Coatings with High 102.6-to-121.6 Nm Reflectance Ratio

Observations in the far ultraviolet (FUV) at wavelengths below similar to 125 nm, which include the H Lyman series and the spectral lines of many other important species, are expected to unveil fundamental information for solar physics and astrophysics. Among these, observations of the solar corona at 102.6 nm H Lyman beta are of high interest, but they may be masked by the strong H Lyman a at 121.6 nm. This goal has been addressed here through the development of novel multilayer coatings with high reflectance at 102.6 nm and at the same time a low reflectance at 121.6 nm; the latter wavelength is mostly absorbed. An efficient reflection/rejection coating is not straightforward because of the lack of high-transmission materials in the short FUV. We have designed and prepared novel multilayers with combinations of the following materials: Al, LiF, SiC and C. Various combinations were found to display a high reflectance ratio at 102.6/121.6 nm when fresh. Some of them resulted in an undesired reflectance increase at 121.6 nm for the samples aged for a few weeks. The most promising multilayers are based on Al/LiF/SiC/LiF (starting with the innermost layer), which resulted in a good performance and a small evolution after months of storage in a desiccator. At the same time, these multilayers may the most efficient reflective narrowband coatings that have been developed with a peak wavelength in the similar to 100-130 nm.