An examination of radiative and nonradiative excitation transport in thin anthracene crystals: Transient grating experiments

Abstract

Picosecond transient grating (TG) experiments have been performed on ultrathin anthracene sublimation flakes at room temperature. The laser excitation wavelength was tuned to the singlet exciton band and the diffracted signal was detected in the reflection geometry. An extension of the expression [1] for the contribution from optical reabsorption to the TG fringe spacing and time dependences is derived. It includes the effects of directional radiation from molecules and a critical angle for total internal reflection. The extension and the original treatment both show that reabsorption will only influence the grating for very large fringe spacings, and therefore does not inhibit the use of the TG for the study of exciton transport. The fringe spacing dependence for small fringes reveals no measurable exciton diffusion in either the a- or b-crystallographic directions. This sets an upper bound on the diffusion coefficient in anthracene at room temperature at D ⩽ 5 × 10 −4 cm 2 s −1.