TRIPLET-STATES AND OPTICAL ABSORPTIONS IN FINITE POLYENES AND CONJUGATED POLYMERS

We study the nature of triplet states in correlated quasi-one-dimensional bands, with emphasis on the half-filled case relevant to conducting polymers. To incorporate both electron-phonon and electron-electron interaction effects, we use a Peierls-extended Hubbard Hamiltonian, which we solve for finite systems via (numerically) exact diagonon (Lanczos) techniques. We extend our results to polymers using both standard finite-size extrapolation techniques and a novel boundary-condition averaging scheme. First, we examine the nature of the lowest triplet state, focusing on the crossover from the free-electron picture of neutral soliton pairs to the strongly correlated limit of spin excitation. We then examine the optical absorption spectra corresponding to (allowed) triplet-triplet transitions; for this purpose it is important that our novel boundary-condition averaging method maintains sharp gap-edge features while reducing spurious rapid frequency variations produced by the finite-size approximations to the band-to-band continuum. We compare our results to data on triplet transitions in finite polyenes and discuss possible additional experimental implications, including the interpretation of recent spin-dependent photomodulation (SDPM) experiments in both trans-(CH)x and non-degenerate ground-state conjugated polymers.