Defrance, P., Claeys, W., Cornet, A., & Poulaert, G. (1981). Electron-impact Ionization of Metastable Atomic-hydrogen. Journal of Physics B: Atomic, Molecular and Optical Physics, 14(1), 111-117. https://doi.org/10.1088/0022-3700/14/1/013 (Original work published 1981)