Optical constants of as-deposited and treated alkali halides, and their VUV quantum efficiency

The optical constants of thin films of CsI, KI, and KBr in the spectral range of 53.6-174.4 nm were obtained from the measurements of reflectivity as a function of the incidence angle. The effect of film heating to 420 K and exposure to UV radiation on the optical constants of the three materials was also investigated. The quantum efficiencies (QEs) of the planar photocathodes made with the three alkali halides, as well as the changes in these QEs after the photocathode treatment similar to that applied to the thin films was measured. KBr was found to be the most stable to heating and irradiation. KI appeared to be close to temperature-stable, while UV exposure affected its optical constants. CsI optical constants were changed after 420-K heating, as well as after UV exposure. The changes in the optical constants were related to the QE changes and the correlation between these variations was determined.