Theory of band and bound states of double triplet excitations in molecular crystals. I

The Hamiltonian of double triplet excitations of a molecular crystal is obtained. The permittivity tensor of a molecular crystal is obtained in the range of frequency of double triplet excitations. This tensor is expressed in terms of the Fourier transform of the retarded Green's function, which is found from a system of algebraic equations. It is shown that in the electric dipole approximation the double triplet states are excited by light through the excitation of a virtual singlet exciton, which determines the polarization of the absorption in the given frequency range.