Electronic properties of aromatic hydrocarbons. III. Diffusion of excitons

Abstract

Non-conducting excited states are known to occur in certain insulating crystals as a result of excitation in the fundamental absorption band. The excited region is called an exciton. Excitons are believed to be mobile and capable of transporting energy through a crystal. In part II the migration of excitons was shown to give a quantitative explanation of the observed transfer of fluorescence excitation in solid solutions of aromatic hydrocarbons. However, in common with other experimental evidence in this field, the exciton motion itself was inferred rather than observed. To demonstrate exciton migration conclusively it is necessary to measure the diffusion constant, or the diffusion length (D$\tau $)$^{\frac{1}{2}}$, where $\tau $ is the lifetime of the excited state. In this paper a method is described by which the migration of excitons across a thin specimen can be observed, and solutions of the diffusion equation with the appropriate boundary conditions are shown. The method has been applied experimentally to anthracene, and the exciton flux as a function of distance agrees with the theory. The observed diffusion length is 460 angstrom. In an isotropic medium this would correspond to a root-mean-square displacement of 1120 angstrom between the points of origin and decay of an exciton. Comparison with the results obtained in part II suggests that in anthracene, which is strongly anisotropic, the migration of excitons occurs most easily between molecules in adjacent b-c planes, i.e. normal to the laminae in the graphite-like structure.