Resonant Energy Transfer between ExcitedFCenters in KI

Resonant energy transfer is observed between excited states of $F$ centers in KI at 7°K by monitoring the decay in time of the excited-state population in samples of high concentration of excited states, $F^{*}$. The large population of excited $F$ centers is obtained with a pulse from a $Q$ switched ruby laser. Nonexponential decay curves—showing an increase over the exponential luminescent decay rate at low concentrations—are found in samples containing more than 2×${10}^{16}$ ($F^{*}$ centers)/cc. It is shown by quantitative comparison of the time decay with theory that the mechanism for resonance transfer is dipole-dipole interaction. For this purpose the rate equation and its solution are derived for the case of transfer among the same states of like species. The individual interaction constant for dipole-dipole interaction is calculated using Dexter's formula. It agrees well with the interaction constant obtained from the experimental results.