Strong evidences for existence of indirect excitons in Gas are obtained a s a result of an analysis of differential absorption and photoluminescence spectra registered in the 4.2 to 100 K temperature range in polarized light. The location of the exciton band in gallium sulphide E,,, = = (2594 & 2) meV and the energy of phonons which take part in indirect transitions are determined. Lines which are interpreted as a result of non-phonon transitions are observed in the absorption and emission spectra. Lines which arise from the decomposition of bound excitons are found in emission spectra as well. It is shown that polarization pecularities of Gas absorption and luminescence spectra are explained in terms of the band scheme of gallium selenide which is structurally analogous to Gas. The possible nature of the anisotropy of excitons in Gas is discussed.

Transmission and photoluminescence spectra are investigated of GaSe1−xTex and Ga1−xInxSe crystals in the temperature range 4.2 to 100 K. It is shown that low temperature radiative recombination in these crystal is mainly due to direct bound exciton decay. In the temperature range 10 K < T < 77 K radiative recombination has an impurity character, and at T = 77 K direct free exciton radiative recombination takes place. The examination of direct exciton zone behaviour when substituting the anions Te, S for Se in GaSe (Se → Te, S) and cations (Ga → In) allows to conclude, that the anions make the major contribution to the formation of electronic states at the direct absorption edge of GaSe and its analogs. It is shown that the bottom of the conduction band at the edge of Brillouin zone of the investigated crystals is formed with considerable participation of cations.

Recently Muller and Hahn /1/ discovered that TlGaSe2 single crystals crystallize in monoclinic structure and have the C 4 space group.

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