Theory for photoinduced ionic-neutral structural phase transition in quasi one-dimensional organic molecular crystal TTF-CA

Using a quasi one-dimensional extended Peierls-Hubbard model, we theoretically study the photoinduced ionic-neutral structural phase transition in an organic molecular chain crystal TTF-CA, by means of the adiabatic approximation and the mean-field theory. This model includes strong intra-chain Coulomb interactions which nonlinearly depend on the distance between TTF and CA molecules, and it also includes a weak inter-chain interaction. We will be concerned with the case that the ionic phase is just below the neutral one, and their energies and charge (spin) distribution are investigated, as well as their energy band structures. Moreover, we calculate the optical absorption spectrum by the classical Monte-Carlo method, including the exciton effect and the thermal lattice fluctuations. We also study the nonlinear lattice relaxation of a charge transfer (CT) exciton, and clarify the adiabatic path from its Franck-Condon state to a macroscopic neutral domain. This domain, being separated from the CT exciton by a high barrier, can not be generated by a single photon resonated to this exciton. On the energy surface of the ground stale, we find various shallow energy minima, which prevent the neutral domain from decay back to the starting ionic ground state.