Geometrical structure of(Tl+)2luminescent center in alkali halides: Presence ofD2handD4hdimer centers

Absorption, emission, and excitation spectra of the ${\left({\mathrm{Tl}}^{+}\right)}_2$ center in KI, KBr, and KCl have been investigated and compared with the excitation spectrum of the ${\left({\mathrm{Au}}^{-}\right)}_2$ center with $D_{2h}$ symmetry in KCl. Dichroism produced in the ${\mathrm{Tl}}_2^{+}$ absorption band after bleaching with polarized light has also been investigated in $\gamma$-ray-irradiated KCl:${\mathrm{Tl}}^{+}$. From these results and from the magnetic-circular-dichroism measurements, it is shown that the ${\left({\mathrm{Tl}}^{+}\right)}_2$ absorption bands observed thus far arise from a $D_{2h}$-symmetry ${\left({\mathrm{Tl}}^{+}\right)}_2$ center. An additional absorption band called $A^{\prime }$, which has been found at a position close to the peak position of the $A$ band, produces an emission band which is different from the previously observed ${\left({\mathrm{Tl}}^{+}\right)}_2$ emission bands. From a systematic investigation of the separation between the $A^{\prime }$ and $A$ bands in various alkali halides, it is shown that the $A^{\prime }$ band arises from a $D_{4h}$-symmetry ${\left({\mathrm{Tl}}^{+}\right)}_2$ center.