Role of long-range forces in H sticking to liquid He

An analysis is presented of the problem of H sticking to liquid He at low temperature. For the bulk liquid, the sticking coefficient is computed to be less than 0.01 at T${10}^{\mathrm{-}4}$ K, varying as ${\mathit{T}}^{1/2}$ for lower T. While apparently contradicted by the recent data of Doyle et al., compatibility is found if cognizance is taken of the fact that the He exists as a film in the experiments. In this case, the underlying substrate greatly enhances the attraction experienced by the H atom. The calculations demonstrate that the effects of retardation and corrections to the distorted-wave Born approximation are significant.