The Transient Absorption Due to Self-Trapped Excitons Localized at Iodine Dimers in Alkali Halide Crystals

The transient absorption spectra of a self-trapped exciton (STE) localized at iodine dimers (I 2 2- * ) in iodine-doped alkali chloride and bromide crystals (AX:I; A=Na, K, Rb; X=Cl, Br) have been investigated. The absorption bands due to electron-transitions appear below 1 eV, and the bands due to hole-transitions around 1.5 and 3.0 eV. The absorption bands due to the electron-transitions are located in much lower energy region than those of intrinsic STEs in nominally pure alkali chloride or bromide crystals. The bands due to the hole-transitions are very similar to those of V k (I 2 - ) centers in alkali iodide crystals. These results suggest that the STE localized at iodine dimers is in the “on-center” configuration and its electronic state is diffused in comparison with that of “off-center” type STE.