In situ removal of carbon contamination from a chromium-coated mirror: ideal optics to suppress higher-order harmonics in the carbon K-edge region

Carbon‐free chromium‐coated optics are ideal in the carbon K‐edge region (280–330 eV) because the reflectivity of first‐order light is larger than that of gold‐coated optics while the second‐order harmonics (560–660 eV) are significantly suppressed by chromium L‐edge and oxygen K‐edge absorption. Here, chromium‐, gold‐ and nickel‐coated mirrors have been adopted in the vacuum ultraviolet and soft X‐ray branch beamline BL‐13B at the Photon Factory in Tsukuba, Japan. Carbon contamination on the chromium‐coated mirror was almost completely removed by exposure to oxygen at a pressure of 8 × 10−2 Pa for 1 h under irradiation of non‐monochromated synchrotron radiation. The pressure in the chamber recovered to the order of 10−7 Pa within a few hours. The reflectivity of the chromium‐coated mirror of the second‐order harmonics in the carbon K‐edge region (560–660 eV) was found to be a factor of 0.1–0.48 smaller than that of the gold‐coated mirror.