Subgap absorption and quantum lattice fluctuation in polymers: effects of the ground-state degeneracy

By using a simple model of soliton–antisoliton pairs to simulate the quantum lattice fluctuations in one-dimensional conjugated polymers, the subgap optical absorption results from the transitions between the lowest unoccupied and the highest occupied levels off the conduction and valence bands, respectively, in the soliton–antisoliton excitations. There is no gap in the absorption induced by the quantum fluctuation. It is shown that the ground-state nondegeneracy suppresses the quantum fluctuations, so that the subgap absorption is reduced and then there appears a clear onset in the absorption spectrum of nondegenerate polymers. The result is in good agreement with experiments.