Indirect-Forbidden Transitions in TlCl and TlBr

The optical absorption spectra of purified TlCl and TlBr have been investigated near the band edge. It was found that the absorption rises as a three half power of the energy near the edge and as a cube of the energy in the high energy region. It is concluded that the edge is due to the creation of the is indirect-forbidden exctions of X 6 + ×R 6 - and the lowest minimum in the conduction band is at the R-point. Theroretical calculation of an indirect-forbidden exciton transition is made. The result is differnt from what has been proposed by the previous works, but it describes well the expermental results. The binding energy of the indirect-forbidden exction in TlBr is determined as 23±5 meV.