Low temperature ion-radical collisions

The chemistry of various gaseous environments is dominated by reactions involving transient, highly-reactive atomic and molecular species; these environments include the interstellar medium (ISM, principally very low density gas clouds between the stars) the upper atmosphere, flames and combustion systems, electric discharges and plasmas. The key highly-reactive species present are either free-radicals - atoms or molecules which generally have an odd number of electrons so that at least one electron is 'unpaired' - or ionic species carrying a positive electrical charge (cations). Such species have a natural tendency to form new chemical bonds, and typically reactions of these species have very low activation energies, or even zero activation energy. This means that they typically have very fast reactions even at low temperatures - indeed many reactions of these species become faster as the temperature lowers. In order to model the chemistry of the environments above (which may contain hundreds of different chemical species), we need to know how fast the reactions of these species are, and how those reaction rates vary with temperature. For the interstellar medium the temperatures are very low (10 - 50 Kelvin) whereas in flames and plasmas the temperature may be very high. Thus knowing the reaction rates at room temperature does not generally provide sufficient information for modelling purposes.

In this work we will measure the rates of reactions between free-radical species and ionic species over wide temperature ranges (from above 300 Kelvin to below 1 Kelvin). There is currently a vast knowledge gap in terms of measuring rates for reactions between two transient species - most work has been done with one transient species and one stable species. The reason for the current dearth of information from laboratory measurements is that both of the transient species involved in the reaction tend to be present in very low concentrations, and therefore current methods lack the sensitivity to detect the occurrence of reactions - the number of reaction product molecules formed per second is likely to be undetectably low.

To make these measurements we will assemble a unique instrument which consists of a 'Zeeman decelerator' for producing the free-radical species with variable kinetic energies (and hence variable temperatures), and a laser-cooled ion trap for producing the cold target ionic species for reaction. The Zeeman decelerator uses the fact that free-radical species are typically magnetic (as they have unpaired electrons) and so their velocities can be controlled using magnetic fields - in this case the fields are created by a linear sequence of 12-100 solenoid coils through which the radicals pass. By decelerating the radical species (H, N and O atoms, or CH3 and CN molecules) we can control their kinetic energy and hence the temperature. For the ionic species we use a radiofrequency quadrupole to trap the ions (which are produced by laser ionization of neutral precursors), and by using laser cooling we can create a low density cloud of atomic ions (Ca+ in this case). The ions condense to form a 'Coulomb crystal' in which the ions take up positions in a regular array and the temperature can be as low as a few milli-Kelvin The Ca+ ions are constantly fluorescing and can be observed individually by imaging microscopy. Molecular ions (in this case CH+, C2H2+, CO2+, or C6H6+) can then be co-condensed ('sympathetically cooled') into the Coulomb crystal and trapped there for periods of hours.

In the experiments, radicals from the Zeeman decelerator interact with trapped ions, and reactions occur producing a new ionic chemical species, which is also trapped. Thus the reaction rate is determined by monitoring product ion formation versus time. The unique ability of our proposed experiment derives from a combination of the very long trapping time of the ions and the capability to observe even single ions in the trap.

The research proposed here is high quality fundamental science, but underpins a number of other applied areas of research, including combustion and plasma modelling, atmospheric and astrophysical modelling, and mass spectrometry processes. The novel types of chemistry explored here will be of importance in identifying unknown reaction pathways in these complex media, and in providing fundamental tests for theory. Computational modelling of the complex reaction pathways requires accurate values for rates over a wide range of temperatures, and we intend to provide these in this work. Outside academia the impact is likely to be felt in three principal areas:

1. Society:
The proposed research has great potential to engage the public in the fascination of fundamental science and we will use the results as a springboard to public engagement work (see Pathways to Impact). The results can be presented in a highly visual format, e.g., single ions can be imaged and reactions observed in real time through the disappearance of these ions. The interpretation of this work gives insights to the 'mysterious world' of quantum physics. We will convey the excitement of the connection between this microscopic world and the macroscopic world of chemical transformations under highly controlled conditions. This field can be used to illustrate a range of concepts in the school curriculum (chemistry and physics), such as kinetics and activation barriers, collision theory of gases, free-radical chemistry, entropy and phase changes, momentum conservation, and wave-particle duality.

2. Economy and industry:
Short term: some equipment to be designed for this project may become viable products especially for specialised companies. The funding for the proposed project is likely to catalyse the development of more devices which may become commodities and may lead to spin-offs. The techniques developed in this project may also be used to identify molecules on a single particle level and thus may be developed into a device to analyse substances with ultra-high sensitivity. Such a device may be used to analyse the tiniest amounts of trace gases - either the ions which are trapped here, or the radical species with which they react. The application of ion-radical reactions for the detection of low concentrations of radicals has been explored in a number of laboratories and applied to atmospheric trace gas measurement. In such an application the concentration of a radical is determined through its reaction with molecular ions, and the product ions are detected via mass spectrometry. To calibrate such measurements, the reaction products need to be identified and the rate of reactions determined accurately.

Long-term: The dream of many chemists is to be able to understand and fully control chemical processes at a molecular level. With a little imagination, we can foresee a world where certain types of chemical transformation are no longer performed in the traditional reaction vessel, but in an environment where each reacting particle and the interactions between those particles are perfectly controlled. The ultracold temperature regime is likely to provide the ultimate medium for control at the molecular level, with sophisticated methods of trapping ions and molecules on chips, or in optical lattices, and single particle control using coherent light and electromagnetic fields. While it is unlikely that such an approach will ever replace bulk chemistry, it may well enable delicate chemical transformations to be performed which are beyond the scope of operating in a thermal entropy-driven environment. Studying chemical processes under ultracold conditions provides a promising approach towards the goal of complete control.

3. Skills:
The current proposal employs cutting-edge techniques in atomic, molecular and laser physics and is an ideal training ground for postdocs and students, who will also benefit from the interdisciplinary collaboration.