V. Fritzsche

Structure determination of Ni(111)c(4 × 2)-CO and its implications for the interpretation of vibrational spectroscopic data

Abstract A detailed quantitative structure determination of the Ni(111)c(4 × 2)-CO structure has been undertaken using scanned energy mode photoelectron diffraction from the C 1s state over a wide range of emission angles. Analyses of these data by approximate direct methods, and by two independent multiple scattering trial-and-error fitting optimisations lead to a consistent structure in which the CO occupies both types of hollow site on the surface in equal amounts with a C-Ni top layer spacing of 1.29 ± 0.05 A. This structure is therefore essentially the same as that for Ni(111)c(4 × 2)-NO, and provides further evidence that simple use of the intramolecular stretching frequencies of such adsorbed molecules, which had been interpreted in both cases as indicative of bridge site adsorption, is not always a reliable indicator of local adsorption site.

The local adsorption structure of acetylene on Cu(111)

Abstract A quantitative structural study of the adsorption of acetylene on Cu(111) has been carried out by scanned energy mode photoelectron diffraction from the C 1s core levels. The molecule is found to be adsorbed with the CC axis almost parallel to the surface with the carbon atoms occupying the two symmetrically distinct three-fold coordinated hollow sites on the surface. The C to top Cu atom layer spacing appears to be slightly smaller (1.38 ± 0.03 A ) above the “fcc” hollow site (above a third-layer Cu atom) than above the “hcp” hollow site (1.44 ± 0.03 A ) . The best fit to theory is obtained with the C atoms exactly in the three-fold symmetric sites of the substrate which constrains the CC bondlength to a value (1.48 ± 0.10 A ) substantially larger than that of the free molecule. This result is consistent with previously published vibrational spectroscopy from this adsorption system which indicates significant lowering of the CC bond order.

The geometric structure of the surface methoxy species on Cu(111)

Abstract The adsorption geometry of the surface methoxy species (-OCH 3 ) on Cu(111) has been determined quantitatively using scanned energy mode photoelectron diffraction in a two-step approach. First, using direct methods it was established that the fragment adsorbs via the oxygen atom at the “fcc” three-fold adsorption site. Subsequently, the structural parameters were refined by a search in multi-parameter space using diffraction spectra obtained in different emission geometries. The oxygen atom is found to be situated 1.32(±0.05) A above the three nearest-neighbour copper atoms with the O-C axis essentially perpendicular to the surface. The O-C distance is 1.42(−0.03/+ 0.10) A; a relaxation as well as a rumple of the outermost Cu layer occur.

Coverage-dependent changes in the adsorption geometry of benzene on Ni{111}

Abstract Photoelectron diffraction in the scanned energy mode has been used to determine the structure of benzene adsorbed on to an Ni{111} surface. In the disordered phase at low coverage the molecule is centered over a bridge site with the CC bonds oriented in the 〈211〉 directions, but in the ordered overlayer at saturation coverage the hcp threefold symmetric hollow site is occupied with the CC bonds oriented in the 〈110〉 directions. The molecular planes are situated 1.92(±0.05) A and 1.91(±0.04) A, respectively, above the Ni atoms which form each adsorption site. There is no significant distortion of the benzene ring. The results are compared with photoemission measurements and quantum chemical calculations.

Is the frequency of the internal mode of an adsorbed diatomic molecule a reliable guide to its local adsorption site

Abstract Although the C-O stretching frequency for the system Ni>{111}c(4>x2)-CO falls in the so-called bridge region, a structure determination with photoelectron diffraction shows that the CO molecules are actually adsorbed in threefold hollow sites. These, and other recent data, seriously call into question the practice of adsorption site assignment solely on the basis of the stretching frequency of an internal mode.

The local geometry of reactant and product in a surface reaction: the dehydrogenation of adsorved ethylene on Ni(111)

Abstract Using C1s scanned-energy mode photoelectron diffraction based both on approximate “direct” methods and full multiple scattering modelling of spectra recorded in different emission angles covering a total spectral data range of 1200 eV, the local geometry of adsorbed ethylene and adsorbed acetylene on Ni(111) have been determined in a detailed quantitative fashion. Ethylene adsorbed at low temperature (120 K) lies with its CC axis parallel to the surface and in an aligned bridge site such that the C atoms lie approximately atop Ni atoms. Heating this surface leads to dehydrogenation of the adsorbed ethylene to adsorbed acetylene, and while the CC axis remains parallel to the surface, the CC bondlength and CNi layer spacing are reduced, and the acetylene occupies a cross-bridge site with the C atoms directly above inequivalent hollow sites on the surface. Both adsorbed species show C bondlengths larger than those of the associated gas-phase molecules, indicating a significant reduction of CC bond order, in agreement with vibrational spectroscopic data. Possible geometrical reaction paths based on a concerted mechanism connecting the reactant and product are discussed.

A photoelectron diffraction study of the structure of the Cu{110}(2 × 1)-CO system

The adsorption site and bond distances for CO adsorbed in the (2 x 1) overlayer on Cu{110} have been determined with scanned energy mode photoelectron diffraction. The molecule is found to occupy an atop site with a metal-carbon separation of 1.87 (±0.02) A. The literature value for the C-O stretching frequency is 2088 cm -1 . These and other results indicate that the traditional correlation between adsorption site and C-O stretching frequency still appears to hold for copper surfaces.

Ethene adsorbed on Cu(110): a combined photoemission and photoelectron diffraction study

Abstract The absorption geometry of ethene on Cu(110) has been investigated using photoelectron diffraction (PhD) in the scanned energy mode as well as angle-resolved valence level photoemission. Two best fit Phf geometries have been found, both of which are compatible with the photoemission data: the molecule is a adsorbed either in an atop site on the close-packed Cu rows at a perpendicular height of 2.08 ± 0.02 A with a CC bond length of 1.32 ± 0.09 A, or in a short bridge site on the Cu rows at a perpendicular height of 2.09 ± 0.02 A with a CC bond length of 1.53 ± 0.13 A. In each case the molecular plane is parallel to the surface, but the CC axis can be twisted azimuthally out of the [1 1 − 0] direction by as much as 24.

Following the changes in local geometry associated with a surface reaction: the dehydrogenation of adsorbed ethylene

Using scanned-energy-mode photoelectron diffraction we have determined the local adsorption geometry of the reactant and product molecules in a model heterogeneous reaction on a Ni(111) surface in which ethylene (C2H4) is dehydrogenated to acetylene (C2H2). Although the C-C axis remains essentially parallel to the surface, a change in adsorption site occurs, indicating that for the case of a concerted reaction mechanism only two distinct pathways are possible. Precise structural information on a model system of this kind has hitherto been unavailable.

Determination of the local adsorption structure of acetylene on Ni(111)

Abstract Scanned-energy mode C is photoelectron diffraction has been applied to the adsorption system of acetylene on Ni(111) to determine the local adsorption structure which is compared with the results of a similar study of acetylene on Cu(111). The adsorption site in both cases is with the molecule essentially parallel to the surface and in a crossed bridge site such that the two C atoms of the molecule occupy inequivalent hollow sites. A substantial, essentially identical stretching of the CC bond relative to the gas phase is found for both surfaces (consistent with the interpretation of vibrational spectroscopic data), despite the marked difference in chemical reactivity of Cu(111) and Ni(111) towards hydrocarbons.