The types of chemical reactions we typically learn in school take place on a Cecil B. deMille, cast-of-thousands scale. They involve huge numbers of atoms or molecules, making it extremely difficult to study exactly what is happening on the quantum level between the individual players. Many of them won't even take place at cold temperatures, where the rate of reaction is slow enough to observe the details. All this makes it very difficult to understand the quantum mechanics of chemical reactions between individual pairs of atoms.

In a new Nature Physics paper, researchers with the Cavendish Laboratory at the University of Cambridge were able to measure the chemical interactions between individual, ultracold ytterbium ions and rubidium atoms. Lothar Ratschbacher, Christoph Zipkes, Carlo Sias, and Michael Köhl studied inelastic collisions between the atoms and ions, in which some of the energy in the system is converted to light or motion. In doing so, they obtained the first low-level analysis of charge exchange, the chemical reaction in which an electron is passed between an atom and an ion.

Ordinary atoms are electrically neutral: they possess equal numbers of protons and electrons. However, when two atoms approach each other slowly (as they do at cold temperatures) something odd happens: the electrons end up repelling each other, preventing chemical interactions.

This doesn't happen with ions—atoms in which one or more electrons are added or subtracted, giving them a net electric charge. When an atom and a positive ion approach slowly, the ion's charge draws part of the atom's electron cloud towards it, leading to an attractive force between them. This force is much weaker than if both atoms were ionized, but at cold temperatures, the two objects can approach each other slowly enough that it becomes significant.

The authors of the new study exploited this property by magnetically trapping ytterbium ions (Yb+) and neutral rubidium atoms (Rb) at very low temperatures. They set the Yb+ to one of two quantum states by exciting them with laser light; these states were chosen because they take a long time to decay to their ground state. This means the ions won't give up this extra energy before they interact with the Rb atoms.

Being in an excited state meant the ions had internal energy—energy unrelated to the motion of the ions, which is expressed as temperature. This is akin to macroscopic objects such as a rolling ball: in addition to the energy of its motion, it has internal energy in the form of the motion of the atoms inside the balls. The internal energy of the Yb+ ions is available during the chemical reactions.

In the experiment, the reactions were all exothermic. This means the additional internal energy from the excited state could be converted to kinetic energy, so that the products of the reaction moved faster after than they did before. (Alternately, the extra energy could be converted to photons—an example of fluorescence.)

In some cases, the kinetic energy was sufficient to kick the reaction products out of the trap entirely; knowing the amount of energy required to do this set a minimum bound on the final kinetic energy. If the products remained inside the apparatus, their final speed was measured, revealing how much energy had been transferred.

The researchers performed the experiment under two conditions. First, the reactions were run in the dark, with the laser shut off after being used to prepare the ions. No photons were present in the trap other than any that might have been emitted by the reaction itself. Second, they kept the laser directed onto the atom and ion, using it to control their interaction. By adjusting the frequency of the laser, the authors altered both the rate of the reaction and the quantum states of the products.

In some trials, one electron was transferred from the Rb atom to the Yb+ ion, so that the rubidium became ionized (Rb+) and the ytterbium became neutral. This reaction is known as charge exchange. In no cases did a molecule form from the reacting objects, though the experimentalists considered that to be a possible outcome.

One interesting thing the researchers noted: they found the relative orientation of the electronic spin and the nuclear spin—known as the hyperfine state—made a difference to the reaction outcome. The properties of an atomic nucleus typically don't play a direct role in chemical reactions, but these results show a clear counterexample to that general assumption.

Between the charge exchange, quenching, hyperfine state, and other properties, this experiment provides an excellent demonstration of how manipulation of quantum states can lead to chemical reactions at very low temperatures. By using single atoms in the reactions, the researchers created a particularly clean experimental environment, avoiding the complications that arise from using the usual large numbers of reacting objects.

Nature Physics, 2012. DOI: 10.1038/nphys2373 (About DOIs).