LEED AND RESONANCE EELS STUDY OF THE (NO)(2) DIMER ON GRAPHITE

The structure of physisorbed (NO)(2) films on graphite has been invest igated using resonance electron energy loss spectroscopy in conjunctio n with low energy electron diffraction (LEED). Various ordered phases were resolved for the first time by the LEED technique as a function o f coverage. Transient negative ion resonance formation was manifested in strong enhancements of the nu = 0-1 vibrational cross section in th e incident electron energy range 11-16 eV. A splitting of this resonan ce by 2.6 +/- 0.2 eV indicates formation of the dimer configuration on the surface. The resonance splitting was observed for all adsorbate c overages studied. Detailed angular distributions have been recorded, w hich showed distinct features for different adsorbed phases. Ab initio self-consistent field calculations allow a qualitative analysis of th e orientation of the NO dimer. Our results are consistent with an orie ntation of the NO dimer plane approximately parallel to the surface in the gamma phase and perpendicular in the beta phase.