Ordering of low energy electronic excitations in α-sexithiophene single crystal

The low energy optical transitions in α-sexithiophene (αT6) single crystal are investigated applying polarized absorption spectroscopy. In contrast to thin films single crystal optical characterization gives rise to well resolved and sharp spectra allowing to derive a vibrational assignment. The absorption spectrum at T=30K shows resonances which can be explained in terms of purely electronic and vibronic transitions. The lowest excited electronic state in αT6 single crystal can be assigned to 21Ag and the narrow resonances in the absorption spectrum can be interpreted as levels arising from vibronic coupling (mixing of electronic states via Herzberg-Teller coupling).