Lifetime Studies on the Relaxed Excited State of Color Centers

Abstract

The improved precision and accuracy of lifetime measurements allowed by the technique of detecting the single-photon delay distribution has been utilized in lifetime studies on $F$, $M$, and $R$ centers in alkali halides. ${\ensuremath{\tau}}_{R}(F)$, the $F$-center radiative lifetime, was measured in KCl at 80\ifmmode^\circ\else\textdegree\fi{}K and at different $F$-center concentrations. Even though the luminescence quantum yield ${\ensuremath{\eta}}_{R}$ varies with concentration, ${\ensuremath{\tau}}_{R}(F)$ is shown to remain constant. This fact has been explained in terms of a tunneling process among nonrandomly distributed $F$ centers. The radiative lifetime of excited $M$ centers has been measured at 80\ifmmode^\circ\else\textdegree\fi{}K in different alkali halides (in nsec): ${\ensuremath{\tau}}_{R}(M)=28\ifmmode\pm\else\textpm\fi{}1$ in KCl; 26.2\ifmmode\pm\else\textpm\fi{}1 in NaCl; 17.2\ifmmode\pm\else\textpm\fi{}1 in LiF; 12.3\ifmmode\pm\else\textpm\fi{}1 in NaF. From the temperature behavior of $\ensuremath{\tau}(M)$ in KCl, the relaxed excited-state deexcitation energy $\ensuremath{\Delta}{E}_{d}$ has been evaluated: $\ensuremath{\Delta}{E}_{d}=0.21\ifmmode\pm\else\textpm\fi{}0.01$ eV. The observed values of ${\ensuremath{\tau}}_{R}(M)$ and $\ensuremath{\Delta}{E}_{d}$ are discussed.