Observation of the Efimov state of the helium trimer

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Science  01 May 2015:
Vol. 348, Issue 6234, pp. 551-555
DOI: 10.1126/science.aaa5601

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Helium caught in the act of triangulating

Helium is the noblest of noble gases, almost completely unattracted to itself or any other chemical element. Of course, when quantum mechanics comes into play, that “almost” is an inevitable caveat. For several decades, researchers have been intrigued by a theoretically predicted Efimov state composed of three helium atoms held loosely together in a triangle. Kunitski et al. now report experimental realization of that state and detection of its acute triangular geometry (see the Perspective by Kornilov). Beyond completing a long quest in helium studies, the results shed light on three-body physics more broadly.

Science, this issue p. 551; see also p. 498


Quantum theory dictates that upon weakening the two-body interaction in a three-body system, an infinite number of three-body bound states of a huge spatial extent emerge just before these three-body states become unbound. Three helium (He) atoms have been predicted to form a molecular system that manifests this peculiarity under natural conditions without artificial tuning of the attraction between particles by an external field. Here we report experimental observation of this long-predicted but experimentally elusive Efimov state of 4He3 by means of Coulomb explosion imaging. We show spatial images of an Efimov state, confirming the predicted size and a typical structure where two atoms are close to each other while the third is far away.

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