Semiclassical formalism of optical absorption

A semiclassical formalism for the optical absorption in a coupled electron-ion system is developed. The formalism assumes (a) an initial periodic trajectory whose frequency is much lower than electronic energies, and (b) a short-memory condition, i.e., the excited-state trajectories diverge away from the initial one within one period. The result shows nonadiabatic features such as level broadening, sidebands, and tails in nonclassical regimes. The formalism is demonstrated on a model for polyacetylene and can account for unusual absorption data.