Hybrid Excitons in Diamond

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 ${{\Gamma }_{25}}^{\prime }\to {\Gamma }_{15}$. 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.