Transient F-H Center Pairs Produced from Self-Trapped Excitons at the Lowest Triplet State in Alkali Chloride Crystals

Time delayed double excitation spectroscopy in the picosecond range has revealed transient F bands due to the F center perturbed by an H center at the next nearest site, [ F - H ] n n n , in RbCl, KCl and NaCl single crystals after excitation of the self-trapped exciton (STE) from the lowest triplet state to higher excited states. The F band consists of a doublet peaking at 1.99 and 2.11 eV (RbCl), at 2.25 and 2.38 eV (KCl) and at 2.70 and 2.86 eV (NaCl) with the ratio of 2:1 in height in every crystal at 108 ps after the excitation. The peak positions and relative height of the doublets are understood qualitatively by the theory for the off-center type S T E . No dichroism of the doublet observed by the light polarized in the [100] or [010] direction suggests rotation of the [ F - H ] axis during relaxation from the electronic b 1 u state, but not necessarily from the hole excited π g state.