A.L.D. Kilcoyne

The structure of the formate species on copper surfaces: new photoelectron diffraction results and sexafs data reassessed

Abstract Photoelectron diffraction from the C 1s and O 1s levels of the surface formate species (HCOO) on Cu{100} and Cu{110} has been measured and shows almost identical modulation structure indicating that the adsorption site is the same on both surfaces. Calculations show that the molecule adsorbs on the (short) bridge site in each case with the oxygen atoms close to atop positions and with a CuO nearest-neighbour distance of 1.98±0.04A and also indicate that the OCO bond angle may be increased to 134°. The bond lengthsagree with previous SEXAFS results but the sites differ and the reason for the discrepancies are discussed by reference to the results from model calculations of the SEXAFS data.

A photoelectron diffraction and nexafs study of the structure of the methoxy species (CH3O−) on Cu{100}

Abstract The structural arrangement of the methoxy species (CH 3 O−) on Cu{100} has been investigated by photoelectron diffraction from the O ls and C ls levels, data being collected in the scanned energy mode for emission along the surface normal, and by O and C K-edge NEXAFS. The results show that adsorption is in a low symmetry site with the oxygen atom lying between the bridge and hollow site (0.9 A from the bridge site) but the molecular axis is oriented essentially parallel to the surface normal (with a possible tilt of about 10 ° only). The photoelectron diffraction and C K-edge NEXAFS data show that the previous assignment of a much greater tilt angle, based on O K-edge NEXAFS data, results from an oversimplified interpretation of the data. Possible reasons for the low symmetry adsorption site are discussed.

The structure of oxygen adsorption phases on Cu(100)

Abstract A study of the structure of the Cu(100) surface in the presence of adsorbed oxygen is presented based on new scanned energy mode O 1s photoelectron diffraction measurements in conjunction with qualitative LEED observations and O K-edge NEXAFS characterisation. The interpretation of these data is discussed in the context of the considerable body of (frequently conflicting) results from other techniques. The best characterised structural phase is the ( 2 × 2 )R45° −O structure and this is found to be best described by a missing-row model. Evidence for this model is derived from a variety of techniques, and some complementary quantitative details of the structure are derived from the present measurements and published quantitative LEED analyses, with excellent agreement between the techniques on the parameter to which both are sensitive, the O overlayer to metal substrate spacing. In this structure the oxygen atoms occupy sites quite close to coplanar with the top Cu atom layer. Both photoelectron diffraction and X-ray absorption spectra show that the local adsorption structure is different at lower coverages characterised by a “four-spot” LEED pattern; in this case there is evidence for chemisorbed oxygen atoms occupying more than one site or having substantial local disorder on the Cu(100) substrate. Some specific local site models of this phase are considered.

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

Abstract 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 angstrom. 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 A, and is accompanied by a top layer expansion of 0.15 ± 0.10 A. The adsorbate atoms lie 0.25 ± 0.05 A above the location of the top layer in the absence of expansion, and are thus 0.10 ± 0.12 A 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.

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.

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

Abstract 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.

Determination of the adsorption site of C, N, and O on Ni(100) and Cu(100) using photoelectron diffraction

Scanned energy mode photoelectron diffraction spectra, collected at normal emission over an energy range typically in excess of 300 eV, are presented for 1s photoemission from O on Cu(100) and from O, N, and C in ordered structures on Ni(100). Double‐scattering curved wave cluster calculations are used to analyze these data. In all cases adsorption in a hollow site is favored, but the only simple case of chemisorption appears to be Ni(100)c(2×2)O. All the other structures appear to involve adsorbate‐induced substrate reconstruction, in which important ingredients appear to be expansion of the top metal layer, and lateral movements within this layer.

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 A, similar to the 〈110〉 spacing of 2.55 A. 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 O, N and C adsorption structures on Ni(100) and Cu(110)

Scanned energy mode adsorbate 1s photoelectron diffraction measurements have been taken from half-monolayer structures of O on Ni(100) and on Cu(110), and from C and N on Ni(100). The sensitivity of these data to the various structural parameters of these surface phases is investigated. In particular, the fact that 180° scattering contributions typically dominate the spectra in this form of the photoelectron diffraction experiment leads to a strong sensitivity to adsorbate–second layer substrate spacings. Even in the case in which the adsorbate and top substrate layers are nearly coplanar, however, some sensitivity to top layer geometry is retained, except in the case of the Cu(110) surface for which the lower rotational symmetry, combined with the plane polarisation of the incident radiation, leads to insensitivity to certain sites.

A simple parallel isochromat detector for inverse photoemission

A simple parallel detection system which allows several bremsstrahlung isochromat (inverse photoemission) spectra to be recorded simultaneously is described. It is based on the dispersive properties of a LiF lens in the vacuum ultraviolet previously described by Childs et al. [13] and operates in the photon energy range of approximately 8.0-11.5eV. In the instrument described three isochromats can be recorded at photon energy separations of 1.5eV. Its utility is demonstrated with measurements on Cu(111), the different photon energies permitting easy confirmation of the contributions of surface-localised states.