Coincidence Measurements on Ions and Electrons

Abstract

Coincidence methods are among the most powerful tools in physics for experiments at the microscopic level, because they select for study single events of reaction by individual atoms or molecules. Coincidence detection of at least two products is essentiai for complete characterization of any process that produces three or more particles, such as double ionization; it also allows particular processes to be studied selectively in experimental situations where events of many different types happen concurrently. Ionization processes, for example, often produce both electrons and ions of many different energies; a coincidence experiment makes it possible to examine the ions that are formed together with electrons of a single chosen energy. The electron and the ion from one event arrive at their respective detectors at different times after their formation, and although the signals at the detectors are not simultaneous (coincident in the normal dictionary sense) they do have a definite temporal relationship. All coincidence experiments involve the search for time-correlated signals such as these from the arrivals of single particles at suitable detectors. Electrons and ions are particularly easy to detect as single particles, but photons and high-energy neutral atoms or molecules can be detected in coincidence too. The majority of experiments in atomic and molecular physics involve coincidences between just two particles, and are called twofold coincidence techniques; triple (three-fold) and higher-order coincidence techniques are in use, but are considerabty more dificult. As primary exciting particles, photons rather than electrons have a distinct advantage; because they are absorbed, not scattered, the total count of final particles is reduced by one for the same final state of the target. They also have precisely controllable energy and polarization, which are highly advantageous in many experiments.