Size of electron-hole pairs in pi-conjugated systems

We have performed momentum-dependent electron energy-loss studies of the el ectronic excitations in sexithiophene and compared the results to those fro m parent oligomers. Our experiment probes the dynamic structure factor S(q, omega) and shows that the momentum-dependent intensity variation of the ex citations observed can be used to extract the size of the electron-hole pai r created in the excitation process.. The extension of the electron-hole pa irs along the molecules is comparable to the length of the molecules. Conse quently, the primary intramolecular electron-hole pairs are relatively weak ly bound. We find no evidence for the formation of excitations localized on single thiophene units.