CONDENSATION COEFFICIENT OF LIQUID HELIUM-2

The condensation coefficient (defined as the fraction of gas atoms striking the liquid-vapour interface which cross it to form part of the liquid) has been measured for liquid helium II from 1°K to the λ point, and was found to be not less than 0·90 and probably unity. The measurements, made by reflecting second sound pulses from the surface, are not in agreement with the macroscopic theories of Osborne and of Chernikova; the former theory gives agreement with experiment when modified to take account of conditions in the vapour a fraction of a mean free path above the surface. The problem of providing a microscopic interpretation of these results is briefly discussed.