Deltafunction model for the helium dimer

The helium dimer 4He2 has recently been detected, confirming earlier ab initio predictions of stability for a single bound state with binding energy of 1.310 mK. The predicted potential minimum is at 2.96 Angstrom, with a radial distribution function peaking at 6.96 Angstrom. We model this system using a Dirac bubble potential, which also admits just one bound J = 0 state. With the bubble located at 6.96 Angstrom, an overlap of 0.9994 with the ab initio wave function is obtained. An average internuclear distance of 52.6 Angstrom is calculated, in good agreement with the ab initio result. The root mean square deviation from the mean, 48.0 angstrom, indicates an enormous spread of the radial wave function. Also consistent with our model is the absence of bound states for the isotopic variants 3He4He and 3He2. Cross sections for helium-helium scattering are also computed, using both a partial-wave expansion and the Born approximation. General trends in the energy dependence of the total cross section are accounted for, in qualitative agreement with experimental results. 13 refs., 4 figs