We have investigated the optical absorption spectra of the CsBr : Au -crystals irradiated with an ultraviolet (UV) light. Before the UV-light irradiation, the absorption bands associated with the isolated Au -ion (monomer) are observed in the energy region of UV. After a slight UV-light irradiation, the absorption intensities of the monomer bands decrease and the absorption bands due to paired Au -ions appear around the monomer bands. Furthermore, UV-light irradiation on the CsBr : Au -crystals brings about the appearance of several new absorption bands in the visible region. From the analogy with the aggregation of the F centers in alkali halide crystals, the new absorption bands are attributed to the aggregate centers of the Au -ions. They can also be regarded as the CsAu nanocluster constructed from the Cs + cations and the Au -anions.
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