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 spectros copy. In contrast to thin films single crystal optical characterizatio n gives rise to well resolved and sharp spectra allowing to derive a v ibrational assignment. The absorption spectrum at T=30K shows resonanc es which can be explained in terms of purely electronic and vibronic t ransitions. The lowest excited electronic state in alpha T-6 single cr ystal can be assigned to 2(1)A(g) and the narrow resonances in the abs orption spectrum can be interpreted as levels arising from vibronic co upling (mixing of electronic states via Herzberg-Teller coupling).