A. I?. LUBCHENKO (a), V. M. NITSOVICH (a), and N. V. TKACH (b) The temperature dependence of the curves of absorption and dispersion of light of ionic crystals in the exciton region is investigated taking into account the interaction of excitons with acoustic and optical phonons. It is shown that the shape of the curves depends on the sign of an effective exciton mass m. In the approximation linear in the phonon operators the maxima of the curves shift with increasing temperature towards lower energies when m > 0, and towards higher energies when m < 0. Es wird die Temperaturabhangigkeit der Absorptions-nnd Dispersionskurven des Lichtesvon Ionenkristallen im Exzitonenbereich untersucht, wobei die Wechselwirkung der Exzitonen mit akustischen und optischen Phononen beriicksichtigt wird. Es wird gezeigt, daB die Kurvenform vom Vorzeichen einer effektiven Exzitonenmasse m abhangt. In der in den Phononenoperatoren linearen Naherung verschieben sich die Maxima der Kurven mit steigender Temperatur zu niedrigeren Energien, wenn m > 0, und zu hoheren Energien, wenn m < 0 ist.

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Institute of Theoretical Physics, Academy of Sciences of the Ukrainian SSR, Kiev Urbac h' s Rule with the Dispersion of Frequencies BY A. F. LUBCHENKO and I. I. FISHUK As is known, the shape of the long-wavelength edge of light absorption by localized excitations and excitons is well described by the formula Gj w 2kT ' d = Q ~t h -Consequently , i =-4 7 Q-%} . 1 B r e they ar ! This expression is analogous to formula (1); the temperature dependence of d is the same as in paper (13). The value of e is defined by the character of the fre-K14 physica status solidi (b) 48quency dispersion of w . SAn analogous expression defines, too, the shape of the short-wavelength e m i s sion edge (if the signis substituted for + before ( Q -3 )).

An investigation is made of the shape of the absorption spectra due to impurity centres in a solid, taking into account the variation of its quasi-elastic constants during phototransitions in impurity centres. An analytical expression for the long-wavelength edge of the absorption curve, similar to the well-known Urbach rule, is obtained.Es wird die Form der" Absorptionsspektren von Verunreinigungszentren in Festkorpern untersucht, wobei die Anderung der quasi-elastischen Konstanten wiihrend der Phototfbergange in den Zentren berucksichtigt wird. Ein analytischer Ausdruck wird fur die langwellige Kante der Absorptionskurve erhalten, der der gut bekannten Urbach'schen Regel ahnlich ist.