Absorption and photoionization of the donor level in CdF$_2$ semiconductor crystals

A model of strong vibronic interaction is proposed to interpret the specific features of infrared absorption and photoionization in CdF$_2$ semiconductor crystals. The model takes into account the polaronic nature of the conductivity in these crystals and the profound configuration shift of the free and bound polaron states. It is shown that the intense infrared absorption band in the crystals is not due to the transitions of charge carriers from hydrogen-like donor levels to the conduction band, but is caused by the phonon replicas of intracenter transitions. The low-temperature photoconductivity (in the temperature range 0–70 K) is a result of tunneling transitions between the phonon states of bound and free polarons, since these states are separated by rather high potential barriers. Overcoming the barriers in both directions is responsible for equilibration in the polaron subsystem upon the photoexcitation of charge carriers. The tunneling character of this process is responsible for the slight variation in the equilibration time in the above-indicated temperature range.