Exchange Interaction between Excitons in the Quasi‐Cubic Model

The general properties of the exchange matrix element between hydrogenic exciton states in a cubic crystal are discussed. It is shown that the high frequency dielectric constant should be applied in the screening of the long range exchange interaction. The exchange effects are then incorporated in the quasi‐cubic model of uniaxial crystals. Including also an external stress the energy levels and oscillator strengths of the exciton states are derived. The theoretical results are consistent with existing optical data on ZnO. By comparison values for the exchange integral and a valence band deformation potential are obtained.