Research Guides

Department of Chemistry University of Oxford

Dr Brianna Heazlewood

I am interested in the field of cold chemistry. Cold environments – typically temperatures less than 1 Kelvin under our experimental conditions – allow us to control the way in which reactions occur, providing information about fundamental reaction processes and underlying potential surfaces. Much of this detail is lost at higher temperatures, where thermal averaging complicates our interpretation of the dynamics at play. In the past few decades, extraordinary progress has been made in the development of techniques to form and trap cold species. Alongside Professor Tim Softley, we employ a number of these techniques (described below) to investigate ion-neutral reactive collisions with unprecedented control and precision.

Upon laser cooling, atomic ions contained within a radiofrequency ion trap can undergo a phase transition, adopting ordered structures referred to as “Coulomb crystals”. These lattice-like structures are highly localised and remain stable for extended time periods, enabling a range of experimental measurements to be carried out.

Figure 1. A schematic illustration of the linear Paul ion trap employed in our experimental set-up. Laser-cooled Ca+ ions are trapped through a combination of radiofrequency and static voltages. The continuously fluorescing Ca+ ions are observed using a microscope and CCD camera.

The exchange of kinetic energy between directly laser-cooled ions (such as Ca+) and co-trapped species (with appropriate properties) has been exploited to “sympathetically” cool molecular ions down to milliKelvin temperatures, yielding multi-component Coulomb crystals. This ability to sympathetically cool molecular ions allows us to investigate a multitude of ion-molecule reactions; we are not limited to direct reactions with Ca+ ions.

The introduction of neutral molecules into the reaction chamber, with selected kinetic energy and internal energy distributions, is a non-trivial process! Two main techniques are employed to introduce translationally cold neutral species into the ion trap reaction chamber: electrostatic velocity filtering and Stark deceleration.

Under the electrostatic velocity filtering approach, only the low kinetic energy component of the Maxwell-Boltzmann distribution of molecules is directed towards the trapped ions; species with higher velocities are lost from the bent quadrupole guide.

Figure 2. Only the low-velocity tail of the initial distribution of molecules is able to be guided around the bend.

An advantage of Stark deceleration (over velocity filtering) is the additional control one has over the internal state distribution of the cold reactant species. The combination of a Stark decelerator and ion trap enables us to move closer to the ultimate goal in these experiments: full quantum state selection over a tunable range of collision energies.

Figure 3. Molecules in low-field seeking states lose kinetic energy as they enter regions of high electric field between the electrode pairs (where their Stark potential energy increases). As the molecules near the top of the potential energy curve, the voltages are rapidly switched (as indicated by the dashed line). This process is repeated at each subsequent pair of electrodes along the length of the decelerator, creating an effective travelling potential well and progressively slowing the selected molecules. 

Precise knowledge – and control – of the internal quantum states of trapped species is required in order to fully characterise a reaction. Ions can be held in radiofrequency ion traps for extended periods of time, making it possible for reactions to be monitored over the course of minutes to hours.  However, the lifetimes of state-selected, sympathetically cooled molecular ions prepared in radiofrequency ion traps can be limited by rapid thermalisation with the background blackbody radiation (BBR) field. We are theoretically investigating cooling schemes for diatomic and polyatomic species of interest, with the aim of driving as much of the population as possible into the ground rovibrational state.

1. "Using a direct simulation Monte Carlo approach to model collisions in a buffer gas cell", M. Doppelbauer, O. Schullian, J. Loreau, N. Vaeck, A. van der Avoird, C. J. Rennick, T. P. Softley and B. R. Heazlewood, J. Chem. Phys., 146, in press (2017).

2. “Ejection of Coulomb crystals from a linear Paul ion trap for ion-molecule reaction studies”, K. A. E. Meyer, L. L. Pollum, L. S. Petralia, A. Tauschinsky, C. J. Rennick, T. P. Softley and B. R. Heazlewood, J. Phys. Chem. A, 119, 12449 (2015).

3. “Coulomb crystal mass spectrometry in a digital ion trap”, N. Deb, L. L. Pollum, A. D. Smith, M. Keller, C. J. Rennick, B. R. Heazlewood and T. P. Softley, Phys. Rev. A, 91, 0334408 (2015).

4. “Accurate determination of the relative concentrations of ammonia isotopologues in a cold, electrostatically guided molecular beam”, E. W. Steer, K. S. Twyman, B. R. Heazlewood and T. P. Softley, Mol. Phys., 113, 1465 (2015).

5. “Low-Temperature Kinetics and Dynamics with Coulomb Crystals”, B. R. Heazlewood and T. P. Softley, Annu. Rev. Phys. Chem., 66, 475-495 (2015).