The sizes and mass spectra of large (N=1900-13 700 molecules) cold (approx3.1K) H_2 clusters have been measured after scattering from CO molecules.Cluster-size measurements after between 2 and 8 collisions indicate that 7% of the H_2 molecules are evaporated.This loss agrees with calculations for the numer of H_2 molecules evaporated by the heat released in the transition from an initial liquid state to a final solid state.Event hough heterogeneous nucleation is initiated after only a few collision with CO molecules,the mass spectr show that additional captured CO molecules coagulate to form large CO clusters with up to n=11 molecules,suggesting that the outer layer is sufficiently liquidlike to facilitate mobility of the CO molecules.Since the calculated H_w cluster temperature (approx3.1K) is below the superfluid transition temeprature predicted for pH_2 with density between 40% and 80% of the triple-point density,a shell-like region of low density near the cluster surface can be expected to the superfluid.
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