Scanned-energy Mode Photoelectron Diffraction Research Articles

Photoelectron diffraction for quantitative determination of adsorption structures on surfaces

The technique of scanned-energy mode photoelectron diffraction for the quantitative determination of surface structures is described and compared in physical approach, methodology, and potential, with that of SEXAFS. The development of both direct methods of data analysis for first-order structure determination, and full multiple scattering simulations, is described, as is one of the first applications of chemical shift photoelectron diffraction for the study of complex coadsorption structures.

Structure determination of an alkali metal-CO coadsorption phase: Ni(111)-K/CO.

The structure of a Ni(111)- $(2\ifmmode\times\else\texttimes\fi{}2)$--K $/n$CO $(n\ensuremath{\approx}2)$ coadsorption phase has been determined using K $2p$, C $1s$, and O $1s$ scanned energy mode photoelectron diffraction, and compared with results from pure CO and K adsorption phases having similar coverages. Coadsorbed K has little influence on the local hollow site geometry of the adsorbed CO molecules; the K remains in the atop site of the pure K layer, but the K-Ni bond length increases substantially (by $0.15\ifmmode\pm\else\textpm\fi{}0.05$ \AA{}), as does the Ni-Ni outer layer spacing. The results conflict with aspects of current interpretations of spectroscopic data from such systems.

The local geometry of reactant and product in a surface reaction: the dehydrogenation of adsorved ethylene on Ni(111)

Using C1s scanned-energy mode photoelectron diffraction based both on approximate “direct” methods and full multiple scattering modelling of spectra recorded in different emission angles covering a total spectral data range of 1200 eV, the local geometry of adsorbed ethylene and adsorbed acetylene on Ni(111) have been determined in a detailed quantitative fashion. Ethylene adsorbed at low temperature (120 K) lies with its CC axis parallel to the surface and in an aligned bridge site such that the C atoms lie approximately atop Ni atoms. Heating this surface leads to dehydrogenation of the adsorbed ethylene to adsorbed acetylene, and while the CC axis remains parallel to the surface, the CC bondlength and CNi layer spacing are reduced, and the acetylene occupies a cross-bridge site with the C atoms directly above inequivalent hollow sites on the surface. Both adsorbed species show C bondlengths larger than those of the associated gas-phase molecules, indicating a significant reduction of CC bond order, in agreement with vibrational spectroscopic data. Possible geometrical reaction paths based on a concerted mechanism connecting the reactant and product are discussed.

STRUCTURAL STUDIES OF MOLECULAR ADSORPTION AT SURFACES USING SCANNED-ENERGY MODE PHOTOELECTRON DIFFRACTION

A short review is presented of the use of scanned-energy mode photoelectron diffraction for the quantitative determination of the local adsorption structure of molecular species on wellcharacterised metal surfaces. Some of the results [notably for NO and CO adsorption on Ni(111)] highlight the importance of such true quantitative methods relative to "fingerprinting" via vibrational spectroscopy which is shown to have led to incorrect local adsorption site assignments. The use of direct methods for approximate site identification, and chemical-shift photoelectron diffraction for studies of more complex coadsorption structures is described with the aid of specific applications to molecular fragmentation and geometrical reaction path determination. The prognosis for further development and application is discussed.

The potassium-induced reconstruction of Cu{110}: the K atom adsorption site

Using scanned energy mode photoelectron diffraction we show that in the potassium-induced reconstruction of the Cu{110} surface the K atoms occupy four-fold hollow sites formed by second layer Cu atoms. This same site, which is the key to the reconstruction, is observed both at low coverages, where the reconstruction is local in nature, and at coverages corresponding to the (1 × 3) and(1 × 2) missing row structures.

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

The adsorption site of pyridine (C 5H 5N) on Ni{111} has been investigated by scanned-energy mode photoelectron diffraction using the N 1s core level. It was found that the nitrogen atom adsorbs close to the atop position with an angle of 18° ( + 2° − 4°) between the NNi bond and the surface normal. The NNi bond length is 1.97 ± 0.03 Å. Since recent X-ray absorption studies show that the π system of the pyridine is also tilted by about 20° away from the surface normal, we conclude that the NNi bond and the ring plane are co-planar.

