Transient Spectroscopy Of Frenkel Excitons In Alpha Hexathiophene Single Crystals

We present the first comprehensive study of ultrafast photoexcitation dynamics in pure single molecular crystals of alpha-hexathiophene. Transient photomodulation spectra of primary excitations were obtained in the spectral range from 0.15 to 2.3 eV. Frenkel excitons were uniquely identified by polarized photoinduced absorption and stimulated emission bands at 1.3 and ~2 eV, respectively. Intramolecular internal conversion was found to be the fastest (~150 fsec) relaxation channel for excitation energies up to 3.6 eV.