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The authors have measured the cross sections for single electron loss and single electron capture for 0.6-2.2 MeV He+ colliding with He, Ne and Ar using the growth rate method and a molecular beam target. The results are compared with other available experimental data. For single electron loss theoretical comparisons are also made with the quantal first Born approximation and some semiclassical calculations. For single electron capture, comparisons are made with the modified classical calculations based on the binary encounter model. To date there seems to be no one theory which accurately describes the data. The theory of Kaneko (1985, 1986) is clearly promising for the neon target at energies greater than 0.15 MeV amu-1 but this could be fortuitous since the theory does not similarly reproduce the data for helium and argon targets. The theory of Kumar and Roy (1979) reproduces the energy dependence of the capture cross sections in the range 0.1-1.0 MeV amu-1 but the absolute values of the cross sections are subject to a scaling of the effective charge of the He. Unfortunately no single value of this effective charge applies to all three target gases.

Autoionisation processes have been investigated in Ar+ + Ar and O+ + Ar collision system using the method of zero-degree spectroscopy. All the spectra were dominated by the ELC cusp, with autoionisation peaks appearing symmetrically on both sides of the wings. The energy of the autoionisation electrons were found to be as low as 8 meV with respect to the parent ion, and the analysis indicated that simultaneous electron transfer and core rearrangement, creating a 3p5(2P½) autoionisation state, is the responsible mechanism in Ar+ + Ar collision. For O+ + Ar, two possibilities : core excitation with core rearrangement creating a 2p2(1D2) autoionisation state, and double core excitation to 3pnl, could explain our data.

A brief historical account of the experimental and theoretical studies of charge exchange and projectile ionisation is shown to lead to the development of the concepts of ECC and ELC. Recent data on cusp peaks are discussed in relation to their overall width, differential cross section and asymmetry. It is concluded that we have obtained reasonable agreement for the cross sections for ECC and ELC over a wide range of velocities and charges. A review of the experimental and theoretical analyses of the cusp shape indicates the need to treat both the electron capture and loss processes to a higher than first order approximation.

The authors present an analysis of electron loss to the continuum (ELC) for 1.6-2.8 MeV He+ colliding with He, Ne and Ar using the first six terms of the multipole expansion method. They give absolute values for B00 and discuss the way in which the method of analysis influences the values of the dipole and quadrupole parameters. The authors' dipole parameter is slightly positive for the He target and becomes progressively more negative for Ne and Ar. Their quadrupole parameter is slightly positive for He and Ne and nearly zero for Ar. For He++He the magnitude of B00 and its velocity dependence are in reasonable quantitative agreement with a recent theoretical calculation based on the second Born approximation. However their dipole and quadrupole parameters are both more positive than predicted.