(0001) and Cr 2 O 3 (0001) surfaces based on low energy electron and surface X-ray diffraction methods. However, the PhD investigation fails to provide definitive evidence for the presence or absence of surface vanadyl (V=O) species associated with atop O atoms on the surface layer of V atoms. Specifically, the best-fit structure does not include these vanadyl species, although an alternative model with similar relaxations but including vanadyl O atoms yields a reliability-factor within the variance of that of the best-fit structure.keywords: surface relaxation; surface structure; vanadium oxide; photoelectron diffraction  present address:

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The structure of molecular furan, C 4 H 4 O, on Pd(111) has been investigated by O K-edge near-edge X-ray absorption fine structure (NEXAFS) and C 1s scanned-energy mode photoelectron diffraction (PhD). NEXAFS shows the molecule to be adsorbed with the molecular plane close to parallel to the surface, a conclusion confirmed by the PhD analysis. Chemical-state specific C 1s PhD data were obtained for the two inequivalent C atoms in the furan, the α-C atoms adjacent to the O atom, and the β-C atoms bonded only to C atoms, but only the PhD modulations for the α-C emitters were of sufficiently large amplitude for detailed evaluation using multiple scattering calculations. This analysis shows the α-C atoms to be located approximately 0.6 Å off-atop surface Pd atoms with an associated C-Pd bondlength of 2.13±0.03 Å. Two alternative local geometries consistent with the data place the O atom in off-atop or near-hollow locations, and for each of these local structures there are two equally-possible registries relative to the fcc and hcp hollow sites. The results are in good agreement with earlier density functional theory calculations which indicate that the fcc and hcp registries are equally probable, but the PhD results fail to distinguish the two distinct local bonding geometries.

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The combination of chemical-state-specific C 1s scanned-energy mode photoelectron diffraction (PhD) and O K-edge near-edge X-ray absorption fine structure (NEXAFS) has been used to determine the local adsorption geometry of the coadsorbed C 3 H 3 and CO species formed on Pd(111) by dissociation of molecular furan. CO is found to adopt the same geometry as in the Pd(111)c(4x2)-CO phase, occupying the two inequivalent threefold coordinated hollow sites with the C-O axis perpendicular to the surface. C 3 H 3 is found to lie with its molecular plane almost parallel to the surface, most probably with the two 'outer' C atoms in equivalent off-atop sites, although the PhD analysis formally fails to distinguish between two distinct local adsorption sites.

The surface relaxations of the rutile TiO 2 (110)(1x1) clean surface have been determined by O 1s and Ti 2p 3/2 scanned-energy mode photoelectron diffraction. The results are in excellent agreement with recent low energy electron diffraction (LEED) and medium energy ion scattering (MEIS) results, but in conflict with the results of some earlier investigations including one by surface X-ray diffraction. In particular, the bridging O atoms at the surface are found to relax outwards, rather than inwards, relative to the underlying bulk. Combined with the recent LEED and MEIS results a consistent picture of the structure of this surface is provided. While the results of the most recent theoretical total-energy calculations are qualitatively consistent with this experimental consensus, significant quantitative differences remain.

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