Effect of Electric Field and Temperature on the Radiative Lifetime of theFCenter

Abstract

The lifetime of the excited $F$ center at 4.2 \ifmmode^\circ\else\textdegree\fi{}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 quantities measured were the luminescent decay time $\ensuremath{\tau}$ (using a pulse sampling technique), the relative luminescent yield $\ensuremath{\eta}$ (using conventional techniques), and the emission-line-shape function. Changes in lifetime were measured for [100] applied dc fields of 0-140 kV/cm at 4.2 \ifmmode^\circ\else\textdegree\fi{}K. These changes were of the form $\frac{\ensuremath{\Delta}\ensuremath{\tau}}{\ensuremath{\tau}}=\ensuremath{-}\ensuremath{\beta}{F}^{2}$, where $F$ is the field in kV/cm, and $\ensuremath{\beta}$ is (5.8 \ifmmode\pm\else\textpm\fi{} 0.5) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}6}$ in KCl, (4.0 \ifmmode\pm\else\textpm\fi{} 0.3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}6}$ in KF, and (3.0 \ifmmode\pm\else\textpm\fi{} 0.3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}6}$ in NaF. Lifetime and yield were measured from 4.2 to 150 \ifmmode^\circ\else\textdegree\fi{}K with approximately 1% precision. In KCl and KF, these data are 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 $\frac{\ensuremath{\tau}}{\ensuremath{\eta}}$. 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 the electric field and the temperature effects on the radiative lifetime are quantitatively consistent with the mixed-state model recently proposed by Bogan in which the lowest emitting state is presumed to be $2s$-like, with a large admixture of $2p$ states. Values for the characteristic parameters, the mixing parameter $\ensuremath{\alpha}$, and the level separation $〈\ensuremath{\delta}E〉$ are determined from these data and agree with values determined in a different manner by Bogan.