PHOTOINDUCED SPONTANEOUS AND STIMULATED-EMISSION IN SEXITHIOPHENE SINGLE-CRYSTALS

Temperature dependent steady-state and transient photoluminescence (PL ) measurements were carried out on sexithiophene (6T) single crystals. As the temperature is lowered, three different sets of equally spaced peaks appear on the steady state PL spectrum. This behavior is interp reted in the frame of the molecular exciton theory. The first energy s et is attributed to transitions to the lower level of the Davydov spli tting of the 1B(u) excited state ui the isolated molecule, whereas the two other sets are ascribed to the crystal defects. An analysis of th e PL emission and excitation spectra allowed us to establish a general exciton energy scheme of sexithiophene crystal. At low excitation lev els, the transient PL follows a single exponential decay, with a decay time of 1.8 +/- 01 ns. When the excitation energy is increased, a sec ond, much faster component is added. Concurrently, the PL spectrum nar rows to a single line centered at the origin of the low energy set of the steady state PL. The width of the emission line is 13 cm(-1) at 10 K. and tends to widen and shift towards lower energies when the tempe rature is raised. All these observations are accounted for in term of stimulated emission. (C) 1998 Elsevier Science B.V.