EXCITON SCATTERING, K-SELECTION RULE, EXCITON BANDWIDTH IN PYRENE MICROCRYSTALLITES, AND LATTICE-RELAXATION ENERGY FOR THE ORIGIN OF V-LUMINESCENCE

The lowest (1)L(b) singlet exciton band of pyrene crystal is studied. Analyzing excitation spectra of pyrene microcrystallites recorded at 2 K, we find that the exciton bandwidth increases superlinearly with mi crocrystallite size toward the maximum value 330 cm(-1), which is inte rpreted as the bandwidth of the (1)L(b) band in bulk crystal. Using th e obtained bandwidth for bulk pyrene crystal and the exciton-phonon co upling constant, the lattice relaxation energy for the (1)L(b) exciton is estimated to be 180 cm(-1) and the self-trap depth for the V state 15 cm(-1). The k selection rule for the exciton creation is lost in s mall size microcrystallites, but it is recovered in microcrystallites larger than 50 Angstrom. Comparing the exciton bandwidth data with tho se for anthracene microcrystallites, it is found that the exciton surf ace scattering, which occurs strongly in anthracene microcrystallites, does not play a significant role in pyrene microcrystallites. The sur face scattering layer thickness in anthracene microcrystallites is fou nd to be 30 Angstrom.