Atomic impact experiments with free helium-3 and helium-4 clusters

Free helium clusters with of the order of 10³ to 10⁷ atoms have been predicted to reach final temperatures of about 0.4 K in the case of⁴He, and of 0.15 K in the case of³He. In the first case, the clusters have to be expected to be superfluid, in the latter case to be normalfluid. Impact experiments with cesium atoms, which are calculated to be strongly repelled from inside liquid⁴He, and slightly from liquid³He, indicate cesium embedding into or onto⁴He clusters. Xenon atoms, calculated to be strongly bound to both quantum liquids, show incomplete momentum transfer to⁴He as well as to³He clusters at higher impact speeds.