An investigation is made of the shape of the emission spectra due to impurity centres in a solid, taking into account the variation of its quasi-elastic constants during the phototransitions. A formula for the short-wavelength edge of the emission spectrum similar to the Urbach expression for absorption is obtained.Es wird die Linienform der Emissionsspektren von Fremddefektzentren in Festkorpern untersucht, wobei die Anderungen der quasielastischen Konstanten wiihrend des Photoiibergangs berucksichtigt werden. Es wird eine Pormel fur die kurzwellige Kante des Emissionsspektrums erhalten, die der Urbach-Formel fur die Absorption iihnlich ist.

The two-particle Green's function method is used to investigate the dependence of light absorption and dispersion due to substitutional impurity centres upon their concentration in the caae of weak electron-phonon coupling, taking into account the change of the quftsi-elastic constants due to the substitution. At low impurity concentrations the halfwidth of the zero-phonon line and the corresponding dispersion curve as well LIB their position in the spectrum are shown to vary linearly with concentration; at higher concentration the absorptios coefficient may deviate from linearity. Consider a cubic crystal with one atom per unit cell which can be substituted by another kind of atom, thus forming a disordered substitutional solid solution. It is evident that upon such a substitution both the mass of the crystal and its quasi-elastic constants change. I n the previous paper [I] the absorption and dispersion of light by such impurity centres in the case of weak electron-phonon coupling was investigated taking into account only the ratio of atomic masses of host and impurity centres. In the present work we assume that , u = --1, where M and rn are atomic masses of the host and the impurity centre, respectively, be negligibly small in comparison with the relative change of the quasielastic constants of the solid solution. We further assume that the impurity absorption region is far from that of the host crystal and the concentration of impurities is so small that the interaction between the centres can be neglected. Then the stationary states of strongly coupled electrons of such a system can be determined in the adiabatic approximation [2]. However, if the spacings between the energy levels of the optical electrons of the impurity atom are of the order of the phonon energy of the ideel host crystal the adiabatic approximation cannot be used to determine their stationary states. Let us assume that in the case under consideration we have to deal with weak electron-phonon coupling. Then writing the potential energy of the nuclear sub-system [3] in TaH EI YMeHbIIIeHHH.

A method of calculating the transport coefficients based on the Kubo formula [l] is presented. The complex admittance is calculated by setting up and solving the equations for the mass operators. A procedure for decoupling the correlation functions is proposed.Es wird eine Berechnungsmethode fiir die Transportkoeffizienten, die auf der Kubo-Formel [l] beruht, mitgeteilt. Die komplexe Admittanz wird durch Aufstellen und Losen der Gleichungen fur die Massenoperatoren berechnet. Es wird eine Methode fur die Entkopplung der Korrelationsfunktionen angegeben.

No abstract