Energy transfer and picosecond relaxation ofFcenters in RbCl, KCl, and KBr

Possible decay mechanisms of excited F centers are investigated in both additively colored and x-irradiated RbCl, KCl, and KBr, containing typically more than 3×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$ F centers and sizable amounts of dimer (${\mathit{F}}_2$) and higher F aggregates. Time-resolved ground-state recovery measurements under F-band excitation at low temperature and optical-absorption data show a picosecond deexcitation process, occurring for a substantial number of F centers and possessing shorter decay times for higher relative concentrations of ${\mathit{F}}_2$ and N centers. This establishes energy transfer from excited F to ${\mathit{F}}_2$ centers, a controversial topic throughout the literature, and towards N centers in KCl to be both competitive relaxation channels. Transient absorption measurements in heavily colored KCl by Gadonas et al., which show increased transparency in the ${\mathit{F}}_2$-band region 500 ps after pulsed F excitation, fully support the above conclusion. A similar behavior was also observed in RbCl and KBr. This additional decay channel could adversely affect laser operation in several color-center lasers.