Spin absorption in triplet exciton systems

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. A theoretical study of the low-temperature paramagnetic resonance lines of linear-chain organic free-radical crystals characterized by triplet excitons is made by means of a generalized moment method. Techniques which permit the determination of the moments of absorption lines at all temperatures, and to any desired order in interaction parameters, are developed. The necessity for the usual truncation of the hamiltonian is avoided with the aid of the absorption operator expansions of Cheng. The method is used to calculate the zeroth through fifth moments of the g=2 doublet absorption lines of crystals of Wurster's blue perchlorate. The hamiltonian for the system includes exchange and anisotropic spin interactions, and coupling to a strong external magnetic field. It is shown that exciton creation, annihilation, and transfer processes have negligibly small effect on the absorption. The calculated exciton density, [rho], and the line width, W, have temperature dependence given by [...]. The inclusion of phonon coupling, through distance-dependent exchange integrals, is shown, by means of a canonical linear transformation, to produce an effective exciton-exciton repulsion; this repulsion decays exponentially with distance. Inclusion of this phonon interaction in the moment calculation leads to exciton density and line width varying like [....] The activation energy, [...]E, can account for the "anomalous" line broadening observed in many triplet systems