Martin Polcik - Academia.edu (original) (raw)

Papers by Martin Polcik

Research paper thumbnail of The structure of oxygen on Cu(1 0 0) at low and high coverages

Surface Science, 2001

The local adsorption structure of oxygen on Cu(1 0 0) has been studied using O 1s scanned-energy ... more The local adsorption structure of oxygen on Cu(1 0 0) has been studied using O 1s scanned-energy mode photoelectron diraction. A detailed quantitative determination of the structure of the 0.5 ML p 2 Â 2 p 2R45°-O ordered phase con®rms 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 signi®cantly larger O±Cu layer spacing, but with some form of local disorder. The best ®t 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 c2 Â 2 domains as seen in a recent scanning tunnelling microscopy investigation.

Research paper thumbnail of The adsorption site of pyridine on Ni{111} determined by low-energy photoelectron diffraction

Surface Science, 1994

ABSTRACT

Research paper thumbnail of High-resolution XPS and NEXAFS study of SO2 adsorption on Pt(111): two surface SO2 species

Surface Science, 1997

The adsorption and condensation as well as the temperature-dependent decomposition of SO2 on Pt(1... more The adsorption and condensation as well as the temperature-dependent decomposition of SO2 on Pt(111) have been studied by high-resolution core-level photoemission and near-edge X-ray absorption fine structure. The data analysis shows that SO2 adsorbs molecularly at 150 K forming a monolayer phase with two SO2 species of different binding energy and quite different molecular orientation. On increasing the temperature to 210 K only SO2 molecules of 0.43 monolayer coverage survive with molecular planes being tilted by c~ = 31 °+ 10 ° with respect to the surface normal. For condensed SO 2 multilayers an SO 4 species is formed close to 300 K.

Research paper thumbnail of Methyl on Cu(111)––structural determination including influence of co-adsorbed iodine

Surface Science, 2002

The adsorption geometry of the methyl species on Cu(1 1 1) with and without coadsorbed iodine has... more The adsorption geometry of the methyl species on Cu(1 1 1) with and without coadsorbed iodine has been determined using scanned energy mode photoelectron diffraction. Under all circumstances the threefold coordinated hollow sites are occupied. At saturation coverage of pure methyl species only the 'fcc' site, directly above a third layer Cu atom is occupied, whereas at half saturation coverage 7070% of the methyl species occupy these fcc hollows and 7030% occupy the 'hcp' sites above second layer Cu atoms. Best agreement between theory and experiment corresponded to a methyl group adsorbed with C 3v symmetry, but the possibility that the species was tilted on the surface could not be excluded. The height of the C above the surface in a pure methyl layer was 1:66 AE 0:02 A A, but was reduced to 1:62 AE 0:02 A A in the presence of co-adsorbed iodine, suggesting that iodine may increase the strength of adsorption. Iodine was also found to occupy the fcc hollow sites with a Cu-I bondlength of 2:61 AE 0:02 A A.

Research paper thumbnail of Multiple-scattering-theory analysis of XANES spectra of SO2 adsorbed on Cu(100)

Research paper thumbnail of Impact of bulk reduction on TiO2(100)/K

Surface Science, 2004

Photoelectron spectroscopy, low energy electron diffraction, and reflection high energy electron ... more Photoelectron spectroscopy, low energy electron diffraction, and reflection high energy electron diffraction have been employed to probe the electronic and geometric structure of the TiO 2 (1 0 0)/K interface. Data have been obtained from two (1 0 0) oriented single crystal samples, which differ in their extent of bulk reduction. It is demonstrated that this parameter is important for both the electronic and geometric structure of ordered TiO 2 (1 0 0)/K surfaces, formed by annealing to $1000 K. Photoelectron spectra, recorded following this preparation, indicate that reduced Ti states are present for the less stoichiometric sample, which exhibits a 1 À1 2 2 surface unit cell. In contrast, the other sample displays a cð2 Â 2Þ overlayer, with no Ti reduction evidenced.

