Homogeneous and Inhomogeneous Triplet Exciton Absorption Lines in Molecular Crystals

Triplet exciton absorption lines in pure anthracene and pyrene crystals are investigated at very high resolution using photoexcitation in the lower temperature (<90 K) regime. The profiles for the lower Davydov components (zero-phonon) are observed to be most sensitive to crystal quality. Linewidths as small as 0.02 cm−1 are observed for the lowest exciton of anthracene with the upper component about 50 times broader. The T-dependences of the linewidths for the Davydov components of anthracene are shown to be explicable in terms of combined linear and quadratic exciton-photon coupling. The T-dependence of the zero-phonon to phonon sideband intensity ratio for pyrene is well described by the linear exciton-photon interaction in which the exciton is taken to be localized.