Trapping Ion-Molecule Reaction Intermediates

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.

Beyond the significant academic impact of the proposed research, this work also has a range of potential non-academic beneficiaries.

1. Commercial beneficiaries:
The cryogenic ion trap apparatus with integrated mass spectrometer could be adopted by researchers in a broad range of scientific disciplines. Mass spectrometry is a universal analytical tool, employed by a diverse range of industries. The cryogenic environment of the ion trap will facilitate long trapping times; optical access will enable laser cooling, fluorescence imaging and the spectroscopic probing of trapped ionic species. Accordingly, there may be potential to commercialise the apparatus. For example, with the ability to detect every ion within a Coulomb crystal string, the techniques developed in this project may facilitate the analysis of trace species in gas samples. An atomic ion with a selected ionisation potential (IP) could be condensed into a Coulomb crystal to selectively charge exchange with the low-IP components of the gas sample, with the resulting ions detected through mass spectrometric means.

2. Policy maker beneficiaries:
This research proposes to establish the role of intermediates in ion-molecule reactions, and in so doing will measure reaction rate coefficients and branching ratios for a number of systems of interest. Not only will this develop our fundamental understanding of how such reactions occur; such measurements will help address the serious lack of experimental rate constants in models of complex gas phase environments. It is an acknowledged problem that many of the reactions occurring in the outer atmosphere and interstellar medium have never been measured under low temperature conditions. Without accurate models of the competing processes occurring in these complex environments, our ability to predict the effect of, for example, releasing certain chemicals into the atmosphere is incomplete. Accurate measurements of reaction rate constants and branching ratios will improve the reliability of these models, allowing scientists to establish the impact of certain actions and potentially assisting policy makers in their regulation of emissions.

3. Beneficiaries within the general public:
Aspects of this research will be communicated to the public through the Oxford Sparks website. Video podcasts and animations enable us to explain why and how we pursue research into ion-molecule reactions to a non-expert audience. There has been significant public interest in our work to date, as there is a fascination about fundamental science and how processes occur on a microscopic level. With fluorescing Ca+ ions in Coulomb crystals, we can literally watch single ions react in real time. Many of the concepts underlying this research are relevant to the physics and chemistry school curriculum. The link between current state-of-the-art research and topics studied at school is discussed in materials provided on the Oxford Sparks website, and will be expanded to include aspects of this research.

4. Researcher skill development:
The research described here is truly state-of-the-art, and will help the UK to maintain a competitive advantage at the forefront of scientific research. A more direct and immediate benefit will be had by the postdoctoral researcher employed to work with me on the project. The cutting-edge techniques and broad relevance of the apparatus will provide an ideal training environment. The PDRA will develop transferrable skills in instrument design and construction, in addition to specialised skills in mass spectrometry, fluorescence imaging, laser-based detection techniques, laser cooling, ion trapping, optics and programming. Visits to collaborators and attendance at conferences will enable the PDRA to develop presentation skills, and to network with other researchers in the field.