1. R.K.M. Landshoff, Phys.Rev., 76, 904 (1949)
2. L. A. Sena, Electrons and Ion Collisions with Gas Atoms (Stolknoveniia elektronov i ionov s atomami gaza), Gostekh-teoretizdat, Moscow, 1948: Zh. eksp. teoret.fiz., 9, 1320 (1939)
3. J. P. Keene, Phil. Mag., 40, 369 (1949)
4. A. Rostagni, Nuovo Cim.
12, 134 (1935);
5. Ibid, Atti.Acc.Sc.Torino, 70, 472 (1935)