Semiclassical formalism of optical absorption and breathers in polyacetylene

A formalism for the optical absorption in a coupled electron-ion system is developed. The formalism assumes an initial trajectory with frequency much lower than electronic energy scales, and a short-memory condition, i.e., the excited-state trajectories diverge away from the initial trajectory within one period. The result shows nonadiabatic features such as level broadening, sidebands, and tails in nonclassical regimes. We apply the formalism to breather modes in polyacetylene and show that they can account for the intragap absorption tail and for the observed photoinduced absorption at 1.35 eV.