The transformation of optical extinction spectra of potassium iodine (KI) films has been investigated under the influence of moisture. When the film is kept in humid air, its fundamental excitonic absorption band appears to shift to the lower-energy side by more than 1 eV. No crystallographic or chemical change is related to the transformation of the optical extinction spectra. On the other hand, scanning electron microscopy revealed the production of an island structure at the film surface in humid air. Application of Mie's scattering theory to such an island structure well explained the variation of the optical extinction spectra. No quantum size effect on the excitonic absorption was observed in KI film thicker than 10 nm.
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