The self-trapping of holes was studied at temperatures ranging from 6 to 300K. It was found that the trapping process involved 3 distinct steps. These were the formation of the 1-center type of small polaron, dimerization into 2-center type holes, and cooling to give the VK center. In NaBr and KBr at 80K, after irradiation with an fs laser pulse, the temporal changes in the optical absorption exhibited a common feature in that an absorption band (with a peak at 2.8eV and the tail of a band which peaked at higher energies) was induced, within 1ps of excitation, as a primary product. The intensity of the 2.8eV absorption band attained a maximum at less than 2ps after excitation, and then started to decrease. The decay of the 2.8eV band led to the formation of a very broad absorption band at about 3ps and, in a later stage that lasted as long as 20ps at 80K, absorption bands which were characteristic of VK centers formed. It was presumed that the transient-hole state which gave rise to the absorption band at 2.8eV corresponded to the primary localized hole center which was associated with absorption bands at 2.1, and above 3eV, in KI.

H.Fujiwara, T.Suzuki, K.Tanimura: Materials Science Forum, 1997, 239-241, 561-4