A systematic study of the near-edge x-ray-absorption fine structure for single-layer coverage of C2H2, C2H4, and, for the first time, C2H6 chemisorbed on Cu(lOO) and Ag(lOO) allows the determination of the molecular orientation and bonding strength. The first surface extended x-ray-absorption fine structure at the carbon K edge yields a nearest-neighbor distance of 1.9(1) A for C2H2 and C2H4 on Cu. A comparison of the a shape-resonance position in the gas phase, chemisorbed on Ag, Cu, and Pt, is presented. PACS numbers: 68.55Jk, 68.45.Da, 78.70.Dm Acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) are three important hydrocarbons which model the triple, double, and single C-C bonds. C2H6 has a fully saturated C-C bond and little dependence of the C-C bond length upon the strength of chemisorption on a metal substrate is expected. The contrary is true for C2H2 with its double n bonding. The chemical reactions of these three species on metal surfaces is of practical importance for catalysis. C2H4 on Pt(lll) rehybridizes upon adsorption below room temperature to the surface, and in the presence of atomic oxygen it combusts to CO2 and H2O at higher temperatures. * Similar reactions are known on Ag(llO).^ A first step in order to understand those processes is the knowledge of the microscopic structure of chemisorption; i.e., the orientation of the molecules on the substrate, the coordination to the substrate atoms (hollow, bridge site, etc.), the bonding length to the substrate, and an eventual change in the intramolecular bond length. Synchrotron radiation, which offers the advantage of a tunable photon energy and a linear polarization, can be used in order to try to answer some of those questions. Recently the near-edge x-ray-absorption fine structure (NEXAFS) of C2H2 and C2H4 on Pt(l 11) has been reported.^ The molecules were found to be lying flat and strongly chemisorbed to the Pt(lll) surface, strongly perturbed by it, and exhibited a longer C-C bond length than on the gas phase. ^ Whether a correlation of C-C bond length and the energy of the a resonance can be deduced in broad generality is subject to debate."^ In this Letter we present the first surface extended xray-absorption fine structure (SEXAFS) and NEXAFS study to obtain the C-Cu distance and the orientation of hydrocarbon molecules on noble-metal substrates. We investigated three similar hydrocarbons (C2H2,C2H4, C2H6) on two different (Ag,Cu) substrates. The adsorption process was characterized by thermal desorption spectroscopy (TDS) measurements, and the energy of the molecular resonances from angle-dependent NEX-AFS measurements was correlated to the strength of adsorption. In particular, we find the molecules lying flat on the Cud00) surface. On Ag(lOO) this is also the case except for C2H2, where the C-C axis makes a finite angle to the surface. C2H6 yields the same a resonance energy on the two substrates as in the gas phase. The TDS data show that the adsorption on Cu is stronger than on Ag. This can be correlated to a monotonic decreas...

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A low-symmetry twofold bridge site of (2x1) oxygen on Cu(110) allows the determination of the azimuthal dependence of the surface extended x-ray-absorption fine structure. For normal incidence and E parallel to the [100] and [110] directions we determine the nearest-and second-nearest-neighbor distances to be 1.84(2) and 2.00(5) A, respectively. We show that the atomic oxygen is occupying the low-symmetry twofold bridge site in the [100] direction 0.35 A above the first Cu layer.PACS numbers: 68.20+t, 78.70.DmThe surface version of the extended xsensitive to long-range periodic structure but only ray-absorption fine-structure (SEXAFS) method to local environment, the present experiments indihas become a powerful technique during the last cate strongly a long bridge site on top of the first Cu five years. 1 " 3 Detailed information on bond layer. lengths and next-neighbor coordinations of ad-The adsorption of oxygen on the Cu (110) surface sorbed atoms on single-crystal substrates can be has been studied by numerous techniques such as gained by employing the polarized synchrotron radiwork-function, low-energy electron-diffraction ation as a "searchlight." Only bonds which have_a (LEED), and Auger electron spectroscopy (AES) sizable component of the electric field vector E measurements, 4 ' 7 low-energy ion scattering, 5 and parallel to the adsorbate-substrate internuclear axis helium diffraction. 