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

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 1π̃ 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 ultraviolet photoelectron spectroscopy, and in XES at different atoms.