INDIRECT EXCITONS AND PHONONLESS TRANSITIONS IN GaS.

An investigation was made of the absorption and luminescence spectra of layered GaS single crystals near the fundamental absorption edge at temperatures of 2-77 degree K. Four steps in the indirect absorption edge were attributed to transitions involving formation of indirect excitons and simultaneous creation of four different phonons. The known energy band structure was used to explain the different polarizations of the exciton absorption steps. Narrow absorption and luminescence lines were observed at T equals 4. 2 degree K near the steps, and these lines were attributed to the formation and annihilation of bound excitons accompanied by the simultaneous creation of phonons. Moreover, resonantly coincident absorption and luminescence lines, due to indirect phononless transitions, were observed. When the temperature was raised from 4 to 77 degree K the free-exciton luminescence lines associated with phonon emission increased in amplitude. An analysis of an exciton-phonon annihilation line profile, carried out at 77 degree K, demonstrated that this profile could be described satisfactorily by the well-known express I(E) varies directly as E** one-half exp( minus E/kT) with a suitable allowance for the Gaussian broadening.