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

We have studied (6,6)-phenyl C-61 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 pi* or sigma* empty conduction band states. The analysis of the resonant Auger processes delivers a powerful tool to distinguish sigma* and pi* states in organic sp(2) hybridized systems. For the s* states, we find the conventional KLL Auger process creating a C2p(4) final state. However, for the pi* 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 (C2p(3)) final state.