Valence and Conduction Band States of PCBM as Probed by Photoelectron Spectroscopy at Resonant Excitation

We have studied (6,6)-phenyl C61 butyric acid methyl ester by near-edge X-ray absorption fine structure and resonant synchrotron radiation X-ray photoelectron spectroscopy. We determine the binding/hybridization character of the participating valence and conduction band states involving π* or σ* empty conduction band states. The analysis of the resonant Auger processes delivers a powerful tool to distinguish σ* and π* states in organic sp2 hybridized systems. For the σ* states, we find the conventional KLL Auger process creating a C2p4 final state. However, for the π* features right at the resonance, we identify a novel Auger decay mechanism. We attribute this process to the decay of an excitonic intermediate state with a long lifetime which causes a three-hole (C2p3) final state.