DECAY PROCESS OF THE CORE EXCITON IN ALKALI HALIDES

The decay o f the Li 1s core exciton in lithium halides and the Na 2p core exciton in sodium halides was investigated by measuring the constant-initial-state (CIS) spectrum and the constant-final-state (CFS) spectrum. The decay probability of the core exciton was estimated from the photoelectron spectra and the absorption spectra. In the case of sodium halides the CIS and CFS spectra were measured with two different incident angles of the exciting light and also with two different temperatures, i.e., room temperature and liquid nitrogen temperature. Two peaks were observed in the CIS and CFS spectra of NaCl and NaBr. The higher energy peak, which appears a t the same photon energy and shows the same temperature dependence as those of the prominent exciton peak in the absorption spectrum, was interpreted as originating from the decay o f the bulk core exciton. It was found that the energy position and the intensity of the lower energy peak show the strong dependence on the polarization of the radiation incident on the sample surface, and it shows no distinct energy shift upon the cooling of the samples. The lower energy peak was interpreted as originating from the decay of the surface core exciton