Polarized absorption and Davydov splitting in alpha-sexithienyl single crystal at 4.2K

The polarized absorption spectra in the be face of an alpha-sexithienyl (T- 6) single crystal have been measured at 4.2K. The origin of the lowest elec tronic transition is at 18360 cm(-1) and has been assigned to the lowest b- polarized a(u) Davydov component of the 1(1)B(u), molecular level. The seco nd optically allowed Davydov component (b(u),) is polarized in the ac cryst al plane and is located at 20945 cm(-1). Therefore the interchain interacti on leads to a Davydov splitting of about 2600 cm(-1). The vibronic manifold in the absorption spectra has been interpreted in terms of the Herzberg-Te ller and Franck-Condon vibronic coupling. In particular, two false origins due to the coupling of the molecular electronic levels 1(1)B(u), and 2(1)B( u) have been identified at 18486 cm(-1) and 18657 cm(-1). The totally symme tric modes involved in the coupling are in excellent agreement with the Ram an scattering data of the single crystal.