The adsorption and condensation of sulphur dioxide on a number of metal surfaces is examined with emphasis on the observed surface structures. First some properties of gas-phase and the characteristic bonding modes of in transition-metal complexes are summarized. The role of fast and high-resolution core-level spectroscopy in the study of the kinetics of surface processes and in the identification of surface species is demonstrated for adsorption and temperature-dependent decomposition on Cu(100). Structural studies of adsorption on Ag(110), Pd(100) and Pt(111) using qualitative techniques are reviewed as well as quantitative structure determinations of condensed and of adsorption on Ni(110), Ni(100), Ni(111) and Cu(100) using x-ray absorption fine structure. The different adsorption and desorption behaviour is discussed in the light of the electronic structure of both the adsorbate and the substrate.

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A phase transition has been observed in the system pyridine adsorbed on Ag(lll) at 100 K by near-edge x-ray-absorption fine-structure measurements in real time. At low pyridine coverages an angle between the ring plane and the surface plane of 45° ±5° was observed. This phase converts sharply at a submonolayer coverage to a phase with an angle between the ring plane and the surface plane of 70° ± 5°. Continued exposure gradually leads to a randomly oriented multilayer.PACS numbers: 68.35.Rh, 68.35.Bs, 78.70.Drn There is widespread interest in the pyridine-silver system because of its importance for surface-enhanced Raman scattering. 1 " 3 In particular, some proposed enhancement models involve charge-transfer excitations from the metal to affinity levels of the adsorbate, 4,5 which are also probed by the near-edge x-rayabsorption fine-structure (NEXAFS) technique. 6 " 8 Pyridine (C5H5N) is electronically characterized by a nitrogen lone-pair orbital. Pyridine chemisorption on metal surfaces may therefore have model character for determining the relative contributions of lone-pair and TT bonding to the chemisorption bond. Here we report the coverage-dependent molecular orientation of pyridine chemisorbed on Ag(lll) at 100 K determined by NEXAFS.Demuth, Christmann, and Sanda 9 studied the chemisorption of pyridine on clean Ag(lll) surfaces at --140 K with vibrational electron-energy-loss (VEELS) and uv-photoemission spectroscopies (UPS). They observed a phase transition at about half a monolayer coverage from a nearly flat-lying 7r-bonded pyridine phase to an inclined N-bonded phase. Similar coverage-dependent orientational phase transitions have been obtained by VEELS for pyridine on Ni(001) 10 and Pt(llO). 11 Orientational phase transitions also occur as a function of temperature. This has been shown for pyridine on Ni(001) by VEELS 10 and for pyridine on Pt(lll) in a recent NEXAFS study. 12 The NEXAFS measurements on Pt(lll) indicated a low-temperature pyridine state with an apparent angle between the ring plane and the surface plane of 52° which converts at 7^300 K to a high-temperature state with a corresponding tilt angle of 74°. This result, as well as UPS and electronic EELS data for pyridine on Ag(lll), 1314 casts some doubt on the existence of flat-lying pyridine molecules on well-defined surfaces at low coverages.NEXAFS studies on molecules which are only weak-ly perturbed by chemisorption are particularly useful in determining the orientation of the molecules relative to the surface. 6 " 8 For low-symmetry surfaces this includes the determination of the azimuthal orientation. 15 The polarization dependence of NEXAFS transitions is due to the validity of dipole selection rules for photoabsorption. The analysis of NEXAFS is especially unambiguous when TT resonances occur resulting from transitions of a Is electron into unfilled antibonding TT states. These TT resonances are rather sharp compared with a-shape resonances so that background subtraction is straightforward. With high-brightness storage ring...

The adsorption of Na on Al(l 11) at room temperature has been studied by surface extended x-rayabsorption fine-structure (SEXAFS) experiments as well as by parameter-free calculations. For coverages of BNU^O. 16-0.33, the SEXAFS analysis shows that Na atoms occupy an unusual sixfoldcoordinated substitutional site. The Na-Al bond length is determined as 3.31 A, consistent with metallic bonding. Ab initio density-functional-theory calculations for several adsorbate geometries show that the substitutional site has the lowest total energy. PACS numbers: 78.70.Dm Studies of alkali-metal (AM) adsorption have played an important role in the development of theories of chemisorption. The model for AM adsorption first proposed by Gurney [1], in which the nature of the adsorbate bonding is determined by the energy position and width of a resonance derived from the alkali valence s level, has strongly influenced subsequent theoretical and phenomenological descriptions of adsorption. Much of the interest has centered on the large change in work function, and phenomena related hereto, and on the characteristic dependence of the work function on coverage, found for many metals and semiconductors. Recently, more detailed insight into the nature of the adsorption bond has been obtained from experimental studies [2] using photoemission, inverse photoemission, electron energy loss, and Penning spectroscopies. These studies appear to confirm the main features of the Gurney model in which the resonance derived from the valence s level of the adsorbed atom is mostly unoccupied at low coverage, but moves down in energy through the Fermi level with increasing coverage due to adsorbate-adsorbate interactions. The interpretation that the bonding is largely ionic at low coverages [3] is, however, the subject of considerable current controversy: An alternative description in terms of the dominance of adatom polarization has been proposed [4,5].With a few exceptions noted below, studies of the adsorbate geometry have not played a central role in discussions of the nature of the adsorption mechanism and very few quantitative determinations of the adatom geometry have in fact been carried out. The recent emphasis of structural studies has rather been, on the one hand, the characterization of the variety of adsorbate phases [6] as a function of temperature and coverage and, on the other hand, studies of the reconstruction [7,8] of fee (110) and bec (100) metal surfaces induced by small concentrations of adsorbed AM's. Thus, theoretical models of AM adsorption have almost by default assumed that the adsorption site is one of high symmetry on an unperturbed substrate. In accord with this assumption, self-consistent calculations [5,9,10] for Na adsorption on Al(lll) arrived at the conclusion that Na occupies the threefoldcoordinated site. LEED structure determinations [6,11,12] for Ni(100)-c(2x2)-Na, Al(100)-c(2x2)-Na, and Rh(100)-c(4x2)-Cs indicate that for these systems the AM atoms occupy the fourfold sites of highest symmetry. The Cu(ll l)...

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