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.

Because of the extremely small binding energy of the helium dimer, the nuclear wave function is delocalized over an extremely large range of separations. One might therefore expect the properties of this extraordinary species to be sensitive to the potential at very large internuclear distances, r, where relativistic corrections to the usual van der Waals interaction may be important. We have estimated the effect of retardation, which changes the r-6 dependence of the potential to r-7 in the limit of large r, and have found that the binding energy and expectation value (r) are indeed significantly affected by its inclusion.

In this note, we evaluate the influence of the relativistic retardation corrections on the dipole–quadrupole, dipole–octupole and quadrupole–quadrupole contributions to the He–He dispersion potential, which affect the attractive r −8 and r −10 terms. These higher-order corrections influence the binding energy of 4He2 an order of magnitude less than does the retardation correction to the leading r −6 dipole–dipole term.