Effect of Electric Field and Temperature on the Radiative Lifetime of the F Center [Thesis]

The radiative lifetime of the excited F center at 4.2°K, and the changes in the radiative lifetime induced by an applied electric field and by increasing temperature, have been measured in KCl, KF and NaF with sufficient precision to test recent models for the relaxed excited states. The results are in semi-quantitative agreement with the predictions of the mixed-state model recently proposed by Bogan and Fitchen. The quantities measured were the luminescent decay time $\tau$ (using a pulse sampling technique), the relative luminescent yield η (using conventional techniques), and the emission line shape function. Lifetimes at 4.2°K were 717 ± 15 nsec. for KCl, 380 ± 20 nsec, for KF, and 117 ± 5 nsec, for NaF. Changes in lifetime were measured for [100] applied dc fields of 0 to 140 kV/cm at 4.2°K. These changes were of the form Δ$\tau$/$\tau$ = - β [E(kV/cm)]² with β = 5.8 ± 0.5 x 10⁶ (kV/cm)^-2 in KCl, 4.0 ± 0.3 x 10⁶ (kV/cm)^-2 in KF, and 3.0 ± 0.3 x 10⁶ (kV/cm)^-2 in NaF. Changes in lifetime and yield were measured from 4° to 150°K with approximately one percent precision. In KCl and KF, these data were used to demonstrate that the only two decay modes from the relaxed excited states are emission and thermal ionization, so that the radiative lifetime is $\tau$/η]. In NaF, another decay mode during relaxation appears to complicate the results, but the radiative lifetime can be extracted using additional data reported by Podini. In all three cases, the radiative lifetime decreases as the temperature increases. Both these effects are quantitatively consistent with the mixed-state model in which the lowest emitting state is presumed to be 2s-like with a large admixture of 2p. Values for the characteristic parameters, the mixing parameter a and the level separation $\delta$E, are determined from these data and are in agreement with values determined in a different manner by Bogan. These results add convincing support for the mixed-state model. Its ability to explain other recent experiments on the excited F center is discussed, as well as some of its limitations which suggest that a vibronic model will be required for a more accurate analysis.