L. Wenzel

Position of the σ-shape and π resonances of C2H2, C2H4 and C2H6 on Cu(100) at 60 K: A NEXAFS study

The position of the π and σ resonances in the C K edge spectra are determined for multilayer and submonolayer coverages of C2H2, C2H4 and C2H6 at 60 K, on a Cu(100) surface. The majority of the molecules are found lying flat on the surface in submonolayer coverages. We determine an elongation of the C-C bond length, for C2H2 and C2H4 on Cu(100) of 0.21(2) A and 0.10(2) A respectively, using the position of the σ resonance on a common energy scale with the gas phase. The results are compared with the ones of strong chemisorption on Pt(111). The σ-shape resonance of C2H6 chemisorbed on a metal substrate is reported for the first time. The molecule shows no elongation.

Carbon K-edge structure of chemisorbed molecules by means of fluorescence detection

Abstract We report the first X-ray absorption fine structure measurements above the carbon K-edge recorded by fluorescence detection (FD). Studies for ethylene chemisorbed on Cu(100) were carried out with high-flux ( > 1010 photons/s) synchrotron radiation using an ultra-high vacuum compatible proportional counter. Our results show that K-edge studies by means of FD allow the measurement of extremely dilute concentrations of C corresponding to about 0.02 monolayers on the surface or 20 ppm in the bulk.

Intramolecular resonances afterK-shell excitation of C2H2n adsorbed on Ag and Cu(100) surfaces

The near edge X-ray absorption fine structure (NEXAFS) at the C K-edge of C2H2, C2H4, C2H6 on Cu(100) and Ag(100) surfaces is studied. The oriented character of the molecules on the surface combined to the dipole selection rules for photon absorption allows an experimental assignment of the nature of the various final states. A careful fitting of the NEXAFS spectra reveals final states of C–H character and shake-up satellites of the C(1 s) to π* (C–C) transition. The asymmetric line shape of the C(1 s) to σ* (C–C) transition is described in a scattering model of the photoelectron from the molecular potential.

Quantitative analysis of the NEXAFS for chemisorbed C2H4 molecules

A quantitative analysis of the NEXAFS for C2H4 molecules on Cu(100) in a single layer coverage is given. A simple quantum mechanical scattering model is used in order to reproduce the asymmetric profile of the σ*-shape resonance using only three independent parameters, that appear to possess a physical meaning. As a result small threshold resonances can be quantitatively analyzed. The angular dependence of all the molecular resonances is presented.

Rydberg and multiple electron excitations of N2 adsorbed on Fe(111): a NEXAFS study

For submonolayer (≤0.2 ML) coverage of N2 on Fe(111), photoabsorption measurements (NEXAFS) at the N K-edge show π* (401 eV) and σ* (419 eV) intramolecular resonances. Photoemission spectra confirm that all the N2 is adsorbed in the γ-state. The angular dependence of the NEXAFS indicates that the N2 molecules are oriented normal to the surface. All resonances appear at the same energies as in gas phase N2; in addition Rydberg-like states at 406.5 eV and 410.2 eV are found, which are not quenched by the adsorption. These observations are consistent with a weak N2-Fe chemisorption bond.

Adsorbate-induced reconstruction on transition-metal and noble-metal surfaces

Abstract The structures of (2×1) O on Ni (110) and c(2×2)N on Ni (100) are studied with a full multiple scattering analysis of the NEXAFS. On Ni (110) the O is found to induce a sawtooth reconstruction with an accompanying O tilt toward the (100) facet. On Ni (100), N occupies a fourfold hollow site and induces a p4g reconstruction of the top Ni layer with the nearest Ni atoms rotated by a tangential displacement of ≈0.7A. Both structures agree with the associated SEXAFS analysis.

Carbon-carbon EXAFS for chemisorbed hydrocarbons on a metal substrate

Abstract C-C SEXAFS of C 2 K 2 , C 2 K 4 , C 2 K 6 on Cu(100) is only detectable for thick films of more than 100 layers. Up to few layers (≈20) the Cu substrate EXAFS always overcomes the intramolecular SEXAFS.

Multiple scattering effects in low Z SEXAFS

Abstract A multiple scattering analysis of O and N on Ni(100) yields on independent determination of the Ni-Ni distance after substrate reconstruction.