Possible Non-One-Electron Effects in the Fundamental Optical Excitation Spectra of Certain Crystalline Solids and Their Effect on Photoemission

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Abstract
Intrinsic optical excitation in solids is considered for the case where localization of the hole or electron is important. It is noted that Koopman's theorem (or the quasiparticle description) will not in general hold in such a case; as a result the optical excitation must be considered in terms of a many-body excitation rather than in the one-electron (or quasiparticle) approximation. The many-body effects are considered qualitatively in terms of the relaxation about a "localized hole" for two extreme cases. The first of these is that in which the relaxation is electronic in nature and takes place in a time comparable to the excitation time. The second is that in which the relaxation is ionic in nature and takes place after the optical excitation event.