Experimental tests of new direct methods for adsorbate structure determination using photoelectron diffraction

Two recently proposed methods for approximate but direct adsorbate site determination using scanned-energy mode photoelectron diffraction data have been tested on experimental data from three different adsorption systems. The three systems studied, O, CO, and C2H4 on Ni(111) involve three different local adsorption sites (single hollow, mixed hollows and near-atop) and have all been subjected to separate full quantitative structure determinations. Both the simple Fourier transform approach of Fritsche and Woodruff, and the projection method of Hofmann and Schindler are found to correctly identify all the adsorption sites. The precision of these approximate structure determinations, and the size of the data bases used, are compared with each other and with other proposed methods.

Structure determination of Ni(111)c(4 × 2)-CO and its implications for the interpretation of vibrational spectroscopic data

A detailed quantitative structure determination of the Ni(111)c(4 × 2)-CO structure has been undertaken using scanned energy mode photoelectron diffraction from the C 1s state over a wide range of emission angles. Analyses of these data by approximate direct methods, and by two independent multiple scattering trial-and-error fitting optimisations lead to a consistent structure in which the CO occupies both types of hollow site on the surface in equal amounts with a C-Ni top layer spacing of 1.29 ± 0.05 Å. This structure is therefore essentially the same as that for Ni(111)c(4 × 2)-NO, and provides further evidence that simple use of the intramolecular stretching frequencies of such adsorbed molecules, which had been interpreted in both cases as indicative of bridge site adsorption, is not always a reliable indicator of local adsorption site.

Photoelectron diffraction investigation of the adsorption site and local structure for potassium on Ni(111)

A recent LEED study of the Ni(111)(2 × 2)-K structure indicates that this is one of a small number of examples of alkali metal adsorption on close-packed metal surfaces now believed to involve atop adsorption. In order to provide an independent test of this surprising result we have conducted scanned energy mode photoelectron diffraction experiments on the K2s and 2p emission at a range of polar emission angles. A simple Fourier transform mapping of the nearest neighbour contribution to these spectra provides direct evidence for the atop adsorption site, and subsequent theory-experiment comparisons using multiple scattering calculations confirm this site and give a nearest neighbour bondlength of 2.86 0.03 Å, essentially identical to the LEED result (2.82 ± 0.04 Å). The new results also indicate a significant contraction (0.17 ± 0.06 Å) of the outermost Ni atom layer, but find essentially none of the “rumpling” obtained in the LEED study.

The geometric structure of the surface methoxy species on Cu(111)

The adsorption geometry of the surface methoxy species (-OCH 3) on Cu(111) has been determined quantitatively using scanned energy mode photoelectron diffraction in a two-step approach. First, using direct methods it was established that the fragment adsorbs via the oxygen atom at the “fcc” three-fold adsorption site. Subsequently, the structural parameters were refined by a search in multi-parameter space using diffraction spectra obtained in different emission geometries. The oxygen atom is found to be situated 1.32(±0.05) Å above the three nearest-neighbour copper atoms with the O-C axis essentially perpendicular to the surface. The O-C distance is 1.42(−0.03/+ 0.10) Å; a relaxation as well as a rumple of the outermost Cu layer occur.

The local adsorption structure of acetylene on Cu(lll)

A quantitative structural study of the adsorption of acetylene on Cu(111) has been carried out by scanned energy mode photoelectron diffraction from the C 1s core levels. The molecule is found to be adsorbed with the CC axis almost parallel to the surface with the carbon atoms occupying the two symmetrically distinct three-fold coordinated hollow sites on the surface. The C to top Cu atom layer spacing appears to be slightly smaller (1.38 ± 0.03 A ̊ ) above the “fcc” hollow site (above a third-layer Cu atom) than above the “hcp” hollow site (1.44 ± 0.03 A ̊ ) . The best fit to theory is obtained with the C atoms exactly in the three-fold symmetric sites of the substrate which constrains the CC bondlength to a value (1.48 ± 0.10 A ̊ ) substantially larger than that of the free molecule. This result is consistent with previously published vibrational spectroscopy from this adsorption system which indicates significant lowering of the CC bond order.

