Observation of Anomalously Low Momentum Transfer in the Low Energy Scattering of Large 4 He Droplets From 4 He and 3 He Atoms

The momentum transferred to large superfluid 4 He droplets (N≈ 1.5 · 10^9 ) in low energy ( ≈ 4–10 K) scattering from 4 He and 3 He atoms was determined from time of flight measurements after the droplets have passed through a low temperature (T = 1.7–4.2 K) scattering box filled with the gas of either He isotope. The results are compared with 3 He droplets scattered from either 4 He or 3 He gas for which all of the incident momentum is transferred, as expected for classical capture of the scattering gas atoms. In the case of the 4 He droplets a smaller momentum transfer is found amounting to 65