Vacuum-ultraviolet optical properties of ion beam assisted fluoride coatings for free electron laser applications

In the frame of a European project aimed at the development of a vacuum-ultraviolet (VUV) free electron laser facility at ELETTRA (Trieste, Italy), ion beam assisted evaporation of fluoride coatings has been revisited as a potential deposition technique for synchrotron radiation resistant mirrors. Magnesium fluoride and lanthanum fluoride single layers and multilayered mirrors have been produced with low energy ion beam assisted evaporation, by using a Mark I gridless ion source. The optical properties in the VUV range down to 160 nm have been studied, and correlated with the ion assistance parameters. The effects of post-growth photonic cleaning on the VUV absorption of the coatings have been investigated. The results demonstrate that the high optical absorption, induced in the mirrors by ion assistance, can be cured by ultraviolet light irradiation. The peak reflectance of the cured mirrors deposited at high ion doses can be recovered up to the value of the ones deposited at low doses. The benefit of growing mirrors at higher doses of ion assistance is that they have higher packing density and show minor degradation against synchrotron radiation.