Lifetime Studies on the Relaxed Excited State of Color Centers

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. τR(F), the F-center radiative lifetime, was measured in KCl at 80^∘{}K and at different F-center concentrations. Even though the luminescence quantum yield ηR varies with concentration, τR(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^∘{}K in different alkali halides (in nsec): τR(M)=28±1 in KCl; 26.2±{}1 in NaCl; 17.2±{}1 in LiF; 12.3±{}1 in NaF. From the temperature behavior of τ(M) in KCl, the relaxed excited-state deexcitation energy ΔEd has been evaluated: ΔEd=0.21±0.01 eV. The observed values of τR(M) and ΔEd are discussed.