The size of electron-hole pairs in pi conjugated systems

TL;DR

This study uses momentum-dependent electron energy-loss spectroscopy to determine the size of electron-hole pairs in pi-conjugated systems, revealing they are extended and weakly bound, comparable to molecular length.

Contribution

It introduces a method to measure electron-hole pair sizes in conjugated molecules using momentum-dependent EELS, showing these pairs are extended and not localized.

Findings

Electron-hole pairs extend along the molecules, comparable to their length.

Pairs are only limited by structural constraints, indicating weak binding.

No evidence of localized excitations on single units.

Abstract

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,omega)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.