  We report the discovery of a novel charge transfer (CT) state at a
molecule-metal interface by the application of resonant photoelectron
spectroscopy (ResPES). This interface feature is neither present for molecular
bulk samples nor for the clean substrate. Within a simplified two-step model
this signal is assigned to a particular final state that is invisible in direct
photoelectron spectroscopy but in ResPES revealed through relative resonant
enhancement. A detailed analysis of the spectroscopic signature of the CT state
shows characteristics of electronic interaction not found in other electron
spectroscopic techniques. Our study demonstrates the sensitivity of ResPES to
such interactions and constitutes a new way to investigate CT at molecule-metal
interfaces.
