Ammonia adsorbed on Cu(110): An angle resolved x-ray spectroscopic and ab initio study

Abstract

We present a study of a monolayer of ammonia (NH3) adsorbed on Cu(110) using core level spectroscopies in combination with ab initio calculations based on density functional theory. In particular, x-ray emission spectroscopy has been applied, providing an unsurpassed view of the electronic structure of NH3 upon adsorption. The saturated NH3 monolayer, Θ∼0.4 ML, is found to induce strong adsorbate–adsorbate interaction, causing the molecules to tilt on the surface. Based on the angular distribution of the x-ray emission (XE) spectra, we have been able to estimate a mean tilt angle from the surface normal of 40°–45° for the saturated monolayer; the accompanying theoretical calculations for up to three NH3 molecules on a Cu21 all-electron cluster model support a tilted structure due to adsorbate–adsorbate dipole, and possibly hydrogen bonding, interactions. Since the creation of a core hole on the nitrogen atom site in the intermediate state of the XE process does not affect the symmetry of the molecule, a separation of valence electronic states having mainly e symmetry (N 2pxy) and a1 symmetry (N 2pz) has been achieved using angle resolved XE measurements. In addition to the electronic states of free NH3, evidence of new, substrate induced, states has been found, interpreted as ammonia 3a1/4a1-Cu 3d valence band hybrids. It is found that back donation into the previously unoccupied ammonia 4a1 orbital, and a simultaneous 3a1 donation into the substrate plays an important role in the surface chemical bond.