A variety of effects are found to contribute to changes in spectral reflectivity found after bombardment with ions of gaseous species; they cannot be assigned solely to the formation of a gas layer. There may be a change in refractive index and thickness associated with a phase change (e.g., quartz); a decrease in refractive index and an increase of thickness by the introduction of microporosity (e.g., SiC, B4C, Al2O3); an increase in refractive index from the introduction of centers, etc. (e.g., SiC, MgO); an increase of refractive index and a decrease in thickness by compaction (e.g., vitreous silica); or a decrease in refractive index caused by changes in composition (e.g., alkali loss in glasses). Often the chromatic fringes are composites of different fringe systems arising in different parts of the illuminated area; and within any part of an area, there may be more than one cause. These effects are observed in the early stages of bombardment manifested by changes in spacing, phase, amplitude, and level of the chromatic fringes. When a gas layer forms (blistering) the large amplitude of the chromatic fringes associated with it usually dominates the spectral reflectivity at large dose, but this is not always the case (e.g., C1 glass).
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