David I. Sayago

Quantitative determination of the local adsorption structure of carbonate on Ag(110)

The local geometry of carbonate (CO3) on Ag(1 1 0), formed by the reaction of CO2 with preadsorbed oxygen, has been investigated using C Is scanned-energy mode photoelectron diffraction. The carbonate species is essentially planar and adsorbs almost parallel to the surface in an off-atop site relative to an outermost layer Ag atom, at a C-Ag layer spacing of 2.64 +/- 0.09 Angstrom, with a well-defined azimuthal orientation. This geometry is best understood in terms of the added-row model proposed by Guo and Madix in which additional Ag atoms lie adjacent to the carbonate, such that bonding can occur through at least one of the oxygen atoms. The distance between this oxygen and its nearest neighbour Ag adatom is 1.90 +/- 0.42 Angstrom. The C-O distances are in the range 1.26-1.30 Angstrom. While the symmetry of the carbonate in the optimum structure is reduced, the D-3h symmetry of the isolated species lies within the limits of Precision

Structure determination of methanethiolate on unreconstructed Cu(111) by scanned-energy mode photoelectron diffraction

The local structure of methanethiolate, CH3S–, on an unreconstructed Cu(1 1 1) surface at low temperature, has been investigated by S 2p and C 1s scanned-energy mode photoelectron diffraction, with chemical state sensitivity. 71(+14/−16)% of the methanethiolate was found to occupy bridge sites, 29±14% to occupy fcc hollow sites and 0+19% to occupy hcp hollow sites. In the bridge site the layer spacing of the sulphur atom to the outermost substrate layer is 1.87±0.03 A giving a Cu–S bondlength of 2.27±0.03 A. The methanethiolate adsorbed in the fcc hollow site has a Cu–S layer spacing of 1.73±0.04 A, corresponding to the same bondlength of 2.27±0.04 A. The S–C bondlength was found to be 1.92±0.10 A. These conclusions are consistent with the results of previous X-ray standing wave and scanning tunnelling microscopy studies for a common model involving co-occupation of bridge and hollow sites, although differing relative occupations and long-range ordering are thought to arise from different preparation conditions. The new data favour a model in which the S–C bond axis of the bridge-bound thiolate is tilted by 45±12° away from the surface normal in the azimuth directed towards the fcc hollow site.

The structure of the Ni(100)c(2×2)–N2 surface: a chemical-state-specific scanned-energy mode photoelectron diffraction determination

Using the chemical shift in the N 1s photoemission peak from the two inequivalent N atoms of N2 adsorbed on Ni(1 0 0) we have performed a scanned-energy mode photoelectron diffraction (PhD) structure determination of the Ni(1 0 0)c(2 × 2)–N2 weak chemisorption system. The N2 is found to adsorb atop surface Ni atoms with the N–N axis perpendicular to the surface at a Ni–N nearest-neighbour distance of 1.81 ± 0.02 A. This is very significantly shorter than the value (2.25 A) found in an earlier published study. An independent density-functional theory slab calculation yields a value of 1.79 A, in excellent agreement with the results of the current experiment. Analysis of the PhD modulations of the N 1s photoemission satellite peak show that these are consistent with this comprising separable components localised at the two N atoms as has previously been assumed in an earlier investigation based on (angle-scan) X-ray photoelectron diffraction. Both experiment and theory indicate a small extension of the N–N distance due to the adsorption (0.03 ± 0.03 A and 0.02 A respecti

Can circular dichroism in core-level photoemission provide a spectral fingerprint of adsorbed chiral molecules?

The results of experimental measurements and theoretical simulations of circular dichroism in the angular distribution (CDAD) of photoemission from atomic core levels of each of the enantiomers of a chiral molecule, alanine, adsorbed on Cu(1 1 0) are presented. Measurements in, and out of, substrate mirror planes allow one to distinguish the CDAD due to the chirality of the sample from that due to a chiral experimental geometry. For these studies of oriented chiral molecules, the CDAD is seen not only in photoemission from the molecular chiral centre, but also from other atoms which have chiral geometries as a result of the adsorption. The magnitude of the CDAD due to the sample chirality differs for different adsorption phases of alanine, and for different emission angles and energies, but is generally small compared with CDAD out of the substrate mirror planes which is largely unrelated to the molecular chirality. While similar measurements of other molecules may reveal larger CDAD due to molecular chirality, the fact that the results for one chiral molecule show weak effects means that such CDAD is unlikely to provide a simple and routine general spectral fingerprint of adsorbed molecular chirality.