Short-pulse optical studies of exciton relaxation andF-center formation in NaCl, KCl, and NaBr

Abstract

Using short-pulse laser techniques, the formation time and absolute production efficiency of $F$ centers in KCl have been investigated on the time scale of ${10}^{\ensuremath{-}11}$ sec. The temperature-dependent yield of $F$ centers observable at 46 psec following two-photon band-gap excitation has been determined over the range 12-880 K in crystalline KCl. The yield of $F$ centers per ionizing event approaches unity near the melting point. The observation of transient 532-nm absorption resulting from ultraviolet pulse irradiation of molten KCl is reported. The formation time, production efficiency, and room-temperature decay time of self-trapped excitons in the lowest triplet state in NaCl have been investigated, as has the onset of 532-nm absorption in NaBr. The observations, in general, place upper limits of a few picoseconds on the time for capture of an electron by a hole in these alkali halides when carrier densities are in the range of 5 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Recently proposed mechanisms of $F$-center formation are discussed in light of the present results. We develop mathematically convenient treatments for convolution of pulse shapes and intrinsic photochemical response in several kinetic models.