Franck–Condon enhancement of χ(3) in a conjugated polymer under double resonance conditions

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Abstract
Stimulated inverse Raman spectroscopy is used to investigate exciton–phonon coupling in the polydiacetylene PTS. The Raman resonances used the v=0 and one v=1 singlet exciton as intermediate states, and the consequent Stark shiftings and splittings of the excitons are measured. We use these optical Stark effects to determine the coupling between the ground state phonons and the v=0 exciton. This coupling is as strong for the combination modes as for the fundamental modes. The coupling between several combinations modes and the v=1 exciton is 4 to 10 times greater than it is for the v=0 exciton, in agreement with calculations of the overlap integrals for each transition.