由于Al膜的保护层MgF2薄膜的光学常数对Al/MgF2高反射镜的性能有极大的影响， 本文研究了获取MgF2薄膜光学常数的方法。用热舟蒸发的方法在室温B270基底上镀制了3块不同MgF2厚度的Al/MgF2反射镜样品，通过掠入射X射线小角反射方法表征样品，获得了膜层厚度和粗糙度。在国家同步辐射实验中心计量站测试了入射角为5°时，样品在105~130 nm波段的反射率。在Al、MgF2膜层的厚度和Al的光学常数已知条件下，依据菲涅尔公式，得出了满足某波长处样品反射率的等值曲线，然后从三条曲线的交点得出了MgF2薄膜在108~128 nm波段的光学常数。对比和分析显示: 利用此方法得到的108~128 nm波段MgF2薄膜光学常数计算的反射率曲线和实际测试得到的反射率曲线吻合较好。

As the optical constant of MgF2 over-coated Al mirror effects its optical performance greatly， this paper explores the method to obtain the optical constant. Three Al/MgF2 mirror samples covered by MgF2 coatings with different thicknesses were deposited on B270 substrates at room temperature by thermal evaporation method. The layer structures of the samples such as the thickness and the roughness were characterized by the grazing incidence X-ray reflectometry. The reflectivities of the samples at a incidence angle of 5° were measured in 105—130 nm region at the National Synchrotron Radiation Laboratory(NSRL). Finally， the optical constant of MgF2 for Al/MgF2 mirror was obtained by Fresnel formulas at given wavelength and corresponding reflectivity when the coating thicknesses of MgF2， Al， and the optical constant of Al have been known. The common intersection point of the curves of three MgF2 coatings with different thickness for Al/MgF2 mirror determines the optical constant of MgF2 coating at given wavelength. The analysis and comparison show that calculated reflectivity from optical constants of this work agrees well with measured reflectivity curves in 108—128 nm region.