Rachel L. Toomes

Structure determination of ammonia on Cu(110) — a low-symmetry adsorption site

Abstract The local adsorption structure of ammonia on Cu(110) has been determined in a quantitative fashion using N 1s scanned-energy mode photoelectron diffraction. While inversion of the photoelectron diffraction spectra using a direct method indicates that the adsorbed NH 3 molecules are near to atop sites, a fuller multiple scattering analysis shows that the molecule is actually displaced 0.37 A off the atop site in a 〈100〉 azimuth. The result is ascribed to adsorbate-adsorbate interactions (steric hindrance) similar to those found in (2 × 1)CO (pmg) structures seen on several fcc (110) surfaces, although in the case of ammonia, it occurs at coverages well below saturation, implying that adsorbate-adsorbate attraction also occurs. These general conclusions are entirely consistent with those of a recent ESDIAD study of this system.

Structural investigation of glycine on Cu(100) and comparison to glycine on Cu(110)

New O 1s and N 1s scanned-energy mode photoelectron diffraction (PhD) measurements and low energy electron diffraction observations from the Cu(100)(2×4)pg phase formed by deprotonated glycine, glycinate (NH2CH2COO–) have been used to determine the local structure of this adsorbed phase. The favored model involves bonding of both O atoms of the carboxylate and the N atom of the amino group in near atop sites with Cu–N and Cu–O distances of 2.05 A. This bonding geometry is similar to that of glycinate on Cu(110), but in this case the fact that the C–C backbone is aligned along 〈100〉 straggling the more widely spaced Cu atoms rows leads to a larger offset from atop of the O atoms. A reanalysis of O 1s PhD data from the Cu(110)(3×2)pg-glycinate surface shows that the two O atoms are inequivalent, with one O being offset by 0.29 A more than the other, leading to a twist of the molecule. The results are discussed in the light of other measurements on these surfaces and recent theoretical total energy calculati...

The structure of oxygen on Cu(100) at low and high coverages

The local adsorption structure of oxygen on Cu(1 0 0) has been studied using O 1s scanned-energy mode photoelectron diffraction. A detailed quantitative determination of the structure of the 0.5 ML (√2×2√2)R45°-O ordered phase confirms the missing-row character of this reconstruction and agrees well with earlier structural determinations of this phase by other methods, the adsorbed O atoms lying only approximately 0.1 A above the outermost Cu layer. At much lower coverages, the results indicate that the O atoms adopt unreconstructed hollow sites at a significantly larger O–Cu layer spacing, but with some form of local disorder. The best fit to these data is achieved with a two-site model involving O atoms at Cu–O layer spacings of 0.41 and 0.70 A in hollow sites; these two sites (also implied by an earlier electron-energy-loss study) are proposed to be associated with edge and centre positions in very small c(2×2) domains as seen in a recent scanning tunnelling microscopy investigation.

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.

The local adsorption geometry of CO and NH3 on NiO (100) determined by scanned-energy mode photoelectron diffraction

The local adsorption structures of CO and NH3 on NiO(1 0 0) have been determined by C 1s and N 1s scanned-energy mode photoelectron diffraction. CO adsorbs atop Ni surface atoms through the C atom in an essentially perpendicular geometry (tilt angle 12±12°) with a C–Ni nearest-neighbour distance of 2.07±0.02 A. NH3 also adsorbs atop Ni surface atoms with a N–Ni distance of 2.06±0.02 A. These bondlengths are only very slightly longer than the comparable values for adsorption on metallic Ni surfaces. By contrast theoretical values obtained from total energy calculations, which exist for CO adsorption on NiO(1 0 0) (2.46 A and 2.86 A) are very much longer than the experimental value. Similar discrepancies exist for the N–Ni nearest-neighbour bondlength for NO adsorbed on NiO(1 0 0). Combined with the published measurements of the desorption energies, which also exceed the calculated bonding energies, these results indicate a significant failure of current theoretical treatments to provide an effective description of molecular adsorbate bonding on NiO(1 0 0).

Structure determination of propyne and 3,3,3-trifluoropropyne on Cu(111)

The technique of scanned-energy photoelectron diffraction has been used to obtain detailed information on the adsorption site and internal structure of propyne (CH3CCH) and 3,3,3-trifluoropropyne (CF3CCH) on Cu(111). Both molecules are bound to the surface via the acetylenic unit in a site analogous to that previously found for ethyne on Cu(111). In particular, the acetylenic unit is parallel to the surface in a cross-bridging position such that one of the C atoms is above a fcc hollow site while the other is above a hcp hollow site, giving a C–C bond length of 1.47 A. The methyl/trifluoromethyl group is strongly tilted away from the surface and is attached with equal probability to the C atoms in the fcc and hcp hollow sites. The molecular plane is perpendicular to the surface. The fluoro group of CF3CCH is preferentially oriented in the cis conformation. From the LEED pattern and the calculated coverage of 0.25 ML, a possible overlayer arrangement for propyne and 3,3,3-trifluoropropyne on Cu(111) has be...

Photoelectron diffraction study of the Ag(110)-(2×1)-O reconstruction

The structure of the (2×1)-O adsorption phase on Ag(110) has been determined using scanned-energy mode photoelectron diffraction. The oxygen atoms have been found to occupy the long-bridge site and are almost coplanar with the top layer of silver atoms. The best agreement between multiple-scattering theory and experiment has been obtained for a missing-row (or equivalently an ‘added-row’) reconstruction in which the first-to-second and second-to-third interlayer spacings are 1.55±0.06 A and 1.33±0.06 A, respectively. Alternative buckled-row and unreconstructed surface models can be excluded.

Is PEXAFS really PhD

Abstract Scanned-energy mode photoelectron diffraction (PhD) measurements from the disordered chemisorption phase formed by oxygen on Cu(111), over a wide range of emission directions, are compared with published surface extended X-ray absorption fine structure (EXAFS) data from the same system. The results show that so-called photoemission EXAFS (‘PEXAFS’) data, even when measured over a wide angular acceptance from a disordered surface, do not reflect the true EXAFS spectrum but are actually dominated by photoelectron diffraction. Quantitative analysis of these data leads to the conclusion that there are likely to be serious systematic errors in all previous PEXAFS analyses.

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.