PHONON STRUCTURES OF FORBIDDEN-INDIRECT TRANSITIONS IN TLCL AND TLBR

Optical absorptions due to the phonon-forbidden indirect X+6 × R-6 exciton in TlCl and TlBr have been studied near the fundamental absorption edge by using a wavelength-moduration technique to resolve their fine structures. The experimental spectral band shape of wavelength-derivative absorptions is well described by a theoretical curve expected for a forbidden indirect exciton transition. It is found that four kinds of momentum conserving phonons at the M-point are associated with this transition, and their energies are in good agreement with the values obtained by the neutron scattering studies. The binding energy of the X+6 × R-6 exciton is determined as 23±3 meV in TlCl and 19±1 meV in TlBr. Phonon modes which are associated with edge emissions observed in these substances are assigned. © 1979, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.