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The fundamental reflectivity spectrum of type-IIa diamond has been studied recently by several workers. Our purpose here is to discuss the edge at 7. 1 eV which has previously been assigned to the direct threshold {₂₅}^'₁₅. We show instead that a hybrid exciton is present near 7 eV, and that the direct threshold probably occurs at about 8. 7 eV. The implications of the revised interpretation for the electronic structure of diamond are also discussed.
J. C. Phillips (Mon,) studied this question.