Interpretation of valence band photoemission spectra at organic-metal interfaces

Adsorption of organic molecules on well-oriented single-crystal coinage metal surfaces fundamentally affects the energy distribution curve of ultraviolet 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 p-conjugated organic molecules. DOI: 10.1103/PhysRevB.87.035413