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Ionisation of aligned atoms and molecules by low energy electron impact

Final Report Summary - ALIGNED E2E (Ionisation of aligned atoms and molecules by low energy electron impact)

The dynamics of electron interactions with atoms and molecules is a rich field for experimental and theoretical exploration. These studies are of both fundamental and practical significance. From a fundamental viewpoint, sophisticated quantum theories must be rigorously tested by experiments that provide high precision data. In this way, models can be extended and refined for application in many areas, with confidence that the results are accurate. From a practical viewpoint, low energy electron impact ionization of atomic and molecular targets plays a key role in many areas, since this energy regime is where the ionization probability is highest. These areas include ionization in the Earth’s upper atmosphere, stellar and planetary physics, lighting and laser technologies, as well as studies of secondary ionization in cancer therapy to name a few. By developing rigorously tested theories that define these interactions, it becomes possible to predict how they occur. The most detailed description of ionization is provided by the (e,2e) technique. This uses an incident electron of known momentum to collide with and eject an electron from the target. The scattering geometry for an (e,2e) event is shown in Figure 1. The two resulting outgoing electrons, i.e. the incident electron scattered in the collision, and the ionized target electron that is ejected, are detected in time coincidence. This ensures the measured events are from a single collision. The momenta of the out-going electrons are measured, so that the collision kinematics are fully determined. The (e,2e) technique therefore provides the most comprehensive data against which different models can be evaluated.

The aims of this project were:
• To implement new experiments to study ionization of spatially aligned atoms and molecules.
• To deliver accurate experimental data for evaluation of the most sophisticated scattering models being developed by theoretical collaborators.
• To study ionization from aligned atoms for the first time, thereby providing a bridge between atomic and molecular collision theories.
• To produce data for the simplest molecule (H2) to determine key quantum mechanical processes for ionization by low energy electrons.
• To measure ionization of polyatomic molecules of relevance to environmental, medical and technological applications so as to aid in the development of new ionization theories for these complex systems.

The experiments carried out during this fellowship have allowed the fundamental nature of electron ionizing collisions to be characterized. All were carried out at low energies, where the ionization probability is highest. The new data from aligned magnesium is a pioneering study that can be extended to other kinematics and geometries, as well as to other targets. Indeed, significant interest from theorists has already been expressed in this area. All experiments carried out in this research programme have strengthened our collaborations with leading theorists throughout the world, and have invoked the development of new models for the ionization of aligned targets. This heralds a new type of study of electron impact ionization for the future, and is expected to lead to further understanding of these important collision processes.