We measured the far-infrared reflectivity spectra of PbTe(Ga) and Pb1−xSnxTe(In) - materials revealing the Persistent Photoconductivity effect at the low temperatures T<Tc; Tc≈80K and 25K, respectively-The reflectivity spectra display the singuliarity nearby the plasmon-phonon minimum. The spectra may be fitted satisfactory by means of the introduction of an additional oscillator into the dispersion relation. The result is interpreted in the framework of the model taking into account the temperature change of the one-electron metastable impurity state Position in the configuration-coordinate space. The oscillator corresponds to the two-electron - one-electron state transitions. Some previously unexplained results find the satisfactory interpretation in the framework of the model Proposed.