Phonon Strucutures of Forbidden-Indirect Transitions in TlCl and TlBr

Abstract

Optical absorptions due to the phonon-forbidden indirect X₆⁺×R₆⁻ 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 X₆⁺×R₆⁻ 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.