# Synopsis: Playing pool with atoms

When an atom is bombarded with just enough energy to fully ionize it, how do the electrons and nucleus break apart from each other? Experimentalists are now able to study such a four-body breakup by bombarding a helium atom with an electron.

One of the most basic ways to learn about few-body interactions in an atom is to break it apart. Understanding this atomic fragmentation is most challenging when the particles emerge with little kinetic energy. Since perturbation theory cannot be used, all of the particles need to be treated on an equal footing, and the behavior of the system at long times and large distances becomes crucial.

For interactions that fall off as $1/{r}^{2}$ or slower—such as that between charged particles—large-distance correlations subtly influence the breakup, but exactly how this occurs has been a subject of controversy for more than 50 years. Writing in Physical Review Letters, Xueguang Ren, Alexander Dorn, and Joachim Ullrich of the Max Planck Institute for Nuclear Physics in Heidelberg use electron bombardment to fully ionize helium at an excess energy of only 5 eV. They find the electrons—two from the helium atom plus the projectile electron—emerge predominantly in the configuration of an equilateral triangle, in accordance with the basic prediction of so-called Wannier-type threshold laws. But they also show that on a more detailed level some unexpected structure persists, indicating that the fragmentation dynamics at these energies is more subtle than the simple Wannier picture would suggest.

The paper is an important step forward in the understanding of threshold fragmentation, but it also raises new questions by showing unexpected complexity in the details of the atomic breakup. – Thomas Pattard

### Announcements

More Announcements »

## Subject Areas

Atomic and Molecular Physics

Mesoscopics

## Next Synopsis

Biological Physics

## Related Articles

Atomic and Molecular Physics

### Viewpoint: Cool Physics with Warm Ions

Ultrafast laser pulses can be used to control and characterize the quantum motion of a single trapped ion over 5 orders of magnitude in temperature. Read More »

Atomic and Molecular Physics

### Synopsis: The Quantum Hall Effect Leaves Flatland

Cold atoms in an optical lattice with a synthetic extra dimension could be used to see the 4D version of the quantum Hall effect.   Read More »

Condensed Matter Physics

### Viewpoint: Emerging Quantum Order in an Expanding Gas

The spontaneous emergence of long-range quantum order, normally the preserve of low-temperature equilibrium states, has been observed in an expanding cloud of potassium atoms. Read More »