Synopsis: A New Negative Ion Takes the Cooling Spotlight

Measurements of the electron binding energy in the negative thorium ion suggest that it may be a good candidate for laser cooling.

Laser cooling—which is routinely performed on atoms and positive ions—works through a photon-driven cycle of transitions between ground and excited states. Most negative ions (or “anions”) are impossible to cool this way, as they have a ground state but no excited states: the loosely bound spare electron is ejected completely upon absorbing a photon. New measurements by Rulin Tang of Tsinghua University in China and colleagues show that the thorium anion (Th) is more strongly bound to its extra electron than predicted, meaning it could be amenable to the technique.

Researchers have previously identified a few negative ions that might be candidates for laser cooling, with the lanthanum anion (La) long considered the most promising (see 6 September 2019 Focus story). Despite possessing the necessary excited states, thorium anions were overlooked, as theoretical predictions had suggested that the extra electron’s binding energy was around 0.3 eV, which is too low for efficient laser cooling. Tang and colleagues measured this binding energy for the first time and found it to be 0.6 eV. They also recalculated the electron orbital assignments for the ground and excited states of Th.

From their results, the researchers predicted that thorium anions could be laser cooled at a photon wavelength of 2.6 𝜇m, reaching a minimum temperature of 0.04 𝜇K. This laser-cooling potential compares well with La, whose minimum predicted cooling temperature is 0.17 𝜇K. Relative to lanthanum, however, thorium’s zero-spin nucleus results in a simpler absorption spectrum, which means the laser pumping setup should be less complicated.

This research is published in Physical Review Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.


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