C. Keller

Surface core level shifts of clean and oxygen covered Ru(0001)

We present the results of high resolution core level photoelectron spectroscopy employed to investigate the electronic structure of clean and oxygen covered Ir(111) surface. Ir 4f7/2 core level spectra are shown to be very sensitive to the local atomic environment. For the clean surface we detected two distinct components shifted by 550meV, originated by surface and bulk atoms. The larger Gaussian width of the bulk component is explained as due to experimentally unresolved subsurface components. In order to determine the relevance of the phonon contribution we examined the thermal behaviour of the core level lineshape using the Hedin-Rosengren theory. From the phonon- induced spectral broadening we found the Debye temperature of bulk and surface atoms to be 298 and 181K, respectively, which confirms the softening of the vibrational modes at the surface. Oxygen adsorption leads to the appearance of new surface core level components at 200meV and +230meV, which are interpreted as due to first-layer Ir atoms differently coordinated with oxygen. The coverage dependence of these components demonstrates that the oxygen saturation corresponds to 0.38ML, in good agreement with recent density functional theory calculations.

X-ray emission spectroscopy of (23×23)R30°CO/Ru(0001): Comparison to c(2×2)CO/Ni(100) and c(2×2)CO/Cu(100)

The atom specific electronic structure of (2∛×2∛)R30°CO on hcp Ru(0001) has been determined with resonantly excited x-ray emission spectroscopy. We find that the general features of the local adsorbate electronic structure are similar to the situation of CO adsorbed on the fcc metals Ni(100) and Cu(100). The interpretation of the surface chemical bond of (2∛×2∛)R30°CO/Ru(0001) based on the direct application of the local, allylic model from on-top adsorption on the fcc(100) surfaces Ni(100) and Cu(100) explains many aspects of the surface chemical bond. However, also nonlocal contributions like adsorbate-adsorbate interaction and the deviation from upright on-top adsorption on the Ru(0001) surface influence observables like the heat of adsorption and the Me-CO bond strength.

Interpretation of x-ray emission spectra: NO adsorbed on Ru(001)

A density functional investigation of the x-ray emission spectrum of NO adsorbed on Ru(001) has been carried out using model cluster calculations. The dipole matrix elements governing the emission probability were evaluated in the frozen ground-state approximation. The resulting simulated spectra exhibit all characteristic features of the experimental data. A detailed analysis of the electronic structure of the model clusters permits a complete rationalization of all observed trends. Furthermore, a picture of the surface chemical bond results in which the classical Blyholder frontier orbital model is extended to a three-orbital description for both the π and σ interactions. Comparison of different adsorption sites reveals that threefold coordinated NO features a stronger orbital interaction with the substrate than NO adsorbed in an on-top position.

X-ray emission spectroscopy of NO adsorbates on Ru(001)

Abstract We report X-ray emission spectra (XES) of the NO adsorbate species on the close-packed Ru(001) surface, and discuss the basis of their interpretation. On this surface NO can exist in two distinct, very different states which coexist in the pure saturated layer and which can be prepared separately by coadsorption with selected superstructures of O atoms. We report symmetry-resolved XES data for these states, and extract atom-specific 2p contributions of adsorbate orbitals which are derived from the molecular 3σ, 4σ, 5σ, 1π, 2π, and 6σ orbitals at the O and N atoms. Their interpretation is facilitated by comparison with very recent density functional calculations [Staufer et al., J. Chem. Phys. 111 (1999) 4704] which use the frozen ground state approximation for the interpretation of the XES data. A consistent picture of the NO–Ru surface bond results which is mainly based on the allyl model, containing charge donation from the adsorbate to the substrate via mixing of the molecular 4σ and 5σ orbitals, and backdonation in the π channels by forming a bonding 1π orbital and a ‘lone pair’ π-type orbital on the oxygen often (somewhat misleadingly) denoted as 2π orbital. Finally we discuss some questions of principle for XES of adsorbates, such as the nature of the core-excited state effective in XES of chemisorbates, the influences of relaxation and screening which are neglected in frozen orbital calculations, and possible differences between the valence hole states observed in ultra-violet photoelectron spectroscopy, and in XES at different atoms.