Isotopic and disorder effects in large exciton spectroscopy

Most of the physical properties of a solid depend to a greater or lesser degree on its isotopic composition. For the first time a systematic analysis is presented of isotopic and disorder effects observed in crystals of various isotopic composition using low temperature large exciton spectroscopy. Substituting a light isotope with a heavy one increases the interband transition energy and the binding energy of the Wannier—Mott exciton as well as the magnitude of the longitudinal-transverse splitting. The nonlinear variation of these quantities with the isotope concentration is due to the isotopic disordering of the crystal lattice and is consistent with the concentration dependence of line halfwidths in exciton reflection and luminescence spectra. The common nature of the isotopic and disorder effects in the crystals of C, LiH, CdS, Cu₂O, and Si and Ge is emphasized.