R. Lindsay

A photoelectron diffraction study of ordered structures in the chemisorption system Pd{111}-CO

The adsorption system Pd{111}-CO gives rise to a series of at least seventeen, sometimes complicated, ordered overlayers. The R30° and c(4×2) phases are formed at coverages of θ=0.33xa0ML and 0.50xa0ML, respectively. The results from surface vibrational spectroscopy have hitherto been interpreted in terms of CO adsorbing on threefold symmetric hollow sites in the R30° phase, but occupying bridge sites in the c(4×2) phase. We show in a quantitative photoelectron diffraction study in the scanned energy mode that, whereas hollow sites are indeed occupied in the ()R30° structure, CO adsorbs in a mixture of fcc and hcp hollows in the c(4×2) phase. Several structural parameters, in particular the Pd-C layer spacings, have been determined. The result is essentially identical to that obtained for the Ni{111}c(4×2)-CO system. It is also inferred that at coverages between 0.33 and 0.50xa0ML “domain wall”, or weakly incommensurate, phases occur in which the percentage of occupied hcp sites increases monotonically as the coverage is raised.

Determination of the local structure of glycine adsorbed on Cu(110)

Abstract Scanned-energy mode N 1s and O 1s photoelectron diffraction has been used to determine the local geometry of glycine adsorbed on Cu(110) in an ordered (3 × 2) phase. The results are consistent with a molecular geometry in which the Cue5f8C axis lies approximately parallel to the surface and the molecule bonds across a pair of [110] Cu surface rows through the two oxygen atoms of the carboxyl group and the N atom of the amino group. The N atom is displaced by 0.24 ± 0.10 A off an atop site along the [110] rows with a Cuue5f8N nearest-neighbour distance of 2.04 ± 0.02 A . The carboxyl bonds to two Cu atoms in a [110] row, with the O atoms displaced 0.80 ± 0.07 A from atop in [001] towards the amino group with a Cuue5f8O nearest-neighbour bond length of 2.03 ± 0.03 A and a resulting tilt of the Oue5f8Cu bond relative to the surface normal of 23 ± 2°. A specific structural model comprising two molecular moieties per unit mesh which has the space group p1g1, consistent with the qualitative LEED observations, is proposed on the basis of these data.

The coverage dependence of the local structure of C on Ni(100): a structural precursor to adsorbate-induced reconstruction

Abstract The local structure around adsorbed carbon atoms on Ni(100) has been determined by Cxa01s scanned-energy mode photoelectron diffraction both at low coverage (0.15xa0ML) and at 0.5xa0ML in the ‘clock’-reconstructed (2×2)p4g phase. At low coverage the C atoms occupy simple undistorted hollow sites; the radial strain of the Ni atoms surrounding the adsorbed carbon, proposed on the basis of STM images, is not found. The comparative study of the (2×2)p4g phase confirms the structural parameters of earlier studies, but allows direct comparison between the two phases free from systematic errors. The Cue5f8Ni near-neighbour distances to both top and second layer Ni atoms are unchanged by reconstruction, but the Niue5f8Ni nearest-neighbour distance in the top layer increases significantly. This implies that the adsorbate-induced compressive stress is associated with Niue5f8Ni, rather than Niue5f8C, repulsion as has previously been proposed. Special care has been taken to check for coupling of structural parameters in the analysis which might influence the precision of these comparisons, and a new methodology for identifying and assessing such parameter coupling is described.

Adsorption site and orientation of pyridine on Cu{110} determined by photoelectron diffraction

The local adsorption geometry of pyridine on Cu{110} has been determined quantitatively using photoelectron diffraction in the scanned-energy mode. At high coverages the molecule adsorbs nearly atop a Cu atom in the close-packed rows with a N–Cu bond length of 2.00 A. Moreover, the Cu–N axis and the molecular (C2) axis are inclined by 8° and 20°, respectively, to the surface normal. The result shows that not only the adsorption site of the emitter (in this case the N atom) but also the position of relatively light scatterers (the C atoms) can be determined by photoelectron diffraction.

The dimers stay intact: a quantitative photoelectron study of the adsorption system Si{100} (2x1)-C2H4

Using the technique of photoelectron diffraction in the scanned energy mode we show that the Si dimer separation on the Si{100} surface following the adsorption of ethene (ethylene) is 2.36(±0.21) A. This value is only very slightly larger than on the clean surface and shows that the dimer remains intact, thus providing a clear quantitative experimental resolution of a long controversy in the literature. The C-C and C-Si separations are 1.62±0.08 A and 1.90±0.01 A, respectively, the former indicating a bond order of less than one.

