Direct measurement of the size of the helium dimer

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Abstract
The relative transmissions of helium dimer and helium atom beams through a set of nanoscale sieves were measured as a function of hole size in the range from 98–410 nm. From the relative transmission coefficients, the mean internuclear distance of helium dimer was determined to be 〈r〉=62±10 Å. This enormous bond length—by far the largest known—confirms recent theoretical estimates of the potential energy well depth ε and the extremely small binding energy Eb of helium dimer. The ranges of these parameters corresponding to the experimental uncertainty in 〈r〉 are ε/k=10.88–10.98 K and Eb/k=0.65–1.30 mK, where k is the Boltzmann constant.