Ning-Xuan Yang, Chen-Zhong Dong, and Jun Jiang. (2012). Electron Impact Excitation Cross Sections of the $1s2s$ $^3S$ Metastable State of Helium.
            <i>Journal of Atomic and Molecular Sciences</i>. <i>3</i> (1).
            49-58.
         doi:10.4208/jams.041811.050911a