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

By using a simple model of soliton±antisoliton pairs to simulate the quantum lattice ̄uctuations 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 ̄uctuation. It is shown that the ground-state nondegeneracy suppresses the quantum ̄uctuations, 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. # 2001 Elsevier Science B.V. All rights reserved.