Th. Lindner

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.

Adsorption and reaction of CO2 on Ni{110}: X-ray photoemission, near-edge X-ray absorption fine-structure and diffuse leed studies

Using three different techniques X-ray photoemission (XPS), near-edge X-ray absorption spectroscopy (NEXAFS) and diffuse LEED we have studied the adsorption and reaction of CO, on Ni(ll0). In agreement with previous angle-resolved photoemission (ARUPS) and vibrational electron energy loss (EELS) data both a linear, physisorbed molecule and a bent, chemisorbed species CO,Sare found. An evaluation of the XPS line intensities shows that the stoichiometry of the chemisorbed species is 1 to 2 in carbon and oxygen. The polarisation dependence of the NEXAFS indicates that the physisorbed molecule lies with its axis parallel to the surface and that the molecular plane of the CO;species is perpendicular to the surface. There is no clear preferential azimuthal orientation. This is in agreement with a diffuse LEED analysis where equal numbers of bent molecules adsorbed on atop sites and oriented along the (100) and (110) directions gave the best fit to the data.

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.

The electronic energy levels of the surface formate species

A recent NEXAFS/SEXAFS study has elucidated the structure of the surface formate species formed by the deprotonation of formic acid on a Cu(110) surface. Using the known orientation of the molecule we now apply selection rules to assign all the features in the valence level photoemission spectrum. The results are compared with quantum chemical calculations.

Nexafs determination of CO orientation on a stepped platinum surface

The near edge X-ray absorption fine structure at the oxygen K edge has been used to determine the orientation of adsorbed carbon monoxide at both step and terrace sites at 80 K on the stepped Pt(533) surface. The angular dependence of both the 2 π intensity and the σ/2π intensity ratio indicates that the terrace CO is bonded essentially normal to the terrace and that the step CO is tilted away by only a few degrees towards the macroscopic surface normal. A dramatically large angle of tilt for step CO, as observed in other systems, was not found. The energy of the σ -resonance is identical for both step and terrace CO indicating that there is little change in the C−O bond order even though temperature programmed desorption shows that the step species is more strongly bound by ∼20 kJ mol −1 (∼0.2 eV molecule −1 ).

An angle-resolved photoemission investigation of CN, HCN and C2N2 adsorbed on Pd{111} using synchrotron radiation

Abstract The electronic valence levels of CN, HCN and C 2 N 2 adsorbed on Pd{111} have been investigated with angle-resolved photoelectron spectroscopy utilising synchrotron radiation. Adsorbate-induced features were observed at 5.0, 6.1, 7.2 and 9.1 eV below E f for CN and at 5.3, 8.5, 11.0 and 13.3 eV below E f for HCN. Only a tentative assignment of the spectral features has been possible since the low symmetry of the parallel-bonded species combined with the random azimuthal distribution prevents a more definitive approach based on selection rules. Preliminary data for C 2 N 2 on the same surface shows three features at 5.9, 6.5 and 8.1 eV below E f ; so far no assignment has been possible.

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 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.