Vucic, S., Potvliege, RM., & Joachain, CJ. (1989). 2nd Born Triple Differential Cross-sections for the Coplanar Asymmetric Ionization of He(2s-1) and He(2s-3) By Fast Electrons and Positrons. Journal of Physics B: Atomic, Molecular and Optical Physics, 22(21), 3519-3527. https://doi.org/10.1088/0953-4075/22/21/014 (Original work published 1989)