Identification and analysis of theTl2+ESR spectrum in KCl:Tl+

The $X$-band electron-spin-resonance spectrum of the ${\mathrm{Tl}}_2^{+}$ center, i.e., an electron trapped by a ${\mathrm{Tl}}^{+}$-${\mathrm{Tl}}^{+}$ impurity dimer in heavily doped KCl: ${\mathrm{Tl}}^{+}$, has been identified by the correlation of its formation and decay properties with those of the known ${\mathrm{Tl}}_2^{+}$ optical-absorption bands. A spin-Hamiltonian fitting further supports this conclusion. A quantitative analysis of the hyperfine components shows that exchange polarization and $s$ mixing both play an important role in the ${\sigma }_g$ ground-state hyperfine interaction. A similar analysis of the $g$ components yields the energies of the excited ${\pi }_g$ states, and this combined with the optical-absorption data permits one to propose an energy-level scheme of the ${\mathrm{Tl}}_2^{+}$ center in KCl.