Effect of Polishing Techniques of Molybdenum Mirrors on Their Optical Stabilityunder Cleaning D<sub>2</sub>–N<sub>2</sub> Plasma
The effect of cleaning with D<sub>2</sub>–(2–4) mol % N<sub>2</sub> glow discharge plasma on the reflectivity of Mo(111) single-crystal mirrors was studied. The surface of each mirror was prepared by mechanical polishing with diamond pastes or diamond powder. Polishing mirrors was accompanied by the formation of a layer of material up to 3 microns thick, different in structure from the bulk metal. The peculiarity of this layer was the gradual improvement of its structure as it moved away from the surface of the mirror and the transition from the amorphous layer to the layer with nanometer-sized crystallites and then to the gradual transformation into the structure of an unbroken single crystal. When polishing, the diamond abrasive was pressed into the surface layer of the mirror. During the plasma cleaning process, carbon particles pressed into the surface layer of the mirror initiated the development of negative roughness (pits or depressions). At the same time, a 5–10 nm layer of molybdenum nitride is formed. In disordered layers of mirrors, a probability of a process of origin and growth of blisters were reduced. Aluminum was chosen as the metal that changes the total reflection coefficient of Mo. The use of D<sub>2</sub>–N<sub>2</sub> mixture to increase the rate of Al sputtering at the last stage of cleaning was replaced by plasma exposure in deuterium with \[{\text{D}}_{3}^{ + }\ ions, which initiated the dissociation of molybdenum nitride and the removal of nitrogen from the surface layer of the mirror. As a result of prolonged plasma exposure, the reflectivity of mirrors polished with diamond paste increased to a stationary value close to the reflection of the reference mirror.