  X-ray absorption spectroscopy yields direct access to the electronic and
geometric structure of hybrid inorganic-organic interfaces formed upon
adsorption of complex molecules at metal surfaces. The unambiguous
interpretation of corresponding spectra is challenged by the intrinsic
geometric flexibility of the adsorbates and the chemical interactions with the
interface. Density-functional theory (DFT) calculations of the extended
adsorbate-substrate system are an established tool to guide peak assignment in
X-ray photoelectron spectroscopy (XPS) of complex interfaces. We extend this to
the simulation and interpretation of X-ray absorption spectroscopy (XAS) data
in the context of functional organic molecules on metal surfaces using
dispersion-corrected DFT calculations within the transition potential approach.
On the example of X-ray absorption signatures for the prototypical case of
2H-porphine adsorbed on Ag(111) and Cu(111) substrates, we follow the two main
effects of the molecule/surface interaction on XAS: (1) the substrate-induced
chemical shift of the 1s core levels that dominates in physisorbed systems and
(2) the hybridization-induced broadening and loss of distinct resonances that
dominates in more chemisorbed systems.
