Vacuum-ultraviolet radiation damage of the KCl surface—application of combined spectroscopic ellipsometry and reflectometry

Abstract

A spectroscopic study (1.7 to 4.0 eV) of the surface damage layer, formed when KCl is exposed to vacuum-ultraviolet radiation, is carried out using ellipsometry and reflectometry. The results indicate the formation of a thin layer (~30–300 Å) with a high density of F-centers (~3 × 10 19 cm −3) and of aggregate centers near the surface. The F-center absorption band in the damage layer appears at 0.04 ± 0.02 eV to lower energy and with 30% larger width, relative to the bulk F-band. An approximate theoretical description of the defect-surface interaction indicates that this shift and broadening is of the correct magnitude and direction to be the result of such an interaction. However, the possibility that the apparent shift may also result from the presence of unresolved aggregate absorptions cannot be ruled out. It is also shown that a thin exponentially inhomogeneous film, such as that assumed to result from photochemical coloration of KCl, can be approximated as an “equivalent homogeneous film” for the purpose of analysis of ellipsometric data.