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The picosecond spectroscopy with the time delayed double excitation technique has revealed that the F - H center pair ([ F - H ]) is converted from the self-trapped exciton ( S T E L ) at the a 1 g state within 20 ps in KCl, NaCl and RbCl at 14 K. Conversion efficiencies from the S T E L to the [ F - H ] and to unknown X states were estimated by assuming two possible candidates of the hole π g state or the higher electronic state in the STE as the precursor for [ F - H ] formation. Percentages of branching from the precursor to the [ F - H ] were 49, 86 and 20% in KCl, NaCl and RbCl, respectively. Rest of the STE were converted to unknown X states. By making use of these results, we criticized the state acceptable as the precursor, i.e., the hole π g state or the electronic states.

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.