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Synopsis: A New Negative Ion Takes the Cooling Spotlight

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发表于 2020-1-14 00:54:05 | 显示全部楼层 |阅读模式
Synopsis:A New Negative Ion Takes the Cooling Spotlight

Laser cooling—which is routinely performed onatoms and positive ions—works through a photon-driven cycle of transitionsbetween ground and excited states. Most negative ions (or “anions”) areimpossible 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 colleaguesshow that the thorium anion (Th−) is more strongly bound to itsextra electron than predicted, meaning it could be amenable to the technique.

Researchers have previously identified a fewnegative ions that might be candidates for laser cooling, with the lanthanumanion (La−) long considered the most promising (see 6 September 2019Focus story). Despite possessing the necessary excited states, thorium anionswere overlooked, as theoretical predictions had suggested that the extraelectron’s binding energy was around 0.3 eV, which is too low for efficientlaser cooling. Tang and colleagues measured this binding energy for the firsttime and found it to be 0.6 eV. They also recalculated the electron orbitalassignments for the ground and excited states of Th−.
From their results, the researchers predictedthat thorium anions could be laser cooled
at a photon wavelength of 2.6 μm, reaching aminimum temperature of 0.04 μK. This laser-cooling potential compares well withLa−, whose minimum predicted cooling temperature is 0.17 μK. Relative tolanthanum, however, thorium’s zero-spin nucleus results in a simpler absorptionspectrum, which means the laser pumping setup should be less complicated.
This research is published in Physical ReviewLetters.


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