T.F.M. Bonsen and L. Vriens, Angular distribution of electrons ejected by charged particles I. Ionization of He and H2 by protons, Physica 47, 307-319 (1970)
J.N. Smith, J. Wolleswinkel and J.H. Los, Residence time measurements for the surface ionization of K on W: the effects of surface contaminants, Surf. Sci. 22, 411-425 (1970)