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

The quantum lattice motion of shea conjugated polymer chains (N less than or equal to 70) described by the Su-Schrieffer-Heeger (SSH) model is studied within the adiabatic approximation. We find that for these short chains only three lattice degrees of freedom are strongly affected by low-energy electronic transitions. Moreover, we show that for N less than or equal to 30 these three degrees of freedom are only weakly coupled to each other. This allows us to perform a calculation of the optical-absorption spectrum of polymer chains in which the lattice is treated quantum mechanically rather than (semi)classically. For the standard set of SSH parameters for trans-polyacetylene, the classical (rigid-band) absorption edge is smeared over an energy interval of 0.5 eV by the lattice quantum fluctuations. The validity of the adiabatic approximation is investigated. Finally, we find a strong size dependence of the onset of the optical-absorption spectrum.