Transient spectroscopy of Frenkel excitons in alpha-hexathiophene single crystals

Photoexcitation dynamics in purr single molecular crystals of alpha-hexathi ophene are studied using ultrafast photomodulation spectroscopy in the spec tral range from 0.15 to 2.3 eV. Frenkel excitons are uniquely described by transient photoinduced absorption and stimulated emission bands at 1.3 and similar to 2 eV, respectively. The transient spectra are interpreted using exciton states identified by one- and two-photon absorption. Exciton dynami cs in pure crystals dramatically differs from that in polycrystalline films , which is attributed to a much lower defect density in single crystals. In tramolecular internal conversion rate is found to be similar to 150 fs for excitation energies up to 3.6 eV. (C) 2000 Elsevier Science B.V. All rights reserved.