Armenta Butt, Sam; (2021) The Reactions of Dications with Neutral Species: Understanding Planetary Ionospheres. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Doubly charged cations (dications) of molecular and atomic species are predicted to be influential in high-energy environments such as the interstellar medium, the ionospheres of planets and satellites, and plasmas. However, definitive detection of dications in these environments are not yet available and the presence of these ions is often overlooked. Early investigations of dication-neutral collisions, often at high collision energies, only resulted in the observation of electron-transfer reactivity. Modern experiments, using lower collision energies, have revealed a range of exotic chemistry such as bond-formation with rare gas elements. This chemistry, coupled with the significant abundance of dications predicted in ionospheres, suggests that these ions could play important roles in atmospheric processes. For example, dications could be involved in the chemistry of complex molecule assembly. The study of dications and their reactions is clearly important to understanding ionospheric processes in planets and satellites including the prebiotic Earth. This thesis explores the bimolecular reactivity of various dications with neutral species in order to better understand the processes occurring in the ionospheres of planets and satellites. The position-sensitive coincidence mass spectrometry technique employed in this work utilises coincident, position-sensitive, detection of ions to reveal comprehensive information concerning the dynamics and energetics of the consequences of dication-neutral reactions. Specifically, the reactions following collisions of Ar2+, S2+ and CH2CN2+ with atoms and small molecules have been investigated. These dication-neutral collision systems exhibit intriguing reactivity clearly demonstrating the diversity of dication chemistry. For example, many of the electron-transfer reactions observed show evidence of proceeding via collision complexes, contrary to the orthodox (direct) mechanism. Of the bond-forming reactions detected, those generating molecular species containing a rare gas, such as ArO+ and ArN+, are the most notable. Despite the observation of the involvement of collision complexes in electron-transfer, many of the bond-forming reactions described in this thesis have been shown to occur via direct mechanisms. The observation of bond-forming reactions and the involvement of collision complexes clearly shows the facility of dications to form associations despite their often-high potential energies.

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