Effect of quantum phonon fluctuations on optical properties of finite semiconducting chains. Adiabatic and nonadiabatic results

We investigate the optical properties of one-dimensional semiconducting chains where the electron-phonon coupling represents the dominant interaction. The isolated monomers possess two electronic orbitals, the lowest being occupied. In addition to hopping, two phonon modes couple to and mix different electronic states. We compute the optical response exactly, by Lanczos algorithm, and compare it to that deduced approximately, within the adiabatic ansatz (i. e. factorizing the wave function of the electron-phonon system). On this basis we discuss the validity of this approximation.