Trapping Ion-Molecule Reaction Intermediates

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Gas-phase reactions between ions and molecules dominate the chemistry of environments such as the upper atmosphere, combustion systems and the interstellar medium. As positively charged ionic species (cations) are highly reactive, many ion-molecule reactions are "barrierless", meaning that they have no activation energy. However, these reaction processes are far from simple; while there may be no energetic barrier to reaction, reactive trajectories typically form van der Waals intermediates and must overcome submerged barriers to form products. Additionally, ion-molecule reactions often display non-Arrhenius behaviour: their reaction rate constants increase with decreasing temperature. Thus ion-molecule reactions play an increasingly important role in low-temperature environments, such as the upper atmosphere and the interstellar medium. There are, however, remarkably few experimental methods for studying ion-molecule reaction intermediates in the absence of solvent or environmental effects - especially when these intermediates are cationic. As a result, ion-molecule reaction mechanisms are still largely unexplained at low temperatures. In this work, we will exploit the numerous benefits of cold, controlled environments to introduce a new analytical instrument for probing reaction intermediates.Experimentally, I will construct a unique apparatus comprising a cryogenically-cooled ion trap and an integrated mass spectrometer. A cloud of Ca+ ions will be held in a radiofrequency quadrupole ion trap. Following laser cooling, these Ca+ ions will condense to form a regular structure termed a "Coulomb crystal". As the laser-cooled Ca+ ions are continually fluorescing, we can directly observe their lattice positions in the Coulomb crystal using a CCD camera. Other non-laser cooled species can be "sympathetically" cooled into the crystal through the efficient exchange of kinetic energy with laser-cooled ions. The cryogenic conditions will ensure that the initial quantum state distribution of sympathetically-cooled molecular ions is maintained. Pre-cooled reactant molecules will be admitted through a leak valve or pulsed valve.I will stabilise the van der Waals reaction intermediates so that they have insufficient energy to surmount the barrier to product formation. This will be achieved through collisions with cryogenic helium buffer gas. Species can be characterised and ion-molecule reactions monitored through a variety of complementary detection methods, including: real-time imaging of the fluorescing ions, time-of-flight mass spectrometry, resonance-enhanced multi-photon ionisation, and resonance-enhanced multi-photon dissociation.In this way, we can provide the first stringent experimental verification of ion-molecule capture theories at low temperatures (T < 20 K), decades after they were first proposed. Capture theories are currently incorporated into important models of the chemistry occurring in the interstellar medium and upper atmosphere - where it is acknowledged that "the fraction of the processes which have been studied at the low temperatures prevalent in cold cores is extremely small. In addition, for those reactions that may proceed to different sets of products, the branching ratios to these different channels are frequently unmeasured" [Space Sci. Rev. 156, p13 (2010)].With the new analytical apparatus proposed here, I can measure the rates of these fundamentally important reaction processes - elucidating the influence of reaction intermediates and submerged barriers on the reaction mechanism for the first time.