  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.
