Exploring chemical reactions at low temperatures

Scientists from Oxford’s Healzlewood group discuss chemistry close to absolute zero.

In the Heazlewood group we are interested in how chemical reactions progress at very low temperatures, close to absolute zero. The study of these reactions is of great importance for the exploration of naturally cold environments, such as the interstellar medium in space or the upper levels of Earth’s atmosphere. Previous research shows that reactions between ions and neutral molecules prevail in cold environments and, surprisingly, some of these reactions become even faster as the temperature decreases. Therefore, the investigation of reactions between these species at low temperatures will aid in the development of models describing the chemistry of cold environments.

To study reactions at such low temperatures, we combine a variety of experimental techniques to trap and cool the reactants. We trap Ca+ ions using a linear Paul trap and then we cool them down using lasers, reaching temperatures that are a fraction of a degree above the absolute zero (that is about -273 Celsius)! At these temperatures, the ions form a Coulomb crystal, a group of ions that adopts a lattice-like structure in the trap. The Ca+ ions are constantly fluorescing due to the cooling lasers, allowing the immediate observation of the ions with a microscope lens attached to a CCD camera (FIG. 1). Other ions can be co-trapped and cooled simultaneously with the Ca+ ions, forming a multi-component Coulomb crystal. These other ions do not emit light and therefore do not appear in our images. A rich variety of chemical reactions can be studied using multi-component Coulomb crystals.

Figure 1: Experimental image of a Ca+ Coulomb crystal taken with our CCD camera. The lattice positions of the constantly fluorescing Ca+ ions can be seen in our images.

Using the above mentioned techniques, we are currently investigating a charge exchange reaction between Xe+ ions and two isotopologues of ammonia molecules, NH3 and ND3.

Xe+ + NH3 → Xe + NH3+

Xe+ + ND3 → Xe + ND3+

The Xe+ ions are trapped and cooled within the Ca+ Coulomb crystal, reaching temperatures close to 10 K. The neutral ammonia molecules, on the other hand, are at room temperature. The ammonia ions, once produced, displace the fluorescing Ca+ ions from the centre of the crystal, forming a dark core in the structure. In this way, the reaction progress can be monitored as a function of the growth of the dark core (FIG. 2).

Figure 2: Sequence of experimental images that show the progress of the charge exchange reaction as a function of the size of the dark core.

The aim of our research is to calculate the reaction rate constants of the charge exchange reaction for both isotopologues. Such information will give us an insight on the kinetics and dynamics of the reaction progress. To achieve this goal, we need to go through several steps.  First, we have to collect numerous measurements for each system, and that takes a lot of time, energy and patience. Then, we analyse the data collected using custom-made software. Finally, we interpret and discuss the results. Each step is challenging since we often need to deal with unexpected problems and assess each new situation carefully. However, the final results are always rewarding and exciting!

Further ahead in our research, we will look into repeating these charge exchange reaction experiments using cold (instead of room-temperature) ammonia molecules. To prepare cold ammonia molecules we are planning to use a decelerator that utilises the Stark effect to remove kinetic energy from polar molecules. Combining the results taken with room-temperature and decelerated ammonia molecules, we aim to explore how the reaction rate depends on the properties of the reactants.

We recently presented some preliminary results of the charge exchange reaction together with our future plans at the RSC Spectroscopy and Dynamics Group meeting (SDG2019) in January 2019. Our group has a long-running tradition in attending the SDG meeting every year. The meeting gathers leading academics of the field, from the UK as well as overseas, and is well attended by PhD and Master students. At the meeting we had the opportunity to discuss our research with world-leading experts and to become familiar with the exciting research that is being conducted by other groups.

A. Tsikritea, L. S. Petralia, M. Hejduk and B. R. Heazlewood