The wavelengths in the region between 35 nm and 50 nm are too short to be reflected at normal incidence by any bulk material and too long for standard multilayer reflective coatings. The practical interest to this range was stimulated by the development of 46.9 nm capillary discharge laser that is working in a repetitive mode with an average power more than 1 mW. The reflective optics are a tool for manipulation of a beam of this laser. The Sc/Si multilayer coating was suggested and realized as a solution to the problem. To increase reflectivity and heat stability, W barriers are introduced between Sc and Si layers. The role of these barriers is to prevent interdiffusion and provide sharp interfaces. It is clear that Sc/Si coatings can provide high reflectivity in a rather limited spectral range. So the next goal is to extend the achieved results to the neighboring spectral intervals. The optics design basically requires the detailed knowledge of optical constants. Experimental data of this kind is very poor. This is explained by the fact that all elements in this spectral range are characterized by extremely short penetration length of a photon. Consequently the surfaces are essentially contaminated. On the other hand, from the theoretical point of view, the interval under consideration lies just between the optical domain where the spectra of solids are formed by excitation of valence electrons and soft X-ray and XUV range where the excitation of core electrons is crucially important. The methods and results of design and study of multilayer coatings containing 3d-transition metals are presented.
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