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Crystal-and evaporated film-type KCI were irradiated with X-rays and studied for optically stimulated exoelectron emission (OSEE). Special attention was paid to sample treatment in a clean high vacuum system and to the charging of samples. Upon studying the effects of dose, acceleration voltage and temperature on the OSEE yield spectra, it was shown that when KCI surfaces are clean and non-electrostatically charged, the yield spectra had no “Selective features”. From the OSEE energy spectra that were measured at temperatures sufficiently high to avoid any charging effects, it was found that the threshold energy increased linearly with the photon energy when the latter exceeded 3.4 eV. These results indicate that a direct photoionization from F centers dominates the OSEE mechanisms for KCI and that a two-step process of photo-thermal ionization from the F centers is not important.

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