Lifetimes of excited states in alkali halide crystals containing F-centers

Measurements were made of the lifetimes of excited F-centers in KC1, NaCl, KBr, and KI crystals by observing the time dependence of the fluorescence and photoconductivity following excitation by a short pulse of light. A special flash lamp was developed for use as the exciting source. It has a width at half maximum of 18 nsec and a useful output of 10^13 photons in the range 2 to 6 ev. The behavior of the lifetime of the excited F-center as a function of temperature can be interpreted in terms of a simple model in which the only two modes of decay are by radiative transition to the ground state or thermally induced ionization. The experimental results are in agreement with this model in their broad features, but differ in details. Using the model g the radiative lifetime of the excited F-center is found to be: KCl – 0.58 µsec, NaCl - 1.00 µsec, KBr – 1.11 µsec, KI = 2.22 µsec. The activation energy for thermal decomposition of the excited F-center in KCl is found to be 0.15 ev. The lifetime of the F-center fluorescence in KCl is independent of concentration below 2 x 10^-16 F-centers/cm, but is 20% smaller at 3 x 10^17 F-centers/cm3. It is not influrenced by the presence of M-centers unless the concentration of the latter is high g in which case the M-luminescence is almost completely quenched and is replaced by M-luminescence. The decay constant of the latter is 60 nsec.published or submitted for publicationU of I Onlythesi