  X-ray photoemission and x-ray absorption spectroscopy are important
techniques to characterize chemical bonding at surfaces and are often used to
identify the strength and nature of adsorbate-substrate interactions. In this
study, we judge the ability of x-ray spectroscopic techniques to identify
different regimes of chemical bonding at metal-organic interfaces. To achieve
this, we sample different interaction strength regimes in a comprehensive and
systematic way by comparing two topological isomers, azulene and naphthalene,
adsorbed on three metal substrates with varying reactivity, namely the (111)
facets of Ag, Cu, and Pt. Using density functional theory, we simulate
core-level binding energies and x-ray absorption spectra of the molecular
carbon species. The simulated spectra reveal three distinct characteristics
based on the molecule-specific spectral features which we attribute to types of
surface chemical bonding with varying strength. We find that weak physisorption
only leads to minor changes compared to the gas-phase spectra, weak
chemisorption leads to charge transfer and significant spectral changes, while
strong chemisorption leads to a loss of the molecule-specific features in the
spectra. The classification we provide is aimed at assisting interpretation of
experimental x-ray spectra for complex metal-organic interfaces.