Research paper thumbnail of Structural characterisation of ultra-thin VOx films on TiO2(110)

Surface Science, 2006

The normal incidence X-ray standing wave (NIXSW) technique has been applied to investigate the st... more The normal incidence X-ray standing wave (NIXSW) technique has been applied to investigate the structure of ultra-thin VO x films grown on TiO 2 (1 1 0) and pre-characterised by core level photoemission. For a film composed of a sub-monolayer coverage of V deposited in ultra-high vacuum the local structure of two coexistent species, labelled 'oxidic' and 'metallic', has been investigated independently through the use of chemical-shift-NIXSW. The 'oxidic' state is shown to be consistent with a mixture of epitaxial or substitutional sites and chemisorption into sites coordinated to three surface O atoms. The metallic V atoms also involve a mixture of chemisorption and second-layer sites above the substrate surface consistent with the formation of small V clusters. VO x films up to $6 atomic layers were also grown by post-oxidation (sequential V deposition and annealing in oxygen) and by reactive evaporation in a partial pressure of oxygen. While films of around one monolayer or less are consistent with epitaxial VO 2 growth, the film quality deteriorates rapidly with increasing thickness and is worse for reactive evaporation. A possible interpretation of the NIXSW data is increasing contributions of V 2 O 3 crystallites. The inferior quality of the reactively evaporated films may be due to an insufficient supply of oxygen.

Research paper thumbnail of Local Structure of CO Coadsorbed with O on Ni(111):  A Temperature-Dependent Study †

The Journal of Physical Chemistry B, 2001

Using C 1s scanned-energy mode photoelectron diffraction (PhD), the local geometry of CO adsorbed... more Using C 1s scanned-energy mode photoelectron diffraction (PhD), the local geometry of CO adsorbed onto a Ni(111)(2×2)-O surface has been investigated as a function of the temperature of dosing or subsequent annealing. Around room temperature the CO adopts local atop sites in agreement with the previous interpretation of vibrational spectroscopy, but at low temperature a substantial fraction of the CO molecules adopt the hollow sites occupied in the absence of the preadsorbed O, generally consistent with the results of an earlier PhD study. Heating such a surface leads to a new state in which only atop sites are occupied, but this appears to be a result of desorption of the hollow species rather than any transformation of sites. The results confirm the qualitative site occupations deduced in a recent study of this system which used C 1s and O 1s photoelectron binding energy shifts to fingerprint the local site changes.

Research paper thumbnail of Structure determination of Cu(210)–(1×2)-O by use of SEXAFS measurements

Surface Science, 1998

Polarisation-dependent surface-extended X-ray-absorption fine-structure experiments have been per... more Polarisation-dependent surface-extended X-ray-absorption fine-structure experiments have been performed on Cu(210)-(1×2)-O. The analysis shows that oxygen chemisorbs in long-bridge sites along the [001] direction with nearest-neighbour and next-nearestneighbour O-Cu distances of 1.81±0.02 Å and 1.91±0.03 Å respectively. A missing-row reconstruction with missing [001] Cu rows is suggested.

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

Surface Science, 2002

The local geometry of carbonate (CO 3) on Ag(1 1 0), formed by the reaction of CO 2 with preadsor... more The local geometry of carbonate (CO 3) on Ag(1 1 0), formed by the reaction of CO 2 with preadsorbed oxygen, has been investigated using C 1s 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 AE 0:09 A A, 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 nearestneighbour Ag adatom is 1:90 AE 0:42 A A. The CO distances are in the range 1.26-1.30 A A. 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.

Research paper thumbnail of Can glycine form homochiral structural domains on low-index copper surfaces?

