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Abstract
We have observed two-pulse Hahn echoes and three-pulse stimulated echoes in the linear chain triplet excitons occurring in single crystals of 1,2,4,5-tetrachlorobenzene. The results indicate that at very low temperatures (1·2–1·6 K) the exciton motion is perturbed by scattering on impurities and traps. When the temperature is increased, a shortening of the phase memory time is observed, which is caused by exciton-phonon interactions. The rate at which the exciton k-states are scattered increases with temperature. When using optical detection we then observe a motionally narrowed resonance line centred exactly around k = ± π/2a. Above 8 K this resonance line broadens again, indicating that the exciton motion becomes incoherent.