Tunable-excitation soft X-ray fluorescence spectroscopy of high-Tc superconductors: an inequivalent-site seeing story

When a molecule is adsorbed on a metal surface by chemical bonding new electronic states are formed. The direct observation and identification of these states has been an experimental challenge. Their signature is often obscured by bulk substrate states. In the following contribution we will show how X-ray emission spectroscopy (XES), in spite of its inherent bulk sensitivity, can be used to investigate adsorbed molecules. Due to the localization of the core-excited intermediate state, XE spectroscopy allows an atom specific separation of the valence electronic states. Thus the molecular contributions to the surface chemical bond can be separated from those of the substrate. Furthermore, angle dependent measurements make it possible to determine the symmetry of the molecular states, i.e. the separation of pi and alpha type states. Density functional theory calculations in the frozen orbital approximation can describe the XE spectra with a good agreement with experiments. In all we can obtain an atomic view of the electronic states involved in the formation of the chemical bond to the surface. We will show how the electronic structure in simple atomic adsorbates on Cu and Ni surfaces can be related to the concept of less or more noble metals. We also show how new molecular states are formed in adsorbed N2 and CO on Ni(100). The resulting strength of the adsorbate bond comes from a delicate balance between pi bonding and sigma repulsion. We can use an additional symmetry selection rule for adsorbed molecules with equivalent atoms where pi and pi* states can be selectively enhanced depending on the nature of the primary excited state. This will be demonstrated for ethylene and benzene adsorbed on Cu(110). The future prospect is illustrated by the adsorption of formate and ammonia, on Cu(110). These adsorbates represent the interaction of functional groups in amino acids, which are an important class of biological molecules involved in building proteins.