To the Theory of the ESR Lineshape of Triplet Excitons in Molecular Crystals: Influence of the Quasi‐Incoherent Exciton Motion and Comparison with Experimental Data

Abstract

AbstractIn a recent paper, on the basis of the Haken‐Strobl model for the coupled coherent and incoherent motion of excitons in molecular crystals, an equation of motion for correlation functions has been derived, describing the influence of the quasi‐incoherent motion of triplet excitons on the ESR lineshape. From this equation analytic expressions for the positions and widths of the ESR lines have been derived by second order perturbation theory. In this paper it is shown that these expressions are in excellent agreement with results obtained from exact computer calculations. The dependence of the positions and the widths of the ESR lines on the orientation of the magnetic high field in the crystal and their extreme values are discussed. From the comparison with experimental data of Haarer and Wolf for the line positions averaged values for the fine structure parameters in anthracene are derived as D = 765 G and E = −98.8 G. An effective hopping rate of T1 Y 4.4 × 1010 s−1 has been estimated from the line widths. Finally it is argued that the influence of librations on the ESR linewidth may be as important as that of the motion of the excitons between inequivalent molecules.