Some features of two-particle exciton-phonon excitations in molecular crystals

In this paper we present the results of a theoretical study of the optical absorption line shapes in the two-particle exciton-intramolecular phonon (IP) excitation regime in molecular crystals. The exciton-IP coupling was handled considering the exciton and the IP as a pair of interacting pseudo-particles and exploring the conditions for branching-off a two-particle bound state (TPBS) from the two-particle-continuum (TP), without invoking the exciton-IP decoupling approximation. We were able to handle both symmetry-allowed electronic transitions as well as symmetry-forbidden, vibronically-induced, electronic excitations. For symmetry-forbidden transitions we have provided a systemic derivation of Rashba's, Koster-Slater type formula where the local perturbation results from quadratic IP coupling. For the case of symmetry-allowed electronic transitions we derived a new approximate expression for the line shape, incorporating both linear and quadratic coupling terms and elucidating the conditions for a peaceful coexistence of TP and TPBS. Next, we have considered the exciton-IP system coupled to intermolecular lattice phonons (LP), taking into account the effects of bath diagonal "strong" coupling and of off-diagonal "weak" coupling. The diagonal exciton-LP coupling results in the appearance of side bands which should be viewed as three-particle resonances corresponding to an exciton + IP + LP. Off-diagonal exciton-LP coupling results in the relaxation of the exciton-IP TPBS to the TP continuum at finite temperatures. Finally, we have explored the effects of static structural disorder on the exciton-IP system, considering the effects of Anderson-type diagonal disorder. Configurational averaging was performed utilizing the coherent potential approximation, while the disorder field was represented in terms of a lorentzian distribution of the site excitation energies. Structural disorder drastically affects the TPBS, while the TP continuum is only slightly affected by disorder scattering.