6 From LEED it is known that will contribute to the amplitude of the EXAFS osoxygen forms a stable (2x1) overstructure at room dilations. There exist two angular dependences: temperature and exposures of 10 to 100 L oxygen. (i) on the polar angle 9 between the E vector and Whether this (2x1) structure in the LEED pattern the surface normal n (detected in most cases) and is caused only by the array of O atoms or by a (ii) on the azimuthal angle in the surface plane.reconstructed first layer of the Cu substrate atoms The latter dependence has not been reported to is not completely clear. The second open question date, because it disappears for high local symmetry for this adsorbate-substrate system is the local posi-(larger than twofold). This ambiguity is lifted for tion of the oxygen atoms. The present experiments low symmetry as for a twofold bridge site. In the are sensitive to the local site since they measure the present Letter we report this observation for the adsorbate-substrate bond length in a direct way. (2x1) structure of atomic oxygen on Cu(110).The fact that for the first time the azimuthal anguThe different SEXAFS spectra for normal incidence lar dependence of EXAFS is detected, and by virtue and E parallel to the [100] ( = 0°) and [110] of this the nearest-and second-nearest-neighbor (0 = 90°) directions enable us to determine uniquedistances are determined independently, strengthly the different bond lengths. ens this result. It was assumed 5 that the O atom sits Secondly, the position of chemisorbed O on between the first and second layers of Cu atoms. Cu(llO) for low (10-L [1 L (langmuir) = 10...

Surface extended-x-ray-absorption fine-structure measurements reveal that formate (HC0 2 ) groups on Cu(100) chemisorb via the two oxygen atoms in adjacent fourfold hollow sites with an average O-Cu nearest-neighbor bond length of 2.38 ±0.03 A. This distance is significantly ( ~-0.4 A) longer than typical O-Cu bonds in bulk compounds and all known surface complexes. The unusually large O-Cu distance is attributed to a steric effect involving the C atom in HC0 2 and the nearest-neighbor Cu surface atoms.PACS numbers: 78.70.Dm, 82.65.Jv The development of various surface structural probes over the last decade has allowed the determination of the precise location of chemisorbed atoms on surfaces. 1 Although it is still difficult to predict atomic chemisorption sites a priori the chemisorption bond lengths appear to be better understood. As shown by Madhukar 2 and Mitchell, 3 experimentally derived bond lengths for a variety of chemisorbed atoms on metal surfaces can be well accounted for (within 0.1 A) by Pauling's bonding concepts in molecular complexes and bulk compounds. 4 In contrast to the case of atomic chemisorption the structural data base for chemisorbed molecules is still relatively small as a result of the difficulty of many techniques to deal with such systems. From the available structural data 5 it appears, however, that the bond lengths between atoms in the molecule and surface substrate atoms are similar to those of the individual atoms in either surface or bulk complexes.Here we report the first observation that the molecular chemisorption bond can be significantly different from the corresponding bond for an atomic adsorbate or for bulk complexes. Surface extended-x-rayabsorption fine-structure (SEXAFS) 6 complemented by near-edge x-ray-absorption fine-structure (NEXAFS) 7 measurements reveal an unusually long O-Cu distance for the formate (HC0 2 ) species on Cu(l 00). The derived bond length is larger by about 0.4 A than the O-Cu nearest neighbor (nn) bond lengths for atomic oxygen and the methoxy (CH 3 0) species on the same surface and for various bulk compounds (e.g., oxides and formates). This unusual bonding is attributed to a steric effect involving the C atom in a bridge position between two Cu surface atoms. The nonbonding C-Cu interaction which coun-teracts the O-Cu bond pushes the molecule away from the surface and leads to the unusually long O-Cu distance.Experiments were performed on beam line 1-1 at the Stanford Synchrotron Radiation Laboratory with use of the grasshopper monochromator (1200 lines/mm holographic grating). SEXAFS and NEXAFS spectra were obtained by partial-electron-yield detection using a retarding voltage of -450 V. 6 The angle of incidence of the x rays on the sample could be varied from grazing (E vector close to surface normal) to normal (E in surface plane). The Cu(100) surface was cleaned by Ar" 1bombardment. Surface cleanliness and order after annealing were checked by Auger spectroscopy and LEED (base pressure 8xl0~n Torr). The formate was prepared 8,9 by p...