Probing chemical bonding in adsorbates using X-ray emission spectroscopy

When a molecule is adsorbed on a metal surface by chemical bonding new elec tronic states are formed. The direct observation and identification of thes e states has been an experimental challenge. Their signature is often obscu red by bulk substrate states. In the following contribution we will show ho w X-ray emission spectroscopy (XES), in spite of its inherent bulk sensitiv ity, can be used to investigate adsorbed molecules. Due to the localization of the core-excited intermediate state, XE spectroscopy allows an atom spe cific separation of the valence electronic states. Thus the molecular contr ibutions to the surface chemical bond can be separated from those of the su bstrate. Furthermore, angle dependent measurements make it possible to dete rmine the symmetry of the molecular states, i.e. the separation of pi and s igma type states. Density functional theory calculations in the frozen orbi tal approximation can describe the XE spectra with a good agreement with ex periments. In all we can obtain an atomic view of the electronic states inv olved in the formation of the chemical bond to the surface. We will show ho w the electronic structure in simple atomic adsorbates on Cu and Ni surface s can be related to the concept of less or more noble metals. We also show how new molecular states are formed in adsorbed N-2 and CO an Ni(100). The resulting strength of the adsorbate bond comes from a delicate balance betw een pi bonding and sigma repulsion. We can use an additional symmetry selec tion rule for adsorbed molecules with equivalent atoms where pi and pi* sta tes can be selectively enhanced depending on the nature of the primary exci ted state. This will be demonstrated for ethylene and benzene adsorbed on C u(110). The future prospect is illustrated by the adsorption of formate and ammonia, on Cu(110). These adsorbates represent the interaction of functio nal groups in amino acids, which are an important class of biological molec ules involved in building proteins. (C) 2000 Elsevier Science B.V. All righ ts reserved.