Photoexcitation energy loss in molecular semiconductors

Abstract Energy relaxation of electron-vibrational excitations through the emission of ground-state phonons is investigated in single sexithiophene crystals. In the experiment, short laser pulses excite vibron states, i.e. excitons bound to intramolecular vibrations. A fraction of the excess vibrational energy is released to the electronic ground state through a vibron fission process in which a non-equilibrium population of C–C stretching vibrations is created. This basic phenomenon was predicted in the 1960s by Davydov and Rasbhe, but, as far as we know, never observed. The hot ground-state intramolecular phonons are probed by a delayed laser beam which stimulates transitions from these vibrational modes to the lowest exciton bands. Hot vibrational states up to the third quantum level are observed.