Core Excitons and the Soft-X-Ray Threshold of Silicon

The photoabsorption coefficient in crystalline Si is computed in the 100-eV region ($L_{\mathrm{II}},L_{\mathrm{III}}$) on the basis of core excitons, using an adaptation of the effective-mass theory. The line shape is in good agreement with experiment, and, using experimental evidence from the phosphorus impurity in Si as to the central-cell correction, we find the absolute value to be compatible with experiment.