Theory of Photoinduced Phase Transition in the Quasi-One Charge Transfer Compound Ttf-Ca

We theoretically study the photoinduced ionic→neutral phase transition in the quasi-one-dimensional molecular crystal TTF-CA. Our theoretical model includes strong intra-chain Coulomb interactions as well as very weak inter-chain interactions. Within the mean-field picture, we investigate the nonlinear lattice relaxation of a charge transfer exciton, and clarify the adiabatic path from its Franck-Condon state to a macroscopic neutral domain. It is found that the lowest state of such a single charge transfer exciton can not relax down to the neutral domain straightly, but a large excess energy is necessary so that it can overcome a high barrier. The aggregation of two identically shaped neutral domains is also discussed.