Scanned-energy Mode Photoelectron Diffraction Research Articles

Structural precursor to adsorbate-induced reconstruction: C on Ni(100)

The local structure around adsorbed carbon atoms on Ni(100) has been determined at low coverage as well as in the 0.5 monolayer $(2\ifmmode\times\else\texttimes\fi{}2)p4g$ ``clock'' reconstruction by scanned energy mode photoelectron diffraction. At low coverage, there is no radial strain of the Ni atoms surrounding the adsorbed carbon, contrary to previous suggestions. None of the C-Ni near-neighbor distances are changed by reconstruction, but the Ni-Ni nearest-neighbor distance in the top layer increases significantly, showing that the adsorbate-induced compressive stress is associated with Ni-Ni, rather than Ni-C, repulsion.

Determination of the adsorption geometry of ethylene on Ni{110} using photoelectron diffraction

Using scanned-energy mode photoelectron diffraction we have determined the adsorption geometry for the c(2×4) phase of ethylene on Ni{110} at saturation coverage of 0.5ML, and at a lower coverage of 0.2ML when no long-range ordered overlayer is seen. The structural optimisation is based on comparison of the experimental data with the results of multiple scattering calculations. For the c(2×4) phase the two molecules per primitive unit mesh occupy slightly different low-symmetry sites approximately midway between short-bridge and atop. There is a significant tilt of the CC axis with respect to the surface plane of about 10°. The CC axes of the molecules are preferentially aligned along the close-packed Ni rows, but are offset in 〈100〉 directions away from the ridges by about 0.2Å. At low coverage the corresponding modulation spectra are very similar to those of the ordered phase; detailed optimisation of the fits to these low coverage spectra confirms that the local adsorption site is also one of low symmetry between atop and bridge.

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

Analysis of scanned-energy mode photoelectron diffraction data from NO adsorbed on NiO(100) confirms that both components of the N1s 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±0.02Å. The N–O bond is tilted away from the surface normal by 59(+31/−17)°.

Structure determination of molecular adsorbates on oxide surfaces using scanned-energy mode photoelectron diffraction

Using N 1s scanned-energy mode photoelectron diffraction (PhD) combined with full multiple scattering simulations the local adsorption site of NO on NiO(100) has been determined. The molecule bonds through the N atom atop a surface layer N atom, while the N–O axis is tilted away from the surface normal by 59(+31/-17)°. The special problems presented by adsorbates on compound surfaces, for the direct inversion of PhD data to provide a first-order site determination, are discussed and some alternative schemes tested.

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

Scanned-energy mode photoelectron diffraction from the N 1s and O 1s core levels has been used to determine the local adsorption geometry of NO on Ni(111) at a coverage of 0.25 ML. 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 N 1s data, and 47(−30/+25)% for the O 1s data). N–Ni bond lengths are almost identical for the two sites: 1.83±0.07 Å (fcc) and 1.85±0.07 Å (hcp). We conclude that for NO adsorption on Ni(111), the fcc and hcp sites are energetically similar.

Photoelectron diffraction study of a catalytically active overlayer: C 2H 2 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 diffraction: this phase corresponds to the threshold coverage for the catalytic conversion of acetylene to benzene. The carbon atoms in the C 2H 2 molecule are located almost over bridge sites with a C–C bond length of 1.34+0.10 Å, 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 ( 3 × 3 ) 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.

Structural determination for H2O adsorption on Si(001)2 × 1 using scanned-energy mode photoelectron diffraction

Using scanned-energy-mode photoelectron diffraction we have determined the local adsorption geometry of the OH fragments adsorbed on Si(100)(2 × 1) surface. On this substrate water is known to adsorb dissociatively even at low temperature (90 K), which gives rise to a surface layer comprising coadsorbed OH and H species. The OH fragments are found to be adsorbed in off-atop sites at a dimerised surface Si atom with OSi bond-lengths of 1.7 ± 0.1Å and bond-angles relative to the surface normal of 22 ± 5°.

