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Surface-extended x-ray-absorption fine-structure studies on the Ni(111)-c(4X2)-CO system show that CO adsorbs in threefold-coordinated hollow sites. This result is in conAict with the adsorption-site determination via molecular vibrational frequencies, which for this system led to an assignment of a bridge site.During the 1970s and 1980s a large number of surface structures were solved by analysis of low-energy electron-diffraction (LEED) intensities. As a result information about adsorbate-substrate bond lengths and adsorption sites became available. ' Relatively few results were obtained for molecular adsorbates, however, for which the LEED analyses are especially demanding. The advent of synchrotron-radiation-based structural methods such as surface-extended x-ray-absorption fine structure (SEXAFS) and energy-scan photoelectron diffraction (PED) in the 1980s did not significantly change the situation regarding these adsorbates. Other surface-sensitive techniques which probe surface structure less directly have therefore been used to gather structural information on adsorbed molecular layers. Thus vibrational spectroscopy, in the form of both infrared refiection-absorption spectroscopy (IRAS) and high-resolution electronenergy-loss spectroscopy (HREELS), has proved particularly successful in assigning adsorption sites for adsorbed diatomic molecules. By comparison with measured vibrational frequencies in inorganic clusters of known structure, the accepted picture seems to be that adsorption on a specific site on a given crystal face should give rise to a typical vibrational frequency falling in a defined and rather narrow range and, furthermore, that the ranges for adsorption on di6'erent sites on the same surface should not seriously overlap.However, only a few adsorption-site assignments based on vibrational spectroscopy have been tested to date by LEED, SEXAFS, ' or PED. "' Whereas the results of the latter methods are in agreement with the results of vibrational spectroscopy for all the CO adsorption structures studied, recent SEXAFS (Ref. 10) and PED (Ref. 12) studies of Ni(111)-c(4X2)-NO have shown that the assignment of adsorption site based on vibrational frequencies for this system is in error. Here we present the results of a SEXAFS study of Ni(111)-c(4X2)-CO which are also in disagreement with the interpretation of vibrational studies, ' ' and thus cast doubt on the general reliability of structural assignments based on vibrational frequencies alone.

Oxygen chemisorption and oxide formation on a Ni{111)surface have been monitored by using the near-edge x-ray-absorption fine-structure technique. The adsorption site of oxygen in the p(2X2) and (&3X&3)R30' superstructures has been determined by a multiple-scattering analysis. In both structures the oxygen occupies a threefold-coordinated fcc site with a nearest-neighbor

The surface carbonate species formed by the reaction of CO2 with preadsorbed oxygen on Ag{ l l O } has been studied with X-ray photoabsorption techniques. The analysis of the oxygen and carbon K near edge structure (NEXAFS) shows that the molecule is adsorbed flat on the surface and that, at least for the 0-resonances, the effective symmetry remains D3h. The observed (1 x 2) surface structure thus results from an adsorbate-induced reconstruction. The analysis of the extended fine structure (SEXAFS) gives an O-Ag bond length of 2.8A.