Abstract
The existence of the excitonic molecule as bound state of Wannier excitons was predicted since 1958. In recent years new lines were observed in the luminescence spectra of crystals excited by high-power lasers. These lines were attributed to the dissociation of the molecule into a free exciton and a photon. We establish the classification of the biexcitonic states taking into account the details of the band structure and the symmetry properties of the crystals. Different numerical calculations of the binding energy of the molecule are presented, some including electron-hole exchange corrections. Discrepancies still exist between different authors. According to the group-theoretical classification of the biexciton states, selection rules of luminescence and two-photon absorption are derived. For two-photon absorption, besides the usual selection rules for dipole transitions, dynamical selection rules were evidenced.
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