Surface Science, 2003

Suggestions that homochiral ordered phases of glycinate, NH 2 CH 2 COO-, may exist on Cu(1 1 0) a... more Suggestions that homochiral ordered phases of glycinate, NH 2 CH 2 COO-, may exist on Cu(1 1 0) and Cu(1 0 0) have been evaluated in the light of experimental evidence from scanning tunnelling microscopy (STM), qualitative low energy electron diffraction and scanned-energy mode photoelectron diffraction. There is clear evidence for heterochiral Cu(1 1 0)(3 Â 2)pg and Cu(1 0 0)(4 Â 2)pg phases. All proposed models for a homochiral (3 Â 2) phase on Cu(1 1 0) are inconsistent with the results of at least one of the experimental studies. A coexistent c(4 Â 2) homochiral phase on Cu(1 0 0) cannot be excluded on a similar basis. However, the only evidence for such homochiral phases, obtained from STM, may be attributed to an experimental artefact associated with an asymmetric tunnelling tip.

Research paper thumbnail of Local adsorption sites and bondlength changes in Ni/H/CO and Ni/CO

Surface Science, 2003

The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) pr... more The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) precovered with atomic hydrogen has been determined by C 1s (and O 1s) scanned-energy mode photoelectron diffraction, using the photoelectron binding energy changes to characterise the different states. The results confirm previous spectroscopic assignments of local atop and bridge sites both with and without coadsorbed hydrogen. The measured Ni-C bondlengths for the Ni(1 0 0)/CO states show an increase of 0.16 AE 0.04 A A in going from atop to bridge sites, while comparison with similar results for Ni(1 1 1)/CO for threefold coordinated adsorption sites show a further lengthening of the bond by 0.05 AE 0.04 A A. These changes in the Ni-CO chemisorption bondlength with bond order (for approximately constant adsorption energy) are consistent with the standard Pauling rules. However, comparison of CO adsorbed in the atop geometry with and without coadsorbed hydrogen shows that the coadsorption increases the Ni-C bondlength by only 0.06 AE 0.04 A A, despite the decrease in adsorption energy of a factor of 2 or more. This result is also reproduced by density functional theory slab calculations. The results of both the experiments and the density functional theory calculations show that CO adsorption onto the Ni(1 0 0)/H surface is accompanied by significant structural modification; the low desorption energy may then be attributed to the energy cost of this restructuring rather than weak local bonding.

Research paper thumbnail of Surface to bulk charge transfer at an alkali metal/metal oxide interface

Surface Science, 2003

Photoelectron spectroscopy has been employed to probe the electronic structure of TiO 2 (1 0 0)c(... more Photoelectron spectroscopy has been employed to probe the electronic structure of TiO 2 (1 0 0)c(2 • 2)K. In sharp contrast to earlier work, no features indicating the presence of Ti 3þ cations are found in the photoelectron spectra. This unexpected result is rationalised on the basis of ab initio periodic slab calculations. These calculations demonstrate that in this system K is fully ionised (K fi K þ), and that the majority of the charge transferred across the TiO 2 /K interface resides on Ti cations, reducing them formally from Ti 4þ to Ti 3þ. However, the excess charge is not restricted to near surface Ti ions, but rather can occupy any Ti site throughout the bulk, and so is unobservable by photoelectron spectroscopy, due to the negligible concentration of Ti 3þ cations in the surface region.

Research paper thumbnail of Adsorption geometry of CN on Cu(111) and Cu(111)/O

Surface Science, 2004

The adsorption geometry of CN on Cu(1 1 1), both with and without predosing with oxygen, has been... more The adsorption geometry of CN on Cu(1 1 1), both with and without predosing with oxygen, has been investigated using N K-edge near-edge X-ray absorption fine structure (NEXAFS) and C 1s and N 1s scanned-energy mode photoelectron diffraction (PhD). The NEXAFS shows clearly that adsorbed onto clean Cu(1 1 1) the C-N axis is closely parallel to the surface, but in the presence of coadsorbed oxygen the average orientation has the axis tilted by 25°away from the surface; this confirms a much earlier report of an oxygen-induced reorientation of CN on this surface based on vibrational spectroscopy. The PhD data show very weak modulations which are rather insensitive to the emission geometry, clearly implying a high degree of disorder or a local adsorption site well-removed from any position of high point group symmetry. The best-fit structure corresponds to the CN lying slightly displaced from the threefold coordinated hollow sites but with the C and N atoms having single Cu atom nearest neighbours at distances of 1.98 ± 0.05 and 2.00 ± 0.05 A respectively.

