  We extend a recently introduced semiclassical approach to calculating the
influence of local lattice fluctuations on electronic properties of metals and
metallic molecular crystals. The effective action of electrons in degenerate
orbital states coupling to Jahn-Teller distortions is derived, employing
dynamical mean-field theory and adiabatic expansions. We improve on previous
numerical treatments of the semiclassical action and present for the
simplifying Holstein model results for the finite temperature optical
conductivity at electron-phonon coupling strengths from weak to strong.
Significant transfer of spectral weight from high to low frequencies is
obtained on isotope substitution in the Fermi-liquid to polaron crossover
regime.
