An approximate method to include second-order effects in the treatment of electron impact ionization processes within a combined distorted wave Born approximation (DWBA)+Rmatrix approach is presented. The sensitivity of the results for triple differential cross sections for electron impact ionization of helium to the description of the initial and final states, and to the inclusion of second-order effects within the R-matrix box, is analysed. It is found that changes in the representation of the initial bound state have the strongest effect on the theoretical predictions.

Total and single differential cross sections for elemon impan ionization of helium are calculated within the R-mabix method. In addition to ionimlion of ground state &oms He (ls*) 'S, we consider the initial metastable states He (1~2s) 'S and He (IsZs) 'S for incident projectile energies ranging from threshold to 350 e?J Our dculalions include the processes which leave the residual ion in an excited state He+(nl) up to n = 3. The results are compared with experiments and other theoretical data.

The general theory of electron-photon coincidence experiments for electron impact ionization plus simultaneous excitation is developed. Orientation and alignment parameters are defined and related to the angular distribution and polarization of the light emitted by the residual ion. As a first guide to experimentalists in this field, we present results for ionization of helium atoms, as calculated within a combined DWBA + R-matrix method.

We have discovered a phase problem in the version of the R-matrix codes that was used to calculate the final state Ψ- with the proper boundary conditions. While there is no effect on total cross sections, the triple-differential cross section graphs presented in our paper need to be corrected. The general tendency is a reduction of the binary peak and an increase of the recoil peak. The corrected results for the `3 + 2' and `1-HF' models are shown in figure 1, in comparison with the first-order and second-order results of Byron et al (1986) and with the experimental data of Müller-Fiedler et al (1985). Note that our revised results are in significantly better agreement with those of Byron et al (1986) and also with those from other calculations (see, for example, Bray and Fursa 1996) than the earlier predictions, particularly due to the increase in the recoil peak. Finally, we point out that the general conclusions given in our paper, regarding the effects of various approximations on the theoretical results, remain valid. Figure 1. Triple-differential cross section for electron impact ionization of He(1s2) at incident energy/ejected energy/scattering angle combinations of 256 eV, 3 eV, 4° (top) and 256 eV, 3 eV, 10° (bottom) as a function of the ejection angle. (3 + 2)-state R-matrix (second order) ————; (3 + 2)-state R-matrix (first order) – – – –; 1-HF R-matrix (second order) - - - - - -; 1-HF R-matrix (first order) - -  - -  - -; second-Born results of Byron et al (1986) - · - · - · -; first-Born results of Byron et al (1986) ········; experiment of Müller-Fiedler et al (1985) ∙. These results replace those given in figures 1 and 2 of Reid et al (1998).

The authors consider the charge cloud distribution of heavy atoms after impact excitation by polarized electrons. A general formula in terms of state multipoles is derived and interpreted with regard to ratios of principal axes and three independent angles of rotation. Finally, the theory is illustrated by numerical results for excitation of mercury and thallium.