Multiple site coincidences and their resolution in photoelectron diffraction: PF3 adsorbed on Ni(111)

Using scanned energy mode photoelectron diffraction from the P2p emission of PF3 on Ni(Ill) the adsorption structure has been determined; PF3 occupies an atop site with a P-Ni nearest neighbour distance of 2.07 +/- 0.03 angstrom. Using the normal emission data alone, comparison of theory and experiment with reliability factors shows a succession of minima as a function of the P-Ni distance. These multiple ''site'' coincidences are very similar to those seen in LEED analyses. The ambiguity is removed not only by the depth of these minima, but more importantly by using selected off-normal emission data sensitive to other scattering path-lengths which also show successive minima, but with a different spatial periodicity.

Multiple site coincidences and their resolution in photoelectron diffraction: PF 3 adsorbed on Ni(111)

Using scanned energy mode photoelectron diffraction from the P 2p emission of PF 3 on Ni(111) the adsorption structure has been determined; PF 3 occupies an atop site with a PNi nearest neighbour distance of 2.07 ± 0.03 A ̊ . Using the normal emission data alone, comparison of theory and experiment with reliability factors shows a succession of minima as a function of the PNi distance. These multiple “site” coincidences are very similar to those seen in LEED analyses. The ambiguity is removed not only by the depth of these minima, but more importantly by using selected off-normal emission data sensitive to other scattering path-lengths which also show successive minima, but with a different spatial periodicity.

Local site identification for NO on Ni(111) in vibrationally distinct adsorption states

Vibrational spectroscopy of NO on Ni(111) shows two distinct N–O stretching bands for adsorption on the clean surface at different coverages which have been attributed to tilted and perpendicular bridging species at low and high coverage, respectively, while an even higher vibrational energy seen when NO is coadsorbed with oxygen has been attributed to atop NO. We have investigated the local adsorption geometry of these species with scanned energy mode photoelectron diffraction from the N and O 1s states, a method previously found to be very sensitive to site changes and capable of quantitative site identification. We find essentially identical photoelectron diffraction spectra from all three species, indicating that there are no site changes to within a few hundredths of an Ångstrom unit. Moreover, a quantitative analysis using double scattering model calculations indicates that in all cases the adsorption site is actually a threefold coordinated hollow and not bridge or atop.

Determination of the orientation of methoxy on Cu(111) using X-ray photoelectron diffraction

The orientation of methoxy (CH 3O−) on copper single-crystal surfaces, has been the subject of some controversy. Here we show, using the particularly direct technique of X-ray photoelectron diffraction, that on Cu(111) methoxy is clearly adsorbed with its OC molecular axis perpendicular to the surface. This agrees with the results of vibrational and electronic spectroscopies, and of scanned energy mode photoelectron diffraction.

The orientation of mercaptide on Cu(111) studied by X-ray photoelectron diffraction polar angle scans

The orientation of mercaptide (CH2S-), and the closely similar methoxy (CH3O-) species on Cu single-crystal surfaces, has been the subject of some controversy. Some near-edge X-ray absorption fine structure results indicate substantially tilted C-S or C-O molecular axes relative to the surface normal, but reflection-adsorption infrared spectroscopy of methoxy on Cu(111), scanned-energy mode photoelectron diffraction of methoxy on Cu(100), and S K-edge surface extended X-ray absorption fine structure of mercaptide on Cu(111) all indicate perpendicular molecular orientations. A technique that provides particularly clear information on molecular orientations is polar angle scanned X-ray photoelectron diffraction, and the authors have applied this method to a study of the molecular orientation of mercaptide on Cu(111). The results show clearly that the mercaptide molecular axis is oriented perpendicular to the surface, a result which is found to be true over a wide range of surface coverages. The origins of the discrepancies between the various techniques are discussed.

