Response to "Comment on 'The Weakest Bond: Experimental Observation of Helium Dimer'"

Xuebin Wu, Chenlei Du, Jian-Bo Deng

Eric S. Meyer, John C. Mester, Isaac F. Silvera

Recently, Luo et al. reported indirect evidence for the formation of dimers in a supersonic expansion of helium. They interpreted their data as confirmation of the existence of a bound state of 4He–4He with a binding energy of order 1 mK. We show that their data are actually consistent with the formation of helium trimers and not dimers. The two control experiments that they performed do not provide any evidence for discarding this alternative hypothesis.

Elias Amselem, Mohamed Bourennane

Fei Luo, George C. McBane, Geunsik Kim +2 more

Helium dimer ion was observed after electron impact ionization of a supersonic expansion of helium with translational temperature near 1 mK. The dependence of the ion signal on source pressure, distance from the source, and electron kinetic energy was measured. The signal was determined to arise from ionization of neutral helium dimer.

LAWRENCE L. JUN. LOHR, S. M. Blinder

Van der Waals attraction between helium atoms can lead to the formation of weakly-bound He 2 diatomic molecules stable at temperatures in the millikelvin range. Helium dimers were first detected experimentally in 1993 and 1994. Sophisticated quantum-mechanical computations confirm that the He–He bond is the weakest known, with a binding energy corresponding to 1.176 mK, about 10 -4 eV. The average internuclear distance is computed to be in excess of 50 Å, which makes He 2 the largest known diatomic molecule as well. The authors have introduced an idealized deltafunction model for the helium dimer that succinctly represents its remarkable properties. Despite the weakness of each individual He–He bond, an intriguing mechanism exists for helium to form trimers and larger clusters with much greater binding energies.

Matthew F. Pusey, Jonathan Barrett, Terry Rudolph