Basal dislocations in anthracene crystals

The low-temperature fluorescence spectra (T = 4.2 K) and etching patterns of anthracene crystals grown from a benzene solution (BSAC) are investigated. It is shown that these spectra exhibit a new series as compared with the fluorescence spectrum of vapour-grown crystals. The series is red-shifted by 130 cm−1 from the exciton band bottom. Its vibrational intervals coincide with the intramolecular vibrations of anthracene. The series is ascribed to basal dislocations of (001) [010] type. After annealing of such crystals for 1 h at T = 398 K basal dislocations vanish and non-basal dislocations of (100) [010] or (201) [010] type appear. This phenomenon is partly reversible: in annealed crystals kept for a week at room temperature nonbasal dislocations vanish and basal appear again, i.e. relaxation processes occur.