Abstract
Absorption and emission spectra of KCl single crystals doped with various amounts of copper have been investigated a t temperatures from 10 to 300 Β°K. In highly doped specimens, four new absorption bands appear in wavelength regions longer than the 260 nm absorption band which has been attributed to the lowest transition within a single Cu^+^ centre. These new absorption bands are found at 288 nm (Bβband), 324 nm (Cβband), 373 nm (D~1~βband), and 382 nm (D~2~βband) a t 77 Β°K. The Bβband is assigned as a transition in paired Cu centres. The D~1~β and D2βbands are attributed t o exciton absorptions in CuCl clusters formed in KCI crystals. As these clusters are small in size, the optical properties of excitons in them are slightly different from those in bulk CuCl crystals: (1) the energy positions of exciton absorption peaks are 20 meV higher than those of bulk CuCl excitons, (2) the absorption lines show relatively large width and asymmetric shapes, (3) the Stokes shifts are large, and (4) there are no bound excitons. These results are discussed as the characterirtic feature of excitons due to the small size of a CuCl microβcrystal.