On the interpretation of valence band photoemission spectra at organic-metal interfaces

TL;DR

This paper investigates how organic molecule adsorption on metal surfaces influences photoemission spectra, emphasizing the identification of true interface states versus artifacts caused by substrate electron scattering.

Contribution

It clarifies the interpretation of photoemission features at organic-metal interfaces, distinguishing genuine hybrid states from substrate scattering effects.

Findings

Interface states with molecular orbital character can be identified at low binding energies.

Features in the d-band region may result from substrate electron scattering, not molecule-substrate interaction.

Proper interpretation of spectra requires careful analysis to avoid misassignments.

Abstract

Adsorption of organic molecules on well-oriented single crystal coinage metal surfaces fundamentally affects the energy distribution curve of ultra-violet photoelectron spectroscopy spectra. New features not present in the spectrum of the pristine metal can be assigned as "interface states" having some degree of molecule-substrate hybridization. Here it is shown that interface states having molecular orbital character can easily be identified at low binding energy as isolated features above the featureless substrate sp-plateau. On the other hand much care must be taken in assigning adsorbate-induced features when these lie within the d-band spectral region of the substrate. In fact, features often interpreted as characteristic of the molecule-substrate interaction may actually arise from substrate photoelectrons scattered by the adsorbates. This phenomenon is illustrated through a series of examples of noble-metal single-crystal surfaces covered by monolayers of large pi-conjugated organic molecules.