Index of Refraction of Y$_{2}$O$_{3}$ in the Extreme Ultraviolet

In applications such as measuring ionized He in the Earth's magnetosphere, it can be useful to be able to detect the relatively weak 30.4 nm line of He$^{+}$ over the stronger 58.4 nm line of neutral He. Work done previously at BYU has calculated that an aluminum/Y$_{2}$O$_{3}$ multilayer mirror should be effective at maximizing reflectance at 30.4 nm while minimizing the 58.4 nm line. We have measured the index of refraction of Y$_{2}$O$_{3}$ to improve the accuracy of these calculations and further our knowledge of Y$_{2}$O$_{3}$ between the wavelengths of 5 nm and 30 nm. We created a sample mirror using electron beam evaporation and measurements were taken using the Advanced Light Source at the Lawrence Berkeley National Laboratory. As this is the first direct measurement of the index of Y$_{2}$O$_{3}$ in this regime, it represents an improvement over previous data calculated using atomic scattering factors.