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 (DFT). In particular,
X-ray emission spectroscopy (XES) has been applied, providing an
unsurpassed view of the electronic structure of NH3 upon adsorption. The
saturated NH3 monolayer, Theta approximately 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 XE spectra, we
have been able to estimate a mean tilt angle from the surface normal of
40-45 degrees 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 backdonation into the
previously unoccupied ammonia 4a1 orbital, and a simultaneous 3a1
donation into the substrate plays an important role in the surface
chemical bond.