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Chemistry of multi-ionized molecules
Multi-ionized molecules (e.g. CF32+, CS23+) are short-lived species with lifetimes which range from seconds to femtoseconds. These ions are extremely energy rich and have remarkably different properties from singly-charged ions. In fact, it has been proposed that doubly-charged molecular ions play a part in the chemistry of a variety of energized media, such as interstellar clouds and planetary ionospheres. We are using coupled mass spectrometry to study the unimolecular and bimolecular reactivity of a wide variety of multi-ionized species and our experiments indicate a rich and varied chemistry. We have also found dication chemical reactions which do not involve the separation of the dipositive charge. For example, SF2+ + Ar - ArS2+ + F (see figure 1-right). We have developed models involving the electrostatic interactions between the reactants and products to explain when the chemical products of dication reactions will be a pair of monocations and when the products will be a dication and a neutral. To probe the dynamics of these dication reactions in more detail we have developed a new experimental technique for studying the relative motions and energetics of the reactions products. This Position-Sensitive COincidence (PSCO) experiment involves detecting the pairs of product monocations formed in dication reactions in a specially designed time of flight mass spectrometer equipped with a position-sensitive detector. These measurements reveal the complete reactivity in a dication-neutral collision system allowing us, for example, to show that N22+ exhibits an extensive bond-forming chemistry, with relevance to the terrestrial ionosphere, and that CO22+ has a similarly extensive chemistry of relevance in the ionosphere of Mars. Our PSCO measurements also allow us to determine the motion (velocity vectors) of the nascent monocation products of each of the bond-forming reactions we detect. From these velocity vectors, via conservation of momentum, we can device the velocity of the neutral species which often accompany such dication chemical reactions. The relationships between these velocity vectors provides a powerful probe of the mechanism of the bond-forming process. Our investigations have shown that many, but not all, dication reactions proceed via formation of a collision complex which lives for markedly longer than it's rotational period and then dissociates via either charge-separation or neutral loss:
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