Triplet exciton spectral line shapes in molecular crystals: 1,2,4,5-tetrachlorobenzene

Abstract

Triplet exciton optical absorption line shapes, widths, and positions have been measured as a function of temperature in the molecular crystal 1,2,4,5-tetrachlorobenzene (TCB). At temperatures above 20 K the absorption line has a gaussian shape and increases in width with temperature. At 14 K the line becomes lorentzian. This change in line shape with temperature is discussed in terms of a model first suggested by Toyozawa. The lorentzian line is taken to be evidence of a delocalized exciton state, while the gaussian high temperature line shape implies the localization of the exciton by lattice phonons. The nature of the difference between the optical line width and the previously reported TCB ESR line width is also briefly discussed.