Site- and Symmetry-Resolved Resonant X-ray Emission Study of a Highly Ordered PTCDA Thin Film

The complex electronic structure of ordered 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) multilayers on Ag(111) is investigated by angle-dependent resonant X-ray emission spectroscopy at the carbon K edge. Beam damage effects were characterized and special care was taken to avoid their influence on the spectra. Appropriate selection of excitation energies and detection geometries allow us to record element-, atom-, and symmetry-specific X-ray emission spectra by making use of dipole selection rules. This specificity, especially the ability to distinguish between σ and π orbitals, gives unprecedented experimental insights into the electronic structure of this large organic molecule. The experimental results are compared with calculated spectra of the isolated molecule, as derived from density functional theory. This allows a detailed discussion of the spectra and separation of the contributions from nonequivalent carbon atoms.