1. Intensity of Optical Absorption by Excitons
2. Effect of pressure on the low-temperature exciton absorption in GaAs
3. The Structure of Electronic Excitation Levels in Insulating Crystals
4. “Direct allowed” means that the band gap is direct (maximum of the valence band and minimum of the conduction band occur at the same point in k-space) and that the electric-dipole transition between valence and conduction band is allowed. All the solids considered in this article have this property. As an example to the contrary, Cu2,O has a directforbiddenband gap. Because of this the absorption coefficients are much lower, allowing the measurements of exciton energies under pressure6,7on comparatively thick samples