Elucidating\nthe Mechanism of Helium Evaporation from\nLiquid Water

We investigate the evaporation of trace amounts of helium\nsolvated\nin liquid water using molecular dynamics simulations and theory. Consistent\nwith experimental observations, we find a super-Maxwellian distribution\nof kinetic energies of evaporated helium. This excess of kinetic energy\nover typical thermal expectations is explained by an effective continuum\ntheory of evaporation based on a Fokker–Planck equation, parametrized\nmolecularly by a potential of mean force and position-dependent friction.\nUsing this description, we find that helium evaporation is strongly\ninfluenced by the friction near the interface, which is anomalously\nsmall near the Gibbs dividing surface due to the ability of the liquid–vapor\ninterface to deform around the gas particle. Our reduced description\nprovides a mechanistic interpretation of trace gas evaporation as\nthe motion of an underdamped particle in a potential subject to a\nviscous environment that varies rapidly across the air–water\ninterface. From it we predict the temperature dependence of the excess\nkinetic energy of evaporation, which is yet to be measured.