Quantum lattice motion and optical absorption in conjugated polymers:Nonadiabatic theory

We calculate the low-energy part of the optical-absorption spectrum of short (N=30) conjugated polymer chains described by the Su-Schrieffer-Heeger model. The calculation is based on a detailed study of the quantum lattice dynamics and accounts for the nonadiabatic mixing of different electron configurations caused by the lattice motion. We consider optical transitions to about 600 mixed electron-lattice excited states. The obtained subgap smearing of the rigid-band absorption edge agrees well with the experiment and with what we found earlier in the adiabatic approximation, while at energies close to the rigid-band transition, the nonadiabatic mixing washes out the spectral structure observed in the adiabatic theory. Finally, we show that for energies well above the rigid-band transition, the absorption spectra of both theories agree, even though the wave functions that they predict are very different. © 1996 The American Physical Society