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

Surface Science, 2003

Using the chemical shift in the N 1s photoemission peak from the two inequivalent N atoms of N 2 ... more Using the chemical shift in the N 1s photoemission peak from the two inequivalent N atoms of N 2 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)-N 2 weak chemisorption system. The N 2 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 A. This is very significantly shorter than the value (2.25 A A) found in an earlier published study. An independent density-functional theory slab calculation yields a value of 1.79 A 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 (anglescan) 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 A and 0.02 A A respectively).

Research paper thumbnail of Local adsorption geometry of 2-methyl-pyridine on Cu(110) determined by photoelectron diffraction

Surface Science, 2000

Using N 1s scanned-energy mode photoelectron diffraction, together with N K-edge near-edge absorp... more Using N 1s scanned-energy mode photoelectron diffraction, together with N K-edge near-edge absorption fine structure, the local adsorption site and molecular orientation of 2-methyl-pyridine adsorbed on Cu(110) has been determined and the results are compared with a similar study of pyridine adsorption on the same surface. The molecule is found to bond to the surface through the N atom that occupies a near-atop site. The molecular plane is almost perpendicular to the surface but twisted into an azimuth away from the two principal high-symmetry directions of the surface. The aromatic ring is also tilted within the molecular plane such as to increase the separation of the methyl group away from the surface.

Research paper thumbnail of Is PEXAFS really PhD?

Surface Science, 2000

ABSTRACT Scanned-energy mode photoelectron diffraction (PhD) measurements from the disordered che... more 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.

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

Surface Science, 2002

The local adsorption structures of CO and NH 3 on NiO(1 0 0) have been determined by C 1s and N 1... more The local adsorption structures of CO and NH 3 on NiO(1 0 0) have been determined by C 1s and N 1s scannedenergy mode photoelectron diffraction. CO adsorbs atop Ni surface atoms through the C atom in an essentially perpendicular geometry (tilt angle 12 AE 12°) with a C-Ni nearest-neighbour distance of 2:07 AE 0:02 A A. NH 3 also adsorbs atop Ni surface atoms with a N-Ni distance of 2:06 AE 0:02 A 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 A and 2.86 A 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).

Research paper thumbnail of Photoelectron diffraction structure determination of Cu(1 0 0)c(2×2)-N

Surface Science, 2001

Using scanned-energy mode photoelectron diraction of the N 1s photoemission, combined with multip... more Using scanned-energy mode photoelectron diraction of the N 1s photoemission, combined with multiple scattering modelling calculations, the structure of the Cu(1 0 0)c2 Â 2-N has been investigated. While the results are broadly consistent with N adsorption in a fourfold coordinated hollow site, almost coplanar with the outermost layer, this model can only be ®tted to the data with unphysically large vibrational amplitudes, strongly suggesting that some static adsorbate-induced substrate distortion is involved. The best-®t structure involves an unusual symmetry-lowering large amplitude (0.34 A) rumpling of the outermost Cu layer. This result is entirely consistent with the results of a recent scanning tunnelling microscopy study of this surface.

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

Surface Science, 2002

The local structure of methanethiolate, CH 3 Son on an unreconstructed Cu(1 1 1) surface at low t... more The local structure of methanethiolate, CH 3 Son 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. 71ðþ14=À16Þ% of the methanethiolate was found to occupy bridge sites, 29 AE 14% to occupy fcc hollow sites and 0 þ 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 AE 0:03 A A giving a Cu-S bondlength of 2:27 AE 0:03 A A.

