Excited state and optical absorption properties of an F center in a lithium fluoride crystal

The method of linear combinations of atomic orbitals has been utilized to perform accurate first-principles calculation of the Γ₁⁺ ground state and the Γ₄^- excited states for an F center in lithium fluoride. The one-electron Hamiltonian includes the Coulomb and exchange interaction (Slater's approximation) due to all of the ions in the crystal and the trapped electron, and incorporates the effects of lattice relaxation of the neighboring ions due to the presence of the defect as well as the electronic polarization effects. The F-center wave functions are expressed as linear combinations of localized orbitals centered at atomic sites up to the sixth nearest neighbors to the vacancy. Solution of the secular equation yields energies for the Γ₁⁺ ground state and Γ₄^- excited states for the F-center electron. Properties of the F-center electron associated with optical absorption, such as absorption energy, orbital g factor of the Γ₄^- excited state, and spin-orbit splitting of the excited state have been calculated and are in excellent agreement with experiment. The oscillator strength for the Γ₁⁺ to Γ₄^- transition has also been calculated and is in agreement with the experimental value within its range of uncertainty. A discussion of the effect of lattice distortion upon the properties of the F-center electron will also be presented.