LOCAL PROBING OF THE SURFACE CHEMICAL-BOND USING X-RAY-EMISSION SPECTROSCOPY

X-ray emission spectroscopy applied to adsorbate systems is shown to r eveal new details of the chemical bond formed at surfaces. An atom and symmetry projected view of the bonding orbitals is obtained. We will present recent studies of N-2 and CO on Ni(100), benzene on Ni(100) an d Cu(110), and glycine adsorbed on Cu(110). New types of molecular sta tes are observed which are directly related to the surface chemical bo nd. The long-accepted Blyholder model which is based on a frontier orb ital concept cannot explain our results for N-2 and CO chemisorption. We find it necessary to offer a new picture where changes in the whole molecular orbital framework have to be considered. We show that both pi and sigma type interactions are important in describing the bonding in benzene to metal surfaces. The future prospect is illustrated by t he adsorption of the simplest amino acid, glycine, on Cu(110). The ads orbate has four different atomic centers where X-ray emission spectra are obtained, providing a unique view of the local electronic structur e.