Research paper thumbnail of The structure of oxygen on Cu(1 0 0) at low and high coverages

Surface Science, 2001

The local adsorption structure of oxygen on Cu(1 0 0) has been studied using O 1s scanned-energy ... more The local adsorption structure of oxygen on Cu(1 0 0) has been studied using O 1s scanned-energy mode photoelectron diraction. A detailed quantitative determination of the structure of the 0.5 ML p 2 Â 2 p 2R45°-O ordered phase con®rms 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 signi®cantly larger O±Cu layer spacing, but with some form of local disorder. The best ®t 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 c2 Â 2 domains as seen in a recent scanning tunnelling microscopy investigation.

Research paper thumbnail of The adsorption site of pyridine on Ni{111} determined by low-energy photoelectron diffraction

Surface Science, 1994

ABSTRACT

Research paper thumbnail of High-resolution XPS and NEXAFS study of SO2 adsorption on Pt(111): two surface SO2 species

Surface Science, 1997

The adsorption and condensation as well as the temperature-dependent decomposition of SO2 on Pt(1... more The adsorption and condensation as well as the temperature-dependent decomposition of SO2 on Pt(111) have been studied by high-resolution core-level photoemission and near-edge X-ray absorption fine structure. The data analysis shows that SO2 adsorbs molecularly at 150 K forming a monolayer phase with two SO2 species of different binding energy and quite different molecular orientation. On increasing the temperature to 210 K only SO2 molecules of 0.43 monolayer coverage survive with molecular planes being tilted by c~ = 31 °+ 10 ° with respect to the surface normal. For condensed SO 2 multilayers an SO 4 species is formed close to 300 K.

Research paper thumbnail of Methyl on Cu(111)––structural determination including influence of co-adsorbed iodine

Surface Science, 2002

The adsorption geometry of the methyl species on Cu(1 1 1) with and without coadsorbed iodine has... more The adsorption geometry of the methyl species on Cu(1 1 1) with and without coadsorbed iodine has been determined using scanned energy mode photoelectron diffraction. Under all circumstances the threefold coordinated hollow sites are occupied. At saturation coverage of pure methyl species only the 'fcc' site, directly above a third layer Cu atom is occupied, whereas at half saturation coverage 7070% of the methyl species occupy these fcc hollows and 7030% occupy the 'hcp' sites above second layer Cu atoms. Best agreement between theory and experiment corresponded to a methyl group adsorbed with C 3v symmetry, but the possibility that the species was tilted on the surface could not be excluded. The height of the C above the surface in a pure methyl layer was 1:66 AE 0:02 A A, but was reduced to 1:62 AE 0:02 A A in the presence of co-adsorbed iodine, suggesting that iodine may increase the strength of adsorption. Iodine was also found to occupy the fcc hollow sites with a Cu-I bondlength of 2:61 AE 0:02 A A.

Research paper thumbnail of Multiple-scattering-theory analysis of XANES spectra of SO2 adsorbed on Cu(100)

Research paper thumbnail of Impact of bulk reduction on TiO2(100)/K

Surface Science, 2004

Photoelectron spectroscopy, low energy electron diffraction, and reflection high energy electron ... more Photoelectron spectroscopy, low energy electron diffraction, and reflection high energy electron diffraction have been employed to probe the electronic and geometric structure of the TiO 2 (1 0 0)/K interface. Data have been obtained from two (1 0 0) oriented single crystal samples, which differ in their extent of bulk reduction. It is demonstrated that this parameter is important for both the electronic and geometric structure of ordered TiO 2 (1 0 0)/K surfaces, formed by annealing to $1000 K. Photoelectron spectra, recorded following this preparation, indicate that reduced Ti states are present for the less stoichiometric sample, which exhibits a 1 À1 2 2 surface unit cell. In contrast, the other sample displays a cð2 Â 2Þ overlayer, with no Ti reduction evidenced.

