Date of this Version
Published in Journal of Physics B: Atomic and Molecular Physics, Vol. 12, No. 23, (Dec. 14, 1979), p. 3993
Correction to article published in J. Phys. B: Atom. Molec. Phys. 8 (1975), pp.1806-9
Re-states the definition of the polarisation p in equation (5), the association of Eb and Ea with Ey and Ex, and the interchange of subscripts y and x in Equation 11.
The substance and conclusions of the papers are unchanged.
We wish to thank Dr Keh-Ning Huang for pointing out this error.