Viehland L.A., E.A. Mason, and J.H. Whealton, Journal Of Physics B: Atomic And Molecular Physics 7, 2433-2439 (1974).
Lee M.S., A.S. Dickinson, and L.A. Viehland, Journal Of Physics B: Atomic, Molecular And Optical Physics 33, 5121-5127 (2000).