Research paper thumbnail of Structural characterisation of ultra-thin VOx films on TiO2(110)

Surface Science, 2006

The normal incidence X-ray standing wave (NIXSW) technique has been applied to investigate the st... more The normal incidence X-ray standing wave (NIXSW) technique has been applied to investigate the structure of ultra-thin VO x films grown on TiO 2 (1 1 0) and pre-characterised by core level photoemission. For a film composed of a sub-monolayer coverage of V deposited in ultra-high vacuum the local structure of two coexistent species, labelled 'oxidic' and 'metallic', has been investigated independently through the use of chemical-shift-NIXSW. The 'oxidic' state is shown to be consistent with a mixture of epitaxial or substitutional sites and chemisorption into sites coordinated to three surface O atoms. The metallic V atoms also involve a mixture of chemisorption and second-layer sites above the substrate surface consistent with the formation of small V clusters. VO x films up to $6 atomic layers were also grown by post-oxidation (sequential V deposition and annealing in oxygen) and by reactive evaporation in a partial pressure of oxygen. While films of around one monolayer or less are consistent with epitaxial VO 2 growth, the film quality deteriorates rapidly with increasing thickness and is worse for reactive evaporation. A possible interpretation of the NIXSW data is increasing contributions of V 2 O 3 crystallites. The inferior quality of the reactively evaporated films may be due to an insufficient supply of oxygen.

Research paper thumbnail of Local Structure of CO Coadsorbed with O on Ni(111):  A Temperature-Dependent Study †

The Journal of Physical Chemistry B, 2001

Using C 1s scanned-energy mode photoelectron diffraction (PhD), the local geometry of CO adsorbed... more Using C 1s scanned-energy mode photoelectron diffraction (PhD), the local geometry of CO adsorbed onto a Ni(111)(2×2)-O surface has been investigated as a function of the temperature of dosing or subsequent annealing. Around room temperature the CO adopts local atop sites in agreement with the previous interpretation of vibrational spectroscopy, but at low temperature a substantial fraction of the CO molecules adopt the hollow sites occupied in the absence of the preadsorbed O, generally consistent with the results of an earlier PhD study. Heating such a surface leads to a new state in which only atop sites are occupied, but this appears to be a result of desorption of the hollow species rather than any transformation of sites. The results confirm the qualitative site occupations deduced in a recent study of this system which used C 1s and O 1s photoelectron binding energy shifts to fingerprint the local site changes.

Research paper thumbnail of Structure determination of Cu(210)–(1×2)-O by use of SEXAFS measurements

Surface Science, 1998

Polarisation-dependent surface-extended X-ray-absorption fine-structure experiments have been per... more Polarisation-dependent surface-extended X-ray-absorption fine-structure experiments have been performed on Cu(210)-(1×2)-O. The analysis shows that oxygen chemisorbs in long-bridge sites along the [001] direction with nearest-neighbour and next-nearestneighbour O-Cu distances of 1.81±0.02 Å and 1.91±0.03 Å respectively. A missing-row reconstruction with missing [001] Cu rows is suggested.

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

Surface Science, 2002

The local geometry of carbonate (CO 3) on Ag(1 1 0), formed by the reaction of CO 2 with preadsor... more The local geometry of carbonate (CO 3) on Ag(1 1 0), formed by the reaction of CO 2 with preadsorbed oxygen, has been investigated using C 1s 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 AE 0:09 A A, 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 nearestneighbour Ag adatom is 1:90 AE 0:42 A A. The CO distances are in the range 1.26-1.30 A A. 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.

Research paper thumbnail of Can glycine form homochiral structural domains on low-index copper surfaces?

