Degradation of optical properties in Mo mirrors under irradiation with low energy helium and deuterium ions

Degradation of the optical properties in polycrystalline Mo mirrors irradiated with 1–5 keV helium or deuterium ions has been studied by in situ reflectivity measurement, SEM, TEM and spectroscopic ellipsometry. It has been shown that the reflectivity in these specimens at a wavelength range of (320–850) nm decreases with increase in the fluence and the energy of the ions, and that the degradation in helium irradiated specimens is greater than that in deuterium irradiated specimens. It has been demonstrated that optical constants vary due to formation of a damaged layer in the surface and subsurface region. Particularly in a helium irradiated specimens, the real part of the dielectric constant changes from negative to positive values and its imaginary part considerably decreases due to the bubble formation. These results suggest that penetration of the laser light is facilitated by the formation of damage such as bubbles and results in the degradation of reflectivity.