Quantum Lattice Fluctuations and the Linear and Nonlinear Optical Susceptibility of Degenerate Ground State Conjugated Polymers

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1>.z> We treat the direct photoproduction of charged solitons as a nonlinear Franck-Condon problem and obtain analytical expressions for the linear optical susceptibility, x (w). With the help of the oscillator strength sum rule, we decompose the Jr-Jr* oscillator strength into two parts; a contribution where the final states are SS pairs and a contribution where the final states are free electron-hole pairs (as in the noninteracting rigid-lattice). The linear optical coefficients calculated from x (w0'), is presented, demonstrating that, for any third-order process, contributions arising from neutral SS pair configurations as intermediate states are one to two orders of magnitude larger than the corresponding rigid-lattice contribution. This mechanism for x is enabled by nonlinear zero-point motion which provides a finite Franck-Condon overlap between the ground and SS excited state lattice wavefunctions. The large contribution to x from the SS intermediate states results from the large transition dipole moment between the free electron-hole pair excited states of Bu symmetry and the Ag symmetric neutral SS excited state. This enhanced transition dipole moment is a consequence of the large virtual shifts of oscillator strength associated with the localized SS electron-lattice configuration. The third-harmonic conversion efficiency x (3w) is further enhanced by a condition unique to degenerate ground state systems, simultaneous two and three-photon resonance.