V. Fernandez

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.

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.

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.

Quantitative structure determination of an NHx species adsorbed on Cu(110)

Abstract Using scanned energy mode photoelectron diffraction we have recently investigated the local structure of NH 3 and NH x ( x = 1 or 2) on Cu(110). NH 3 was found to occupy the atop site on the closed-packed Cu rows in the first layer. The NH x species, discussed in more detail in the present paper, occupies the short bridge site on the outermost 〈110〉 rows, at a height of 1.40 ± 0.02 A above the surface, resulting in a CuN nearest neighbour distance of 1.89 A.

Structure determination of a COO coadsorption phase on Ni(111)

Abstract The structure of the Ni(111)(2 × 2)OCO adsorption phase, formed by CO adsorption onto a Ni(111)(2 × 2)O phase, has been determined by scanned-energy mode photoelectron diffraction of both the C 1s and O 1s signals. Contrary to earlier interpretation of vibrational spectroscopic data, the CO molecules occupy the same threefold coordinated hollow sites which they adopt on the clean Ni(111) surface, with a NiC bondlength of 1.95 ± 0.04 A and a CO bondlength of 1.17 ± 0.03 A . Changes in bondlength and the vibrational amplitude of the CO molecule against the surface are related to the weaker CO-metal bonding in the presence of the atomic oxygen. The chemisorbed oxygen atoms occupy the same “fcc” hollow sites, directly above third layer Ni atoms, both before and after CO adsorption.

Quantitative determination of molecular adsorption structures using photoelectron diffraction: the methoxy species

Abstract Photoelectron diffraction in the scanned-energy mode is now an accepted method in surface structural analysis. Here we demonstrate its use in the determination of the local adsorption geometry of the methoxy species CH 3 O- on a Ni(111) surface. The bonding site of the O atom at an “fcc” hollow site directly above a Ni atom in the third layer is obtained from the experimental data using direct methods, and the exact NiO spacing and degree of local substrate distortion, together with the molecular orientation, are found by the use of multiple scattering simulations. A re-analysis of earlier data obtained for the same species on Cu(111) confirms the results of this earlier study and has allowed an evaluation og possible subtle differences in the adsorbate-induced substrate distortion despite the occupation of the same adsorption site and very similar O-metal nearest neighbour distances.

An integrated approach to adsorbate structure determination using photoelectron diffraction : direct imaging and quantitative simulation

Abstract Using scanned-energy mode photoelectron diffraction (PhD) we have developed a two-stage methodology for the quantitative determination of the local geometry of molecular adsorbates on surfaces. The first stage involves the inversion of the experimental adsorbate photoelectron diffraction spectra using a direct method to obtain an “image” of the nearest neighbour substrate atoms. The underlying physics is essentially the same as that of inversion in photoelectron holography, but our method has been shown to be effective for many cases and does not require the collection of additional data. The second stage is to optimise the detailed structure indicated by this “image” using an iterative trial-and-error comparison of the same experimental spectra with the results of simulations based on multiple scattering calculations. Specific examples of this approach which relate to ammonia, CO and hydrocarbon surface chemistry are outlined in this short review; NH3 and NHx (NH or NH2) adsorbed on Cu(110), CH3O on Ni(111) and C2H2 on Ni(111).

Quantitative structural study of the coadsorption of CO and K on Ni(111) using photoelectron diffraction

Abstract Scanned energy mode photoelectron diffraction measurements from the K 2p, O 1s and C 1s states of a Ni(111)(2 × 2)-K/ n CO ( n ≈ 2) coadsorption structure have been coupled with multiple scattering simulations to determine the local adsorption structure. This structure is compared with the structure of the Ni(111)c(4 × 2)-CO phase obtained from a similar analysis of C 1s and O 1s photoelectron diffraction data (which extends earlier work based on the C 1s signal alone), and of the structure of the Ni(111)(2 × 2)-K phase based on a previously-published analysis of K 2p and K 2s photoelectron diffraction. In the CO coadsorption phase the K atoms retain the atop site of the (2 × 2)-K phase but with a K Ni bondlength increased by 0.15 ±0.05 A, and with the outermost Ni layer contraction of the (2 × 2)-K phase replaced by an outer layer expansion. The CO molecules occupy both types of hollow site on the surface with a C Ni outermost layer spacing of approximately 1.30A, but the inferred C O bondlength shows no statistically significant change from its value in the absence of coadsorbed K.

Structure determination of using scanned-energy mode photoelectron diffraction

A scanned-energy mode photoelectron diffraction study of the surface with adsorbed has provided quantitative determination of key structural parameters previously only predicted from theoretical calculations. The N atoms are found to occupy off-atop sites at a dimerized surface Si atom with an N - Si bondlength of and bond angle relative to the surface normal of . The positions of Si atoms in the dimer to which the adsorbates are bonded are found to indicate that the marked asymmetry of this dimer on the clean surface is lost as a result of the adsorption. The conclusions are discussed in relation to published results from less direct experimental probes and the results of theoretical model calculations.

The structure of the surface methoxy species on Ni{111}

Using photoelectron diffraction in the scanned energy mode the structure of the surface methoxy species (CH 3 O-) on Ni{111} has been determined. Bonding occurs at the fcc threefold symmetric hollow site with a Ni-O bond length of 1.94 A; the O-C axis is perpendicular to the surface.