Diffusion of H atoms on the surface of liquid 4He

The diffusion constant D for H atoms bound to the surface of liquid 4 He is calculated from the Boltzmann equation in the relaxation time approximation. The dominant process for momentum relaxation is found to be the emission and absorption of ripplons. Long wavelength ripplons are shown to couple very weakly to bound H atoms, implying a strong temperature dependence of the relaxation time at low T, [Formula: see text] for T < 10 −3 K. For the experimental temperature range, 0.1 < T < 0.3 K, [Formula: see text] is rather long, by which we mean that the mean free path [Formula: see text] is several thermal De Broglie wavelengths, Λ, i.e.,l > 30 Λ for T < 0.3 K. We conclude that surface spin relaxation calculations based on binary collisions of freely propagating H atoms are appropriate for H on liquid 4 He and that the hydrodynamic contribution to1/T 1 , due to interactions with ripplons, is negligible.