Exciton absorption parameters in TlGaS2 crystals

It is shown that, because the shape of the exciton absorption curve in crystalline TlGaS₂ is described by the Fano antiresonance profile, the experimentally observed exciton peak corresponds to a modified state which is the result of the configuration interaction of a discrete state (exciton) with the quasi-continuum of conduction-band states. The oscillator strength for the transition to the discrete (“pure”) exciton state is calculated as F ₀=1.22×10^?2. The exciton transition selection rules are calculated for two assumed symmetry groups, D _2h and D _4h . An analysis of the selection rules for the dipole-allowed exciton transition permits one to conclude that the symmetry group for the TlGaS₂ crystal is D _2h .