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Consequences of electron-molecule collisions.
Reactions of ions dominate the chemistry of many media, such as planetary ionospheres, plasma etchers and interstellar space. In these energized media primary ions are generated from neutral molecules by energetic photons and electrons. These primary ions then react with neutral species to create secondary ions. In order to model the chemistry of such ionized media it is important to know (i) which primary ions are formed when the available neutral molecules are ionized and (ii) how those primary ions go on to react. Dealing with the first point seems simple, especially for the relatively small molecules that often predominate in gas-phase media. To this end, partial ionization cross sections (numbers which quantify the different ions formed in ionizing events) have been measured for many years, following electron-impact ionization of molecules, using conventional mass spectrometers. Due in part to our recent work, it has become apparent that even for simple molecules (e.g. N2O, Cl2) the existing partial ionization cross sections are often in error. These errors arise because the mass spectrometric experiments used to identify and quantify the ions did not detect the translationally energetic ions formed from multiple (and to some extent single) ionization. We have developed an experiment involving two-dimensional mass spectrometry which can efficiently detect and identify all the ions formed following an ionization event. This procedure produces the reliable relative partial ionization cross sections which are required for an accurate representation of the primary ion distribution in an energized medium. In addition, our experimental data, which contains the intensities of ion pairs and triples, as well as single ions, also quantifies the ionization level which is responsible for any fragment ions. For example, we can determine how many NO+ ions from the ionization of N2O come from fragmentation of N2O+ and how many from N2O2+. That is, we measure charge-state specific partial ionization cross sections. The data produced by our experiments can be used to upgrade models for the ionic abundances in energized media. In addition the experiment is a powerful probe of the properties of the multiply charged molecules generated by electron ionization. For example , we can see the multitude of different dissociation pathways that occur when you doubly ionize C2F6 and have also studied the ionization of a variety of reactive molecules.
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