Surface Science, 2003

Suggestions that homochiral ordered phases of glycinate, NH 2 CH 2 COO-, may exist on Cu(1 1 0) a... more Suggestions that homochiral ordered phases of glycinate, NH 2 CH 2 COO-, may exist on Cu(1 1 0) and Cu(1 0 0) have been evaluated in the light of experimental evidence from scanning tunnelling microscopy (STM), qualitative low energy electron diffraction and scanned-energy mode photoelectron diffraction. There is clear evidence for heterochiral Cu(1 1 0)(3 Â 2)pg and Cu(1 0 0)(4 Â 2)pg phases. All proposed models for a homochiral (3 Â 2) phase on Cu(1 1 0) are inconsistent with the results of at least one of the experimental studies. A coexistent c(4 Â 2) homochiral phase on Cu(1 0 0) cannot be excluded on a similar basis. However, the only evidence for such homochiral phases, obtained from STM, may be attributed to an experimental artefact associated with an asymmetric tunnelling tip.

Research paper thumbnail of Local adsorption sites and bondlength changes in Ni/H/CO and Ni/CO

Surface Science, 2003

The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) pr... more The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) precovered with atomic hydrogen has been determined by C 1s (and O 1s) scanned-energy mode photoelectron diffraction, using the photoelectron binding energy changes to characterise the different states. The results confirm previous spectroscopic assignments of local atop and bridge sites both with and without coadsorbed hydrogen. The measured Ni-C bondlengths for the Ni(1 0 0)/CO states show an increase of 0.16 AE 0.04 A A in going from atop to bridge sites, while comparison with similar results for Ni(1 1 1)/CO for threefold coordinated adsorption sites show a further lengthening of the bond by 0.05 AE 0.04 A A. These changes in the Ni-CO chemisorption bondlength with bond order (for approximately constant adsorption energy) are consistent with the standard Pauling rules. However, comparison of CO adsorbed in the atop geometry with and without coadsorbed hydrogen shows that the coadsorption increases the Ni-C bondlength by only 0.06 AE 0.04 A A, despite the decrease in adsorption energy of a factor of 2 or more. This result is also reproduced by density functional theory slab calculations. The results of both the experiments and the density functional theory calculations show that CO adsorption onto the Ni(1 0 0)/H surface is accompanied by significant structural modification; the low desorption energy may then be attributed to the energy cost of this restructuring rather than weak local bonding.

Research paper thumbnail of Surface to bulk charge transfer at an alkali metal/metal oxide interface

Surface Science, 2003

Photoelectron spectroscopy has been employed to probe the electronic structure of TiO 2 (1 0 0)c(... more Photoelectron spectroscopy has been employed to probe the electronic structure of TiO 2 (1 0 0)c(2 • 2)K. In sharp contrast to earlier work, no features indicating the presence of Ti 3þ cations are found in the photoelectron spectra. This unexpected result is rationalised on the basis of ab initio periodic slab calculations. These calculations demonstrate that in this system K is fully ionised (K fi K þ), and that the majority of the charge transferred across the TiO 2 /K interface resides on Ti cations, reducing them formally from Ti 4þ to Ti 3þ. However, the excess charge is not restricted to near surface Ti ions, but rather can occupy any Ti site throughout the bulk, and so is unobservable by photoelectron spectroscopy, due to the negligible concentration of Ti 3þ cations in the surface region.

Research paper thumbnail of Adsorption geometry of CN on Cu(111) and Cu(111)/O

Surface Science, 2004

The adsorption geometry of CN on Cu(1 1 1), both with and without predosing with oxygen, has been... more The adsorption geometry of CN on Cu(1 1 1), both with and without predosing with oxygen, has been investigated using N K-edge near-edge X-ray absorption fine structure (NEXAFS) and C 1s and N 1s scanned-energy mode photoelectron diffraction (PhD). The NEXAFS shows clearly that adsorbed onto clean Cu(1 1 1) the C-N axis is closely parallel to the surface, but in the presence of coadsorbed oxygen the average orientation has the axis tilted by 25°away from the surface; this confirms a much earlier report of an oxygen-induced reorientation of CN on this surface based on vibrational spectroscopy. The PhD data show very weak modulations which are rather insensitive to the emission geometry, clearly implying a high degree of disorder or a local adsorption site well-removed from any position of high point group symmetry. The best-fit structure corresponds to the CN lying slightly displaced from the threefold coordinated hollow sites but with the C and N atoms having single Cu atom nearest neighbours at distances of 1.98 ± 0.05 and 2.00 ± 0.05 A respectively.

