Abstract
The Lorentz-oscillator model applied to excitons in crystals gives an optical-absorption peak (∝ the oscillator strength F) arising from a transition of an incident photon into an exciton with the same momentum, because of its conservation rule. In this situation, however, the transition should not take place since there exists no sufficient density of final states. Then, the model loses the self-consistency. Optical absorption by excitons causes a spatial decay of an incident photon in crystals. It should enable the photon to interact with excitons with various momentums, and to make a transition to them. Then the rate of the decay equal to the absorption coefficient can be determined self-consistently. This new model shows that even at zero degree Kelvin the exciton-absorption peak has a nonvanishing width, proportional to F 2 3 . Its integration intensity is still proportional to F, but only 1 3 of that given by the oscillator-strength sum rule of the Lorentz model.
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