Irradiation effects of low energy helium ions on optical reflectivity of metallic mirror

Polycrystalline molybdenum specimens kept at room temperature and 873 K has been irradiated by 1.2 keV- and 8 keV-He⁺ up to 3 × 10²² He⁺/m². Optical reflectivity has been measured with a spectrophotometer for the wave length between 190 and 900 nm. Surface morphology and chemical analyses has been examined with a scanning electron microscope, an atomic force electron microscope and X-ray photoelectron spectrometer. Loss of metallic gloss and remarkable reduction of reflectivity (190-900 nm) occurs by the irradiation of only 1 × 10²² He⁺/m². Reflectivity decreases with increasing ion energy, irradiation temperature and dose of helium ions. At rather low dose, it is reasonable to suppose that the reflectivity reduction may be due to the surface roughening and fine bubble formation. With increasing dose, roughness became smaller, while reflectivity decreased further. The further reduction of the reflectivity at high dose is considered to be due to the porous structure with nano-scale helium bubbles.