Nonlinear absorption and luminescence in organic molecular crystals.

摘要

This dissertation presents results of a study of the enhanced two-photon induced luminescence in certain molecular crystals consisting of intramolecular charge-transfer compounds and discusses the results of the study of spontaneous light emission from a system of interacting Frenkel excitons.;In part I of the dissertation we explain enhanced two-photon absorption in certain centrosymmetric organic molecular crystals in which intramolecular charge transfer takes place. Specifically we suggest that charge transfer causes the interacting molecules in a unit cell to have non-zero dipole moments in the excited state of the molecules contributing to a resonance in the two-photon absorption coefficient. An anti-parallel orientation of the molecular dipole moments in the unit cell allows for formation of excimer states but disallows the production of SHG in the same materials. Our estimate of the ratio of the two-photon to the one-photon absorption coefficient for heteriltricyanobutadiene crystals suggests a nanosize optical response resulting from a significant charge delocalization in molecular chains which form the crystals.;In part II we investigate frequency resolved spontaneous light emission (SLE) from a system of interacting Frenkel excitons excited with high intensity optical radiation. Application of non-equilibrium diagram techniques allows us to address the problem of computation of the SLE cross section by solving the Dyson equation. In the lowest order for the self-energy part, the forward scattering and the luminescence components of the SLE cross section are distinguished. The angular distribution of the luminescence is analyzed in detail for the simplest case of a cubic crystal. It is also shown that the multi-photon absorption process produces an exciton density which affects the absorption and the emission lineshape (exciton dynamic Stark effect) as well as the non-radiative dephasing rate. This dependence corresponds to a nonlinear relation between the luminescence and the incoming radiation intensities.