Exciton scattering and the dephasing of ESR and optical absorption lines: The 1,2,4,5-tetrachlorobenzene triplet exciton problem

Abstract

The relationship between exciton scattering induced dephasing of the ESR and the optical transitions of triplet exciton bands is examined by properly including exchange in the line shape formalism. First, a simple heuristic model demonstrates that while the optical dephasing time can be associated with the exciton scattering rate, the ESR dephasing time is more closely associated with the distribution of final states in the scattering process. The full exchange problem is then examined and a formal solution is obtained for the low power steady state ESR spectrum. Application to the 1,2,4,5-tetrachlorobenzene triplet exciton system shows that the exciton ESR spectrum with its associated 1 μsec dephasing time can be reproduced using a 20 psec exciton scattering rate obtained from the optical dephasing time. Exciton scattering occurs to a small region of k space (∼5% of the band) surrounding the initial k state. These results resolve the apparent conflict between the optical and ESR spectra in this system and indicate that exciton transport, while not strictly coherent, is nonetheess predominantly wavelike in character.