EXCHANGE FORMALISM APPLIED TO ONE-DIMENSIONAL TRIPLET EXCITONS - OBSERVATION OF RESTRICTED AND TEMPERATURE-DEPENDENT K TO K' SCATTERING IN THE ZERO-FIELD OPTICALLY DETECTED MAGNETIC-RESONANCE SPECTRUM OF 1,2,4,5-TETRACHLOROBENZENE

Zero-field optically detected magnetic resonance (ODMR) spectra of the D-|E| transition in 1,2,4,5-tetrachlorobenzene excitons have been obtained for a variety of temperatures (1.2-4.2°K), microwave powers (0.1-5000 mW), and modulation frequencies (25-1000 Hz). These data have been fit to a theoretical model for band-to-band transitions of one-dimensional triplet excitons that explicitly incorporates the effects of k to k′ scattering, microwave power, and interaction of the band with a thermal bath. It is shown that a proper interpretation of the data can be made only if one considers the actual observables used in monitoring the zero-field spectrum. Analysis of the data indicates the low-temperature k to k′ scattering is restricted to k′ states that lie within roughly one third of the total bandwidth from k. This restricted or nonuniform range is temperature independent below 2.2 °K, but increases above this temperature. At temperatures below 1.35°K, the ODMR line shape depends on the microwave modulation frequency used. Explicit consideration of the actual distribution of one-dimensional chain lengths and potentially chain-length dependent effects that occur in a real crystal are also discussed, and questions concerning the validity of an exciton lifetime" are raised. © 1980 American Institute of Physics."