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The effect of the crystal lattice on plasma oscillations is investigated. The results which have been obtained by Wolff on a semiclassical basis are derived quantum-mechanically. The interaction between electrons and plasma oscillations causes interband transitions besides intraband transitions. The interband transitions give rise to frequency broadening in addition to a shift in frequency. This explains the differences in behavior between various metals observed by Ruthemann, Lang and other workers.
Hideo Kanazawa (Tue,) studied this question.