Introduction

Understanding the collision processes that lead to ionization of atoms and molecules is of great importance, not only in furthering fundamental quantum physics, but also for many applications in technology, industry and science. Applications include lighting, laser development and plasma etching, and these types of collisions occur in stellar and planetary atmospheres, the upper atmosphere of the earth, in a wide range of biological systems, and in the production of greenhouse gases. Indeed, in any area where ionization by electron impact occurs, it is essential to understand the collision mechanisms to fully describe the system under study.

The probability of ionization depends on several factors, including the incident energy of the electron projectile, and the nature of the target that is being ionized. Since electrons cannot be created or destroyed during these collisions (the energy is almost always too low for such a process to occur), the incident electron will be scattered from the target through some angle with respect to the incident direction. Ionization then results in one or more electrons being ejected from the target, and these electrons will also emerge at different angles, depending upon the collision dynamics.

In the types of processes discussed here, we confine our studies to single ionization (with the subsequent release of only one electron from the target), and only consider the interactions when the incident electron has relatively low energy compared to the ionization potential of the target. We here define this as the region from threshold (where the ejected and scattered electrons have almost no energy after the collision) through to intermediate energies, where the incident electron has energies typically several times that of the ionization potential (IP). In this energy regime, the electrons spend sufficient time in the region of the target that impulsive approximations (as successfully used at high incident energies) are no longer applicable. It is hence necessary to carefully consider different mechanisms that lead to ionization.