Ultrafast decay dynamics of excited and charged states in alpha-sexithienyl film as revealed by femtosecond transient absorption and picosecond fluorescence spectroscopy

Primary processes of photoexcited states in an alpha-sexithienyl film are investigated by femtosecond transient absorption spectroscopy and picosecond time resolved fluorescence spectroscopy. Four species are observed in the transient absorption measurement. A broad absorption that shows a very rapid relaxation (within a few picoseconds) is ascribed to a singlet Frenkel exciton state. An oscillating structure is also apparent immediately after excitation and is ascribed to the Stark effect induced by a charged species (ion pair state). The latter state decays within 200 ps and is replaced by a different oscillating structure that is due to a thermal effect induced by a dissipation of excess energy. Another band owing to a tripler state appears via a very rapid conversion from a higher singlet exciton state. Fluorescence decay curves are well fitted with lifetimes of the charged state, which indicates that most of the emission is brought about by the charge recombination process, and furthermore, a charge-transfer (CT) emission band that has not been reported is observed.