P. Mandal, K. Roy and N.C. Sil, Phys. Rev. A 33 (1986) 756.
D.R. Schultz and C.O. Reinhold, J. Phys. B 23 (1990) L9.
N.C. Sil, K. Roy and P. Mandal, in:Abstracts of Proc. 17th Int. Conf. on Physics of Electronic and Atomic Collisions, Brisbane (Hilger, Bristol, 1991) p. 354.
K.F. Canter, P.H. Lippel, W.S. Crane and A.P. Mills Jr., in:Positron Studies of Solids, Surfaces, and Atoms, eds. A.P. Mills Jr., W.S. Crane and K.F. Canter (World Scientific, Singapore, 1986) p. 199.
A. Kövér, G. Laricchia and M. Charlton, J. Phys.B (1994), in press.
R.D. DuBois and M.E. Rudd, Phys. Rev. A 17 (1978) 843.
S.K. Srivastava, H. Tanaka, A. Chutjian and S. Trajmar, Phys. Rev. A 23 (1981) 2156.
L. Dou, W.E. Kauppila, C.K. Kwan and T.S. Stein, Phys. Rev. Lett. 68 (1992) 2913.
R.A. Sparrow and R.E. Olson, private communication; to be published