Calculating binding energy shifts in core level XPS from first principles: a case study of adsorbates on Cu(111)

X-ray Photoelectron spectroscopy is one of the most widely used experimental techniques in surface science, yet in spite of over forty years of research, the interpretation of core level spectra of chemically complex systems remains extremely challenging. Whilst theoretical calculations could help in this regard, at present almost all published core level XPS spectra are analyzed without the aid of first principles modelling.

In order to challenge this paradigm, we have used Density Functional Theory to calculate C 1s and O 1s core-level binding energy shifts for a series of small molecules adsorbed onto the Cu(111) surface. Theoretical binding energies were obtained as the total energy difference between the ground state and the core-ionized state, under the assumption of a fully-screened core hole. We have shown that this method yields excellent agreement, when the results for isolated molecules are compared against gas phase spectra. For the adsorbed species, good overall agreement with experiment is observed, and in cases where there are large discrepancies between different published experimental binding energies, the theoretical results can be used to judge, which peak assignments are most likely to be accurate.