A.W. Robinson

Single local site structure for vibrationally distinct adsorption states : NO on Ni(111)

Abstract Scanned energy mode N and O 1s photoelectron diffraction data collected from NO on Ni(111) under conditions corresponding to three distinct N—O vibrational frequencies, previously assigned to different (bridge and atop) local adsorption configurations, show that NO in all cases occupies the same site. Comparison of these data with the results of theoretical simulations indicates that the common site is a threefold coordinated hollow. These results indicate that there is a need to reevaluate the range of applicability of the widely used method of adsorption site assignment based on the numerical values of internal molecular vibrational frequencies.

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.

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 Angstrom 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.

The low temperature CO/H coadsorption state on NiP{100}: X-ray absorption and photoemission studies

Angle-resolved photoemission and X-ray absorption spectroscopy have been used to re-examine the electronic structure and orientation of the low-temperature CO/H-coadsorption state observed on the N{100} surface. Although the C-O axis remains perpendicular to the surface, the photoemission spectrum is different from that of the pure Ni{100} system, showing some evidence for satellite structure. A comparison is made with CO on Cu{100}, a similar weak chemisorption system.

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.