Location of the low-energy ${}_{}{}^1{}_{}{}^{}$${\mathit{A}}_{\mathit{g}}$ state in a polythiophene oligomer by two-photon absorption spectroscopy: α-sexithienyl

The two-photon excitation spectrum in polycrystalline α-sexithienyl (${\mathit{T}}_6$) thin films has been investigated at 4.2 K in the spectral range between 910 and 1180 nm of the fundamental laser radiation. The intense two-photon absorption band at 18 350 ${\mathrm{cm}}^{\mathrm{-}1}$ is assigned to the $2^1$${\mathit{A}}_{\mathit{g}}$ exciton band origin. The comparison of the one-photon and two-photon excitation spectra shows that the lowest ‘‘gerade’’ exciton level lies at 898 ${\mathrm{cm}}^{\mathrm{-}1}$ above the lowest one-photon-allowed 1 ${}_{}{}^2{}_{}{}^{}$${\mathit{B}}_{\mathit{u}}$ exciton level.