Two types of self-trapped excitons in alkali halide crystals

We have carried out ab initio many-electron variational calculations of the adiabatic potential-energy surface (APES) for the lowest triplet state of the self-trapped exciton (STE) in KCl and LiCl. For KCl, it is found that at the APES minimum, the ${\mathrm{Cl}}_2^{\mathrm{-}}$ molecular ion comprising the STE hole is displaced along the 〈110〉 axis by about 0.90 Å from its symmetrical position. The STE electron and hole are shifted in the direction opposite to that of the ${\mathrm{Cl}}_2^{\mathrm{-}}$ displacement. The calculated optical-transition energies due to electron and hole excitations of the STE at the APES minimum, and the luminescence energy due to the transition to the crystal ground state agree well with the experimental results. It is found that the 〈110〉 displacement of the ${\mathrm{Cl}}_2^{\mathrm{-}}$ molecular ion at the APES minimum from its symmetrical position for LiCl is 0.07 Å, much smaller than that in KCl, and that the direction of the shift of the electron and hole is opposite to that for KCl; the electron and hole are localized near one of the ${\mathrm{Cl}}_2^{\mathrm{-}}$ ions located closer to the lattice site. It is shown that, for a small shift of the ${\mathrm{Cl}}_2^{\mathrm{-}}$ molecular ion from its symmetrical position, the states in which electron and hole are shifted to opposite directions appear in both LiCl and KCl crystals within energy intervals less than 0.8 eV. It is pointed out that the configuration interaction between the two states with the electron and hole shifted in opposite directions should be included for more precise APES calculations at small off-center displacements, and that the electron-hole correlation is important to determine the electronic structure of the STE.