Abstract
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.
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