On the optical absorption of free polarons at weak coupling

The optical absorption of Frohlieh polarons as a function of the frequency of the incident light is calculated within the Lee‐Low‐Pines formalism. A simple calculation, which is developed here in full detail, gives a result for the absorption coefficient which is exact in order a. Gurevich et al. [1] had obtained this perturbation result using involved calculations based on Green's functions. This result is also extracted here from the “dissipation” calculation of Feynman ct al. [2]. An analytical expression for the optical absorption between 0 < α < 1 is proposed. For α = 1 the main correction to the perturbation result is e decrease of about 15% around the maximum of the absorption. This seems to explain the difference between the experimental absorption detected in CdO (α = 0.75) by Finkenrath et al. [14] and the perturbation result. The effect of a static electric field on the optical absorption coefficient is examined. For low temperatures a shift towards lower frequencies of the threshold for absorptiou is predicted. This shift depends on the mobility and effective mass of the polaron.