Alcove

Science, culture, complexity

Tag: diatomic carbon

  • Rapid rotation explains unusual stability of C2 anion

    In various settings, including chemical reactions in the lab, inside nuclear reactors, and in outer space, scientists have found C2– anions living for as long as three milliseconds before decaying to a more stable state — and they haven’t been able to explain why. Normally particles, atoms, and molecules make these transitions to lose energy and become more stable. And normally the C2– molecule has around 4 eV less energy than the C2– anion, so the latter decayed to the former within one-trillionth of a second. The puzzle was that scientists didn’t know of a mechanism that allowed C2– to not decay to C2 for more than 3 ms, a timespan more than a billion-times longer.

    In two new papers published on October 31 (here and here), researchers from Austria, the Czech Republic, and Germany reported “strong evidence” for an idea scientists first had in the late 1990s: that the delay had something to do with rotation. Scientists have previously found rapidly rotating molecules in space — including when radiation breaks up water molecules in the interstellar medium and in the dynamic neighbourhood of a newborn star.

    The study team found that when the C2– complex rotates fast enough to increase its rotation quantum number 𝑁 beyond 155, it acquires a “centrifugal potential” that rearranges the lower-energy states to which C2– can decay. In particular, the team’s theoretical calculations revealed that a different state other than the C2 state to which it normally decays has lower energy, and dropping to the C2 state becomes unfavourable. More specifically, if the C2– anion had 𝑁 values in the 165-183 range, the normal decay to C2 requires electrons to have at least six units of angular momentum. If 𝑁 is lower than 165, the rearrangement of energy states doesn’t forbid the rapid drop to C2.

    In other words, the spinning molecule spits out a spinning electron to move to a more stable configuration — and even then not before living to the ripe old age of 3 ms. This so-called rotation-assisted stability of the C2– anion isn’t entirely new. Other scientists have previously found dihydrogen and dideuterium anions (H2– and D2–) to be more stable as well when 𝑁 = 20-40. Using and theory and experiments, the European team found C2– acquired the same stability gain at 𝑁 of 155 or more because it’s heavier and has a higher rotational constant (“a fundamental parameter describing the rotational energy levels of a molecule,” per Meta AI).

  • 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.