The R' Center in LiF: An Optical Study of a Jahn-Teller Distorted System [Thesis]

A series of experiments are described in which the optical absorption and emission spectra of the R' center in LiF at low temperatures are perturbed by applied electric, magnetic, and uniaxial stress fields to determine the structure of the center and the effects of Jahn-Teller distortion in its excited state. The magnetic circular dichroism and linear Stark splitting observed for the 8328 Å zero-phonon line confirm that the R' center has C_3v symmetry and that this transition is 3_A₂ → ³E, as expected for the model of a triangle of three F centers with one extra trapped electron. The R' center undergoes dynamic Jahn-Teller distortion in the ³E excited state, as proposed by Fetterman. This is revealed experimentally in the characteristic double-peaked band shape of the R' absorption, the polarization of the R' emission under uniaxial stress, and the strongly-reduced orbital Zeeman effect. These effects are analyzed quantitatively in terms of the well-known LHOPS model for the linear Jahn-Teller effect in an X₃ molecule and are all consistent with the choice of model parameters k² = 5 for the strength of the interaction, and $\hslash$ω = 0.049 eV for the frequency of the mode responsible for the distortion. Other predictions of the model based on these parameters, such as the linearity of the Stark and Stress splitting of the zero-phonon line in absorption are qualitatively confirmed. No evidence of "warping" effects due to higher order terms in the Jahn-Teller coupling is observed.