Study of the Reflection Spectrum of Crystalline Anthracene: Evidence for the Existence of Defects

This paper reports the results of a study of the reflection spectrum of crystalline anthracene at 4 and 77°K. At 4°K the reflection spectrum has, on the low-energy side of the transition to the first exciton state, a number of narrow regions of low reflectivity. At 77°K these antireflection lines do not appear. A crystal may be cycled repetitively between 4 and 77°K, reversibly exhibiting this phenomenon. It is argued that the observed antireflection lines are associated with exciton states bound to lattice defects, and that the lattice defects arise because of the inclusion of chemical impurities in the crystal. The observed antireflection lines can be correlated with lines in the low-energy emission spectrum of anthracene; our data are sufficient to resolve some previous difficulties of interpretation in the assignment of the luminescence spectrum. Some aspects of the interpretation of the linewidths and line intensities, and of the relationship between the bound excitons and the free exciton band, are discussed.