Frenkel excitons in a vibrating molecular crystal

This paper is concerned with a study of the interaction between collective electronic excitations and collective vibrational excitations in a molecular crystal. A strong coupling theory is developed and is used to calculate the vibrational structure of the collective excitation in the Frenkel limit. The frequency dependence of the self-energy of the excitation and dispersion in the optical phonon branch of the vibrational spectrum are included in the analysis. The line shapes in the absorption spectrum of a molecular crystal are considered for the two limiting cases of exciton-phonon coupling-namely weak and streng coupling. One of the topics considered in detail is the damping of the zero phonon transition.