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

The low energy optical transitions in alpha-sexithiophene (alpha T-6) 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 alpha T-6 single crystal can be assigned to 2(1)A(g) 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).