Introduction

The ionization of atoms and molecules by electron impact is of considerable technological and theoretical relevance. From the practical perspective it plays a central role in many atmospheric, industrial and environmental processes. Examples include the physics and chemistry of the upper atmosphere, the operation of discharges and lasers, radiation-induced damage in biological material and plasma etching processes [1–3]. The extent to which such processes can be controlled and/or optimized is limited by our ability to describe the underlying physical mechanisms which drive them. To refine our understanding, new experimental and theoretical results are required. From a broader perspective, the process of electron-impact-induced ionization of atoms and molecules provides an ideal testbed to refine models for the few- and many-body behaviour of identical particles whose interaction is mediated through the Coulomb potential. Moreover, as the ionization process is extremely sensitive to the electronic structure of the target, comparison of experimentally derived ionization cross sections with calculation provides a powerful means to refine models for the target electronic structure. Historically, (e,2e) measurements can be divided into two categories, namely, those whose primary aim is the determination of the target electronic structure and those whose focus is revealing underlying ionization mechanisms. For the former case (so-called electron momentum spectroscopy (EMS) studies), measurements are performed at relatively high impact energies and for roughly equal energies for the two scattered electrons. Under such conditions the ionization mechanism is quite well understood, with the primary electron interacting predominantly with a single bound target electron.