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. ${\tau }_R\mathrm{}\left(F\right)\mathrm{}$, the $F$-center radiative lifetime, was measured in KCl at 80°K and at different $F$-center concentrations. Even though the luminescence quantum yield ${\eta }_R$ varies with concentration, ${\tau }_R\mathrm{}\left(F\right)\mathrm{}$ 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°K in different alkali halides (in nsec): ${\tau }_R\mathrm{}\left(M\right)=28\mathrm{}\pm 1$ in KCl; 26.2±1 in NaCl; 17.2±1 in LiF; 12.3±1 in NaF. From the temperature behavior of $\tau \mathrm{}\left(M\right)\mathrm{}$ in KCl, the relaxed excited-state deexcitation energy $\Delta E_d$ has been evaluated: $\Delta E_d=0.21\mathrm{}\pm 0.01$ eV. The observed values of ${\tau }_R\mathrm{}\left(M\right)\mathrm{}$ and $\Delta E_d$ are discussed.