On the origin of unexpected core level shifts of tetrapyrrole molecules on surfaces

Abstract

Several studies have addressed the presence of unexpected components in the molecular level X-ray Photoelectron Spectroscopy (XPS) spectra, specially in the N1s signal, when tetrapyrrole molecules are deposited on different surfaces in Ultra High Vacuum (UHV) conditions. In this work, we employ a set of DFT-based tools to unravel the origin of an unpredicted peak in the N1s core level spectra of metal phthalocyanines (in particular, ZnPc) adsorbed on Cu(110). We analyze the different possible chemical or electronic mechanisms that might explain this experimental result. On one hand, we prove that surface effects, such as LUMO splitting and surface screening, cannot account for the huge experimental CLSs. On the other hand, we show that, when adsorbed at low temperatures, these molecules capture residual H atoms from the surface, giving rise to hydrogenated molecular species without de-metalation of the molecule and, thus, to an extra component in the molecular CLS spectra. Only upon annealing, and subsequent H release, do the molecules recover their nominal structural and electronic properties. Our results thus demonstrate that residual hydrogen, which is known to be the primary residual gas in UHV systems, might have a significant effect on the measured electronic properties of molecule-surface complexes.