While specific values of f i and τ τ i are not universal and change within collections of specimens, typical values confirm the above regularities. In CdS, with E i ≈ ≈ 6 meV, were observed impurity-exciton oscillator strengths f i ≈ ≈ 10 . The value f i > 1 per a single impurity center should not be surprising because the transition is a collective process including many electrons in the region of the volume of about a i 3 >> v . High oscillator strength results in low-power optical saturation and radiative life times τ τ i ≈ ≈ 500 ps. Similarly, radiative life times of about 1 ns were reported for impurity excitons in GaAs. The same mechanism is responsible for short radiative times down to 100 ps for excitons confined in CuCl microcrystallites.