Photoelectron diffraction study of a catalytically active overlayer: C2H2 on Pd{111}

A quantitative structure determination of a newly discovered (2◊2) adsorption phase of acetylene chemisorbed on Pd{111} has been performed by scanned-energy mode photoelectron diVraction: this phase corresponds to the threshold coverage for the catalytic conversion of acetylene to benzene. The carbon atoms in the C 2 H 2 molecule are located almost over bridge sites with a C‐C bond length of 1.34+0.10 A ˚ , the centre of the molecule being positioned almost over a hollow site. Of the two hollow sites the hcp site (directly above a second layer Pd atom) is favoured, particularly by a subset of the data most sensitive to this aspect of the structure, but the full analysis indicates that the fcc site (above a third layer Pd atom) cannot formally be excluded. The adsorption site adopted by acetylene in the higher coverage (E3◊E3)R30° phase on Pd{111} is essentially identical. This is the dominant structure in the coverage regime which is catalytically active for the conversion of acetylene to benzene. The implications of these findings for acetylene coupling reactions over Pd{111} are discussed.

CN coordination in the adsorption system Ni(110)c(2×2)–CN: an unexpected geometry

Abstract N K-edge NEXAFS and Nxa01s and Cxa01s scanned-energy photoelectron diffraction have been used to determine the orientation and local adsorption geometry of CN adsorbed on Ni(110) in the c(2×2) phase. The NEXAFS results show that the C–N molecular axis lies approximately parallel to the surface in agreement with several prior studies of CN adsorbed on metal surfaces, but also show that it is approximately aligned in the [001] azimuth, perpendicular to the close-packed rows of the surface, in contradiction to earlier conclusions based on angle-resolved valence level photoemission data. Photoelectron diffraction shows the CN to lie approximately atop a second-layer Ni atom such that the C atom has similar distances to this second-layer Ni atom and two top layer Ni atoms, whereas the N atom bridges two top layer Ni atoms in the next close-packed surface atom row. This geometry appears to have no analogue in previous structure determinations of adsorbed diatomic species. In view of the intense fractional order LEED beams seen for this phase, several possible substrate restructuring models have been considered.

The local adsorption geometry of benzene on Ni(110) at low coverage

Abstract Using scanned-energy mode photoelectron diffraction we have determined the local adsorption geometry of benzene adsorbed on Ni(110) at a coverage of approximately 0.1 monolayers. The molecule is found to adsorb with its molecular plane parallel to the surface, centred on a four-fold coordinated hollow site, with two opposite Cue5f8C bonds perpendicular to the close-packed 〈110〉 rows of surface nickel atoms and a C–Ni nearest-neighbour layer spacing of 1.81±0.02xa0A. The unusually high precision in the determination of the Cue5f8C distance in the benzene ring (1.45±0.03xa0A) if the molecule is assumed to retain its full gas-phase symmetry shows that there is a significant increase in this distance relative to the free molecule (1.39xa0A).

Molecules on oxide surfaces: a quantitative structural determination of NO adsorbed on NiO(100)

Abstract Analysis of scanned-energy mode photoelectron diffraction data from NO adsorbed on NiO(100) confirms that both components of the Nxa01s photoemission doublet observed for this system do originate from the same surface species, and provides a quantitative determination of the local adsorption geometry. NO is found to adsorb N atom down ontop a Ni atom, with a N–Ni bond length of 1.88±0.02xa0A. The N–O bond is tilted away from the surface normal by 59(+31/−17)°.

The structure of NO on Ni(111) at low coverage

Abstract Scanned-energy mode photoelectron diffraction from the Nxa01s and Oxa01s core levels has been used to determine the local adsorption geometry of NO on Ni(111) at a coverage of 0.25xa0ML. The optimum surface geometry consists of NO molecules adsorbed in both the fcc and hcp three-fold hollow sites in a ratio of approximately 50/50 (the fcc site occupation is 50(−20/+17)% for the Nxa01s data, and 47(−30/+25)% for the Oxa01s data). N–Ni bond lengths are almost identical for the two sites: 1.83±0.07xa0A (fcc) and 1.85±0.07xa0A (hcp). We conclude that for NO adsorption on Ni(111), the fcc and hcp sites are energetically similar.