Taking tenuous helium molecules for a spin

Emission spectrum of helium.

The emission spectrum of helium. Laser pulses can expose quantum aspects of pairs of helium atoms. Credit: Dept. of Physics, Imperial College/SPL

Atomic and molecular physics

Zapping helium ‘dimers’ with lasers allows a glimpse into a fleeting relationship.

Helium atoms are stand-offish, rarely interacting with atoms of other elements or with one another. But physicists know that helium atoms cooled close to absolute zero can be coaxed into forming fragile pairs, or dimers, of a particularly quantum nature.

To better understand these tentative dance partners, Maksim Kunitski and Reinhard Dörner at Goethe University in Frankfurt, Germany, and their colleagues have given helium dimers a ‘kick’ by exposing them to a strong laser field. The team shot a laser pulse at helium dimers, imparting angular momentum to the pairs. A second, shorter laser pulse quickly knocked electrons off the atoms. The remaining positively-charged ions repelled one another, flying apart so they could be measured.

By varying the time between the first and second laser pulses, the group could see the dimer’s quantum response. The team’s technique could be used to study the less-explored helium trimer — a grouping of three helium atoms — as well as other low-energy molecules. It could also open a window onto the dynamics of exotic quantum states.

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