The trouble with laser-cooling anions

For scientists to use lasers to cool an atom, the atom needs to have two energy states. When laser light is shined on an atom moving towards the source of light, one of its electrons absorbs a photon, climbs to a higher energy state and the atom as a whole loses some momentum. A short span of time later, the electron loses the photon in a random direction and drops back to its lower energy state, and the atom’s momentum changes only marginally.

By repeating this series of steps over and over, scientists can use lasers to considerably slow atoms and decrease their temperature as well. For a more detailed description + historical notes (including a short profile of a relatively forgotten Indian scientist who contributed to the development of laser-cooling technologies), read this post.

However, it’s hard to use this technique with most anions – negatively charged ions – because they don’t have a higher energy state per se. Instead, when laser light is shined on the atom, the electron responsible for the excess negative charge absorbs the photon and the atom simply ejects the energised electron.

If the technique is to work, scientists need to find an anion that is bound to its one excess electron (keeping it from being electrically neutral) strongly enough that as the electron acquires more energy, the atom ascends to a higher energy state with it instead of just losing it. Scientists discovered the first such anion in the previous decade – osmium – and have since added only three more candidates to the list: lanthanum, cerium and diatomic carbon (C2). Lanthanum is and remains the most effective anion coolable with lasers. However, if the results of a study published on November 12 are to be believed, the thorium anion could be the new champion.

Laser-cooling is relatively simpler than most atomic cooling techniques, such as laser-assisted evaporative cooling, and is known to be very effective. Applying it to anions would expand its gamut of applications. There are also techniques like sympathetic cooling, in which one type of laser-cooled anions can cool other types of anions trapped in the same container. This way, for example, physicists think they can produce ultra-cold anti-hydrogen atoms required to study the similarities between matter and antimatter.

The problem with finding a suitable anion is centred on the atom’s electron affinity. It’s the amount of energy an electrically neutral atom gains or loses when it takes on one more electron and becomes an anion. If the atom’s electron affinity is too low, the energy imparted or taken away by the photons could free the electron.

Until recently, theoretical calculations suggested the thorium anion had an electron affinity of around 0.3 eV – too low. However, the new study found based on experiments and calculations that the actual figure could be twice as high, around 0.6 eV, advancing the thorium anion as a new candidate for laser-cooling.

The study’s authors also report other properties that make thorium even more suitable than lanthanum. For example, the atomic nucleus of the sole stable lanthanum isotope has a spin, so as it interacts with the magnetic field produced by the electrons around it, it subtly interferes with the electrons’ energy levels and makes laser-cooling more complicated than it needs to be. Thorium’s only stable isotope has zero nuclear spin, so these complications don’t arise.

There doesn’t seem to be a working proof of the study’s results but it’s only a matter of time before other scientists devise a test because the study itself makes a few concrete predictions. The researchers expect that thorium anions can be cooled with laser light of frequency 2.6 micrometers to a frosty 0.04 microkelvin. They suggest doing this in two steps: first cooling the anions to around 10 kelvin and then cooling a collection of them further by enabling the absorption and emission of about 27,000 photons, tuned to the specified frequency, in a little under three seconds.