On the Bi‐Exciton Formation in Crystals

Abstract The binding of two free excitons into a bi‐exciton due to direct exciton‐exciton interaction is studied. The process of bi‐exciton formation via exciton–phonon interaction is not taken into account. By group theory methods the wave functions of two interacting excitons are constructed for a model applicable to CuCl and Cu 2 O crystals. Making use of these functions, the Fourier transform of exciton‐exciton interaction energy is calculated. By the Green's function method the dissociation energy of the bi‐exciton is obtained in the adiabatic approximation (the exciton‐exciton interaction energy is much smaller than the ionization energy of the exciton). A numerical estimate and comparison with experimental values is made for CuCl crystals.