DENSITIES OF STATES OF THE (1)L(B) EXCITON BAND AND LUMINESCENCE PROCESSES IN PYRENE

Densities of states and bandwidths of (1)L(b) excitons in pyrene micro crystallites embedded in PMMA films are studied, using a technique app lied to anthracene microcrystallites. The excitation spectra consist o f two components. One referred to as component A shows practically an exponential decrease with increasing energy. Component A is interprete d as owing to the wavevector k = 0 component in the density of states of microcrystallites; the spectral width of component A is equal to th e exciton bandwidth. The exciton bandwidth varies with the microcrysta llite size, and the exciton bandwidth 2B for bulk crystal is estimated to be at most 330 cm(-1). For large microcrystallites, the Delta k = 0 selection rule for optical transitions becomes established, causing observed exciton bandwidth tend to zero. On the contrary, component 13 which overlaps with component A appears as a broad band. Its intensit y is relatively weak but it is not negligible for large microcrystalli tes where component A is weak. With observed exciton bandwidth 2B 330 cm(-1) for bulk crystal, the self-trap depth E(SF) and the lattice rel axation energy E(LR) of the shallow self-trapped state (V state) are d erived to be E(SF)less than or equal to 15 cm(-1) and E(LR)less than o r equal to 180 cm(-1), respectively, in connection with the (1)L(b) ex citon band. Owing to these numerical results the excitonic state and e xciton relaxation processes in pyrene crystals are understood quite sa tisfactorily.