- m at . s of t ] 1 9 Ju l 1 99 9 The size of electron - hole pairs in π conjugated systems

We have performed momentum dependent electron energy-loss studies of the electronic excitations in sexithiophene and compared the results to those from parent oligomers. Our experiment probes the dynamic structure factor S(q, ω) and we show that the momentum dependent intensity variation of the excitations observed can be used to extract the size of the electron-hole pair created in the excitation process. The extension of the electron-hole pairs along the molecules is comparable to the length of the molecules and thus maybe only limited by structural constraints. Consequently, the primary intramolecular electron-hole pairs are relatively weakly bound. We find no evidence for the formation of excitations localized on single thiophene units. 1 There is still growing interest in the physical properties of various π conjugated systems because many polymers and oligomers, have become promising candidates for applications in devices such as organic light emitting diodes [1,2], field effect transistors [2,3], optical converters [4] or molecular switches [5]. In addition, π conjugated molecules with a finite, well defined chain length can serve as model systems for the investigation of general and fundamental properties of the whole material class. Moreover, π-conjugated systems bridge molecular and extended electronic states and therefore also allow one to study special cases of Heisenberg, Hubbard or other models for narrow band solids. Any potential application in electronic or optical devices and their optimization requires an understanding of the electronic structure of the system in question. Hereby, the excited electronic states are of particular interest in polymers or oligomers as they are directly related to processes such as light absorption and emission, photoconductivity or exciton formation. One of the remaining questions is to what extent exciton formation plays a role in the excited state and how large is the related exciton binding energy. Exciton binding energies ranging from 0.1 to more than 1 eV have been discussed previously [6–13]. Directly connected with the exciton formation and binding energy is the spatial extension of a possible excitonic state, i.e. the size of the electron-hole pair formed in the excitation process. In this contribution we present a detailed analysis of optically allowed and forbidden electronic excitations of higly oriented sexithiophene (α-6T) thin films which have been studied using electron energy-loss spectroscopy (EELS) in transmission. The results are compared to those from parent π conjugated oligomers. EELS is a measure of the dynamic structure factor S(q, ω) and thus probes the form …