Exciton scattering, κ selection rule, exciton bandwidth in pyrene microcrystallites, and lattice relaxation energy for the origin of V luminescence Original Research Article

The lowest 1Lb 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 microcrystallite size toward the maximum value 330 cm−1, which is interpreted as the bandwidth of the 1Lb band in bulk crystal. Using the obtained bandwidth for bulk pyrene crystal and the exciton-phonon coupling constant, the lattice relaxation energy for the 1Lb exciton is estimated to be 180 cm−1 and the self-trap depth for the V state 15 cm−1. The κ selection rule for the exciton creation is lost in small size microcrystallites, but it is recovered in microcrystallites larger than 50 Å. Comparing the exciton bandwidth data with those for anthracene microcrystallites, it is found that the exciton surface scattering, which occurs strongly in anthracene microcrystallites, does not play a significant role in pyrene microcrystallites. The surface scattering layer thickness in anthracene microcrystallites is found to be 30 Å.