The radiation emitted by an ion collisionally excited by an electron beam is anisotropic. We discuss two schemes, one for use with LS-coupling cross sections and one for intermediate coupling, whereby theoretical resonant-transfer-excitation cross sections can be modified to take account of this anisotropy when making comparisons with experimental results deduced from observations of radiation at a particular beam angle. We have carried out such calculations for a number of ions initially in an S state, viz. ${\mathrm{F}}^{6+}$, ${\mathrm{Ca}}^{\mathit{q}+}$ (q=16–19), and ${\mathrm{Nb}}^{\mathit{q}+}$ (q=29–31). In all cases, neglecting hyperfine structure, the allowance for anisotropic radiative emission at right angles to the beam axis was found to result in only a small increase (10–15 %) in the cross section over results that assumed isotropy. Allowance for hyperfine structure reduced even this anisotropy by 20% for ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}_{}^{6+}{}_{}{}^{}$ (I=1/2) and by 80% for ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ (I=9/2) ions; results for ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ (I=0) ions remain unchanged.

• Received 23 April 1990

DOI:https://doi.org/10.1103/PhysRevA.42.3795