Core excitons in solids.
The theory of core excitons in insulators and semiconductors is reviewed, and the validity of the effective-mass approximation is discussed. It is shown that deep core excitons with binding energies of the order of 1 eV can only occur in semiconductors as the result of a breakdown of the effective mass approximation. A possible mechanism for such a breakdown is seen in dynamical correlation effects, which can be computed with the model of the electronic polaron. This may produce an effective electron-hole interaction much stronger than the fully screened interaction over a distance comparable with the exciton radius. It is shown, however, that the conditions for such an occurrence are not met in semiconductors because of their small electron mass and their large dielectric functions, so that effective mass theory should apply. It is hinted that in special cases additional effects due to interaction between equivalent minima or to time-dependent screening may produce an increase in binding energy that increases the electron-hole interaction and precipitates a shallow-deep instability in the binding energy. Only future detailed calculations will show if this is a realistic possibility. Experiments to check the theory are suggested.
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Bassani