A photoelectron diffraction study of the Ni(100)(2 × 2)-C(p4g) and Ni(100)(2 × 2)-N(p4g) structures

Scanned energy mode photoelectron diffraction spectra have been recorded from the C and N 1s levels of the adsorbate in the Ni(100)(2 × 2)-C(p4g) and Ni(100)(2 × 2)-N(p4g) structures at polar emission angles of 0°, 18° and 44° and are compared with the results of model calculations. The results allow a determination of the structure of these two phases which appear to be identical to within an estimated accuracy of a few hundreths of an ångström. The analysis supports the “clock reconstruction” model of these surfaces with the adsorbate atoms located in 4-fold coordinated hollows at the centres of rotation of the top layer Ni atom movements parallel to the surface. The magnitude of these parallel displacements is found to be 0.55 ± 0.20 Å, and is accompanied by a top layer expansion of 0.15 ± 0.10 Å. The adsorbate atoms lie 0.25 ± 0.05 Å above the location of the top layer in the absence of expansion, and are thus 0.10 ± 0.12 Å above the expanded top layer. Within these parameter ranges there is some evidence that the degree of distortion (particularly perpendicular to the surface) may be slightly larger in the N phase than in the C phase. These values compare very well with LEED analyses, although the result for the N appears to be at variance with SEXAFS results.

The structure of Cu(110) (2×3)–N; pseudo‐square reconstruction of a rectangular mesh substrate

The structure of the Cu(110)(2×3)–N phase has been investigated by parallel studies using 3‐keV Li+ and 2‐keV He+ ion scattering and scanned energy mode normal emission N 1s photoelectron diffraction. The Li+ ion scattering results provide clear evidence for an adsorbate‐induced reconstruction of the Cu surface and, in particular, indicate a shortening of the Cu–Cu atomic spacing along the 〈100〉 azimuth to a value of 2.7±0.1 Å, similar to the 〈110〉 spacing of 2.55 Å. A model of the surface consistent with this result involves a pseudo‐square Cu top atom layer with N atoms occupying alternate hollow sites leading to a local structure which is a 6% distorted Cu(100)c(2×2)–N phase. He+ ion scattering data provides support for the N hollow adsorption sites. Scanned energy mode photoelectron diffraction is insensitive to the nature of the reconstruction but provides information on the local adsorption sites and, hence, on the lateral registry of the reconstructed overlayer and the underlying substrate.

Photoelectron diffraction study of the local adsorption site in the Cu(110)(2 × 3)-N structure

Normal emission scanned energy mode photoelectron diffraction from the 1s state of N adsorbed on Cu(110) to form a (2 × 3) surface mesh is analysed using curved wave double scattering simulations. Much of the main structure of the experimental data can be reproduced by models involving simple overlayer chemisorption but these sites give no clue as to the origin of the (2 × 3) periodicity. A more radical reconstruction model supported by low energy ion scattering measurements is also found to give satisfactory agreement with the photoelectron diffraction data. This reconstruction involves a surface layer almost identical to that found in Cu(110)(2 × 2)-N, with a local structure very similar to that in bulk copper nitride.

A photoelectron diffraction study of Cu{110}-(2×1)-O

The adsorption of atomic oxygen on Cu{110} has been studied using scanned energy mode photoelectron diffraction from the O 1s level in the range 80–400 eV. Experiments were carried out for normal emission as well as for emission angles of 39°, 57° and 67° in the azimuth. We find that the oxygen is adsorbed in the long bridge site at a distance of 1.98 A from copper atoms in the second layer. This is consistent with oxygen lying 0.23 A above the surface with a Cu-O bond length of 1.82 A, if the spacing between the top two copper layers is identical to that of the bulk. We cannot, however, rule out the possibility of subsurface oxygen, were there a substantial relaxation of the spacing between the first and second layers of copper atoms. The use of R -factors to compare experimental data and calculated diffraction curves is evaluated. We conclude that both the Zanazzi-Jona and Pendry R -factors may be successfully employed to determine the best agreement between calculation and experimental data.