies upon the emergence of the 3-fold hollow. To assess the origin of the frequency shifts we measured isotopic CO mixtures. HREEL spectra of 12CO and 13CO mixtures at total CO coverage of 0.33 ML proof that the frequency shifts arise purely from dipole-dipole coupling. We also used a fixed low 13CO coverage to monitor the frequency shifts in a changing 12CO matrix. The internal 13C-O frequency remains at its singleton frequency until 0.5 ML total CO coverage, then a pronounced shift to higher frequencies begins. This testifies that at low coverage (<0.5 ML) frequency shifts arise from dipole-dipole coupling while at high coverage (0.5 - 0.75 ML) also chemical effects contribute to a large extent.