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Ionization cross sections for positrons impacting on atomic hydrogen have been measured for kinetic energies in the range 15-700 eV. This has been done in a crossed-beam geometry where a magnetically guided positron beam intersects a hydrogen gas jet emanating from a radio frequency discharge tube. Electron impact ionization cross sections were also measured with the same apparatus thus facilitating comparison with, and normalization to, published results. The positron-atomic hydrogen results are found to be significantly lower than those obtained by Spicher et al. (1990).

Measurements of the cross section for non-dissociative single ionization and the cross sections for the creation of charged fragments have been performed for 50-6000 keV antiproton and proton impact on N2, CO, CO2, and CH4. The results support the understanding of the ionization phenomenon that has been achieved via measurements with fundamental charged particles on atoms. The present high-energy antiproton fragmentation data supply a stringent test of the validity of the published electron-impact fragmentation data which, unfortunately, most of them fail.

A simple, convenient rf discharge source for the production of an intense beam of hydrogen atoms is described. The design and operation is such that the discharge tube can be operated over a period of several thousand hours, producing an intense beam with dissociation ∼95%.

The results of electron-photon coincidence experiments have given a new insight into the excitation of atoms by electron impact. This article reviews the way in which this new experimental technique has provided an exacting and fundamental test for the theory of the excitation process. The basic experimental method is first described, and a comprehensive treatment of excitation of the nlP states of helium is given. The extension of the technique to the study of excitation of the n = 2 states of atomic hydrogen is then presented. Coincidence measurements also provide information on the role of spin in the collision, and this subject is treated in detail for excitation of heavy atoms by both unpolarised and spin-polarised beams of electrons. Finally, the electron-photon coincidence method permits observation of the time evolution of excited atomic states, and this review describes the attempts made to observe quantum beats in the radiative decay.

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Using beams of polarised electrons and polarised hydrogen atoms we measured the spin antiparallel-parallel ionisation asymmetry from threshold to 500 eV. Our measurements are generally more precise than those of earlier studies and, more significantly, are characterised by an energy spread that is at least an order of magnitude smaller than that of the earlier work. We compare our results with several theoretical models as well as with the results from similar experiments using polarised alkali atoms.

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