Chemistry near Absolute Zero
The Absolute Zero of Temperature – zero Kelvin, or -273.15 Celsius – is a concept that was already a cause for robust scientific discussion in the 17th century, in the earliest years of the Royal Society, through the work of Robert Boyle and others. By the 19th century it was recognised that this characterises a state in which matter has its lowest possible energy and entropy. In practice Absolute Zero can only be approached asymptotically rather than attained, and in the modern field of Ultracold Matter, we refer to ‘the milliKelvin regime’, the microKelvin regime’ and so on, as temperatures are lowered below 1K. In the natural world, the lowest known observed (inferred) temperatures are in the Boomerang nebula at around 1K, while the cosmic microwave background indicates that the average temperature of the entire universe is 2.73K. In the interstellar region, very low density gas clouds have temperatures in the 10-50K range, while on Earth the lowest natural temperature recorded (in Antarctica) was a relatively warm 184K (-89.2C).
In the laboratory, the techniques of modern physics, particularly laser cooling, allow us to obtain samples of gaseous atomic matter (commonly alkali metal atoms) for which the temperature can be pushed well down into the sub-Kelvin regime. The microKelvin range and below has been achieved in many laboratories, leading to observation of remarkable physical phenomena such as Bose Einstein condensation – a quantum state of matter where the atoms exhibit coherent behaviour analogous to the properties of light in laser radiation. And in the last 20-30 years, very considerable efforts worldwide have been dedicated to creating samples of molecules at sub-Kelvin temperatures – generally simple molecules such as ammonia, formaldehyde, hydroxyl radicals (OH), dimers of alkali metals (e.g., KRb or Rb2) – and also ionic species, particularly cations. These developments prompt the question - can chemical reactions occur at such low temperatures? And if so what are the characteristic chemical behaviours of molecules in such a low-temperature regime? .
In the classical theory of Chemical Kinetics, it is recognised that chemical reactions occur through the collisions of molecules, and that the kinetic energy associated with these collisions is expended to break chemical bonds, in order that new bonds (and hence new molecules) can be created. This concept of the activation energy of chemical reactions was captured in the Arrhenius equation in the 19th century. It follows that chemical reactions proceed more slowly as the temperature is lowered (and collisions are less energetic) and near absolute-zero would occur at such a negligible rate that in essence chemical reactions no longer occur.
However, this is an entirely classical concept and does not take into account the fact that as the temperature is lowered the laws of quantum physics start to dominate behaviour; the wave-particle duality of matter must be invoked, as captured in the deBroglie equation p = h/l. As a particle (atom or molecule) moves more slowly, its momentum (p) is lowered, and hence the wavelength (l), associated with the wave that describes its motion, becomes longer. A simple molecule such as fluoromethane, CH3F moves at around 400 ms-1 in a room temperature gas, and the associated wavelength is 0.03 nm – about one order of magnitude smaller than the size of the molecules. Under such circumstances, the classical nature of the particle’s motion comes to the fore. However, if the temperature is lowered to 30 mK, the mean velocity reduces to 4ms-1 and the wavelength increases to 3 nm, about one order of magnitude greater than the molecular scale. As a consequence, the quantum, wave-like nature of the molecular behaviour dominates. This allows the wavepacket to spread across the activation energy barrier rather than having to surmount it, and thus the concept captured in the Arrhenius equation breaks down. In addition, there are a number of categories of reaction that have very low energy barriers to reaction, or even negligible barriers – a key one being reactions between ionic species and neutrals – and this again changes the underlying physical behaviour of the reacting species.
My research interests are concerned with exploring this new physical regime for chemistry using novel experiments and theory. I do not currently have an experimental laboratory in Birmingham, but collaborate closely with others, particularly Dr Brianna Heazlewood, now at Liverpool. Ionic-neutral reactions have been a particular area of interest, and over the last 15 years we have developed experiments [2,3] in which the ‘reaction vessel’ is a laser-cooled radiofrequency ion trap mounted inside a high vacuum chamber. Calcium atoms are ionized with a laser to produce Ca+ cations and the ions are laser cooled to temperatures in the milliKelvin range. Under these circumstances the ions form a ‘Coulomb crystal’ – a 3-dimensional regular array of ions, in which the natural repulsion between the ions is balanced by the trapping radiofrequency field. Although the crystal has a solid-like microscopic structure, observed by imaging the fluorescence from individual ions, the density is extremely low, and thus this is really a crystal in the gas phase. Individual ions are resolved in the images, and the ions can be trapped and observed on a timescale of hours.
In order to observe reactions at milliKelvin tempartures, the ion trap chamber is connected to another vacuum chamber containing one of a number of devices for creating a cold, low-velocity beam of neutral molecules; for example, an electric-quadrupole-guide velocity selector, a Stark decelerator, or a Zeeman decelerator. These devices create low-velocity molecules through the use of either high electric fields or high magnetic fields to control the velocity. A simple reaction such as Ca+ + CH3F → CaF+ + CH3 can be monitored by observing the disappearance of the calcium ions in the Coulomb crystal as a function of time. Typically, Coulomb crystals of a few hundred ions are used, but in some examples the experiment begins with just two trapped ions and the time is measured for just one of the ions to react. More complex reactions can be observed, involving molecular ions, by first sympathetically cooling molecular ions into the calcium ion framework, with mass-spectrometric detection of reaction products. Recent work in Dr Heazlewood’s laboratory has demonstrated that a remarkably large ‘inverse isotope effect’ can be observed in simple charge transfer reactions of ammonia isotopologues with rare gas ions .
The study of chemical reactions in this sub-Kelvin regime is an example of basic discovery science. It is in this regime, devoid of the effects of thermal averaging that are present at higher temperatures, in which there is the greatest potential to gain complete control over the chemical reaction process at a molecular level.
 “Towards chemistry at absolute zero” B.R. Heazlewood and T.P. Softley, Nature Chemistry Reviews, 5, 125 (2021).
 "Cold Reactive collisions between Laser Cooled Ions and Velocity-Selected Neutral Molecules", S. Willitsch, M. Bell, A. Gingell, S. R. Procter and T. P. Softley, Physical Review Letters, 100, 043203 (2008).
 “Low-temperature kinetics and dynamics with Coulomb crystals” B. R. Heazlewood and T. P. Softley, Ann. Rev. Phys. Chem. 66 475 (2015).
 “Strong inverse kinetic isotope effect in ammonia charge transfer collisions”, L.S. Petralia, A. Tsikritea, J. Loreau, T.P. Softley, B.R. Heazlewood, Nature Communications, 11, 1 (2019).