Phenomenological representation of triplet-exciton kinetics

A general density-matrix formulation is derived in a way suitable for the analysis of optical and magnetic effects due to the triplet-exciton kinetics in molecular crystals, in the presence of a microwave field inducing transitions among the spin levels. The expansion of the density matrix as a power series of the microwave field intensity provides a hierarchy of systems of equations, the knowledge of higher-order corrections requiring the solutions of all lower-order terms. The solutions of the zeroth-, first- and second-order equations are relevant for the calculation of polarization and delayed fluorescence effects. ESR lineshapes and DF ODMR spectral profiles, respectively.