Electronic structure of the R center in LiF

Abstract

The electronic states associated with the R1 and R2 transitions of the R color center in LiF are studied. The R center is taken to be three F centers in an equilateral triangle configuration as suggested by Pick. The F center is taken to be an electron trapped at a fluorine vacancy. The ions surrounding the vacancy are taken to be point charges and a pseudo-potential is introduced to account for the structure of nearest neighbor ions. The F electron is described by a 2s Slater type orbital and the ground state energy is calculated and found to be -5.66 ev which is in good agreement with previous work. The Heitler-London approach is used to construct R center states from the previously obtained F wave functions. Group theory is used to approximately solve the secular problem. The energies of the trial states are calculated and the states are then assigned to the transitions of interest. The ground state and the R1 and R2 excited states are of symmetry type E. The R1 and transition energies are found to be 3.62 ev and 2.88 ev. These values are within ten per cent of the observed values. The calculation is extended to LiCl and the model is found to be in accord with the Ivey relations.