Excitation Dynamics in Single Molecular Crystals of alpha-hexathiophene from Femtoseconds to Milliseconds

We identify odd and even parity states and measure transient photomodulation spectra of Frenkel excitons in alpha-hexathiophene single crystals. The lowest Frankel excitons are characterized by a stimulated emission band at ~2 eV and an absorption band at 1.3 eV. Intramolecular internal conversion governs transitions from upper excited states to the lowest exciton and occurs in 100-200 fsec. We observe exciton trapping on the nsec time scale; as a result, the msec photomodulation spectrum is dominated by the absorption of trapped excitons.