THE ROLE OF SOLITON ANTISOLITON CONFIGURATIONS WITH AG-SYMMETRY IN THE NONLINEAR OPTICAL-RESPONSE OF POLYACETYLENE

The contribution of soliton-antisoliton (SSBAR) configurations with A(g) symmetry to the third-order nonlinear optical response is investigated for conjugated polymers having a degenerate ground state. The results indicate that nonresonant terms in the expansion of chi(3) (-3-omega:omega, omega, omega) involving the A(g) symmetric neutral SSBAR pair configuration as an intermediate state are one to two orders of magnitude larger than the corresponding rigid band contribution. This mechanism for the third order response is enabled by nonlinear zero-point motion which provides a finite Frank-Condon overlap between the ground and SSBAR excited state lattice configurations. The contribution to chi(3) (3-omega) from the neutral SSBAR configuration is due to the large transition dipole moment and is further enhanced by a condition unique to degenerate ground state systems, simultaneous two- and three-photon resonance.