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

Scanned energy mode photoelectron diffraction measurements from the K 2p, O 1s and C 1s states of a Ni(111)(2 × 2)-K/ nCO ( 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 Å, 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 1.30Å, 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 adsorbed molecules

Electron and X-ray diffraction techniques now make it possible to determine the structural parameters of molecules adsorbed on single crystal metal surfaces in a straightforward way. Moreover, scanned energy mode photoelectron diffraction and diffuse low energy electron diffraction also provide access to adsorption layers that do not show long range order. Even the scanning tunnelling microscope can deliver some degree of quantitative information, in that adsorption site determination is now possible. Recent examples of the analysis of the structure of adsorbed molecules by these techniques include diatomic molecules and associated coadsorption systems, molecular fragments created in simple heterogeneous reactions and hydrocarbons.

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

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 Å 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.

Local Structure ofNH2onSi(100)−(2×1)and its Effect on the Asymmetry of the Si Surface Dimers

A scanned-energy mode photoelectron diffraction study of the $\mathrm{Si}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surface with adsorbed $\mathrm{NH}{}_{2}$ provides quantitative determination of key structural parameters previously only predicted from theoretical calculations. In particular, the $\mathrm{N}$ atoms occupy off-atop sites at a dimerized surface Si atom with N-Si bond lengths of $1.73\ifmmode\pm\else\textpm\fi{}0.08\AA{}$ and bond angle relative to the surface normal of $21\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}4\ifmmode^\circ\else\textdegree\fi{}$. The adsorption greatly reduces the Si dimer asymmetry relative to that of the clean surface.

Photoelectron diffraction determination of the structure of the Cu(100)c - Mn surface phase

The structure of the - Mn surface phase has been determined using scanned-energy mode photoelectron diffraction (PhD) from the Mn core level. The results confirm the earlier finding of a quantitative LEED study, namely that the surface layer comprises an ordered, strongly corrugated two-dimensional alloy in which the Mn atoms occupy substitutional sites. By using near-grazing as well as near-normal emission directions, the PhD data are found to be sensitive to the locations of both the alloy (approximately coplanar) and the underlying substrate Cu atoms relative to the Mn emitter. This can be seen both in direct `projection method' data inversion maps of the emitter environment and in full multiple-scattering simulations. The PhD data indicate a large outward relaxation of the Mn atoms relative to the surrounding alloy-layer Cu atoms , but this is consistent with the previous LEED study within the quoted precisions. There are, however, statistically significant differences between the LEED and PhD conclusions regarding some of the structural parameter values, most notably the layer spacing of the Mn atoms to the underlying pure Cu substrate layer.

A scanned-energy mode photoelectron diffraction study of the structure of Ni(111)(2 × 2)-O

A scanned-energy mode photoelectron diffraction study has been conducted of the Ni(111)(2 × 2)-O chemisorption phase using O 1s modulation spectra recorded in a range of different emission directions. The O atom is found to occupy the “fcc” hollow site, directly above third layer Ni atoms. The exact O-Ni outermost layer spacing and the possible magnitude of outermost substrate layer “rumpling” and lateral distortions have been investigated through comparisons with the results of multiple scattering theory calculations. The values obtained are compared with those given by several recent LEED studies, and the relative sensitivity of the two methods to some of these parameters is discussed. In particular, the problem of parameter-coupling in structure determination is investigated. The combination of the present results and those of the most recent LEED studies indicate that there is no significant lateral distortion of the outermost Ni layer.

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

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; NH 3 and NH x (NH or NH 2) adsorbed on Cu(110), CH 3O on Ni(111) and C 2H 2 on Ni(111).

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

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 Å.

Structure determination of a [formula omitted] coadsorption phase on Ni(111)

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.

Spatial probability distribution of adsorbate atoms.

In order to access the dynamics of surface diffusion or of surface chemical reactions it is necessary to determine the spatial probability distributions of the adsorbed atoms or molecules rather than their average positions. Here we demonstrate how such a distribution can be determined from scanned-energy mode photoelectron diffraction data using the maximum entropy method. As a test case we have chosen the distribution of the carbon atom in the methoxy species adsorbed on Ni(111). Both simulated and experimental data are used.

Direct methods for adsorbate structure determination using photoelectron diffraction

We describe the application of two direct methods for the transformation of scanned-energy mode photoelectron diffraction spectra which are capable of providing useful adsorption site identifications, and which form part of an integrated methodology for quantitative adsorbate structure determination. We draw some comparisons between the principles and proven success of this approach and that of ‘photoelectron holography’. Specific examples given include the adsorption sites of intact and partially reacted ammonia-derived species on Cu(110), the methoxy species, CH 3O-, on Ni(111), and the growth on Fe on Cu(111).

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 × 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) Å). 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.