Maksim Kunitski, Goethe Universität, Frankfurt, Germany, and colleagues have experimentally demonstrated the existence of so-called Efimov trimers for the first time. In this exotic state, atoms in the excited state form trimers. The atoms are unusually far apart from each other and should not be expected to be bonded. This situation, generated by quantum effects, had long been postulated, but so far had not been proven experimentally. Such trimers occur in ultra-cold quantum gases of cesium atoms. Unfortunately, they are so unstable that they disappear from the atom trap without being seen.
The team has forced ultra-cold helium gas through a tiny nozzle into a chamber with vacuum. Under these conditions, short-lived molecules of two, three or more helium atoms form in the helium beam, among them Efimov trimers. By scattering the molecular beam at an ultra-fine grid, the researchers separated the trimers and then shot a laser beam at them. The atoms of the fragile trimers were ionized and the molecules separeted explosively caused by the mutual repulsion. Using a special ion detector, the researchers captured the helium ions and could reconstruct the shape and properties of the original Efimov trimer from their position relative to each other.
In the Efimov state, the three helium atoms are not more than 100 Å apart from each other. The distance between the atoms differs within the molecule: two atoms are close together, the third forms an isosceles triangle with them.
- Observation of the Efimov state of the helium trimer,
M. Kunitski, S. Zeller, J. Voigtsberger, A. Kalinin, L. P. H. Schmidt, M. Schoffler, A. Czasch, W. Schollkopf, R. E. Grisenti, T. Jahnke, D. Blume, R. Dorner,
Science 2015, 348, 551–555.