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

Surface Science, 2003

Using the chemical shift in the N 1s photoemission peak from the two inequivalent N atoms of N 2 ... more Using the chemical shift in the N 1s photoemission peak from the two inequivalent N atoms of N 2 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)-N 2 weak chemisorption system. The N 2 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 A. This is very significantly shorter than the value (2.25 A A) found in an earlier published study. An independent density-functional theory slab calculation yields a value of 1.79 A 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 (anglescan) 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 A and 0.02 A A respectively).

Research paper thumbnail of Local adsorption geometry of 2-methyl-pyridine on Cu(110) determined by photoelectron diffraction

Surface Science, 2000

Using N 1s scanned-energy mode photoelectron diffraction, together with N K-edge near-edge absorp... more Using N 1s scanned-energy mode photoelectron diffraction, together with N K-edge near-edge absorption fine structure, the local adsorption site and molecular orientation of 2-methyl-pyridine adsorbed on Cu(110) has been determined and the results are compared with a similar study of pyridine adsorption on the same surface. The molecule is found to bond to the surface through the N atom that occupies a near-atop site. The molecular plane is almost perpendicular to the surface but twisted into an azimuth away from the two principal high-symmetry directions of the surface. The aromatic ring is also tilted within the molecular plane such as to increase the separation of the methyl group away from the surface.

Research paper thumbnail of Is PEXAFS really PhD?

Surface Science, 2000

ABSTRACT Scanned-energy mode photoelectron diffraction (PhD) measurements from the disordered che... more 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.

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

Surface Science, 2002

The local adsorption structures of CO and NH 3 on NiO(1 0 0) have been determined by C 1s and N 1... more The local adsorption structures of CO and NH 3 on NiO(1 0 0) have been determined by C 1s and N 1s scannedenergy mode photoelectron diffraction. CO adsorbs atop Ni surface atoms through the C atom in an essentially perpendicular geometry (tilt angle 12 AE 12°) with a C-Ni nearest-neighbour distance of 2:07 AE 0:02 A A. NH 3 also adsorbs atop Ni surface atoms with a N-Ni distance of 2:06 AE 0:02 A 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 A and 2.86 A 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).

Research paper thumbnail of Photoelectron diffraction structure determination of Cu(1 0 0)c(2×2)-N

Surface Science, 2001

Using scanned-energy mode photoelectron diraction of the N 1s photoemission, combined with multip... more Using scanned-energy mode photoelectron diraction of the N 1s photoemission, combined with multiple scattering modelling calculations, the structure of the Cu(1 0 0)c2 Â 2-N has been investigated. While the results are broadly consistent with N adsorption in a fourfold coordinated hollow site, almost coplanar with the outermost layer, this model can only be ®tted to the data with unphysically large vibrational amplitudes, strongly suggesting that some static adsorbate-induced substrate distortion is involved. The best-®t structure involves an unusual symmetry-lowering large amplitude (0.34 A) rumpling of the outermost Cu layer. This result is entirely consistent with the results of a recent scanning tunnelling microscopy study of this surface.

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

Surface Science, 2002

The local structure of methanethiolate, CH 3 Son on an unreconstructed Cu(1 1 1) surface at low t... more The local structure of methanethiolate, CH 3 Son 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. 71ðþ14=À16Þ% of the methanethiolate was found to occupy bridge sites, 29 AE 14% to occupy fcc hollow sites and 0 þ 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 AE 0:03 A A giving a Cu-S bondlength of 2:27 AE 0:03 A A.