F-Center Formation in NaCl: Picosecond Spectroscopic Evidence for Halogen Diffusion on the Lowest Excitonic Potential Surface

We have used picosecond white-light spectroscopy to study evolution of absorption in NaCl after pulsed band-gap excitation. Spectra at 80 and 300 K, initially very similar, subsequently develop features of well separated $F-H$ pairs (300 K) or of self-trapped excitons (80 K). We interpret the spectra in terms of diffusion of the $H$ center in the neighborhood of the complementary $F$ center, corresponding to reversible interconversion on the lowest potential surface connecting the exciton and the $F-H$ pair.