Reflectivity of LiquidHe4Surfaces toHe4Atoms

We explain recent results on the reflectivity of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ surfaces to externally incident ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms in terms of a Van der Waals interaction and strong coupling to quantized surface-tension waves which suppresses all other influences on the reflectivity. The model gives numerical agreement with experiment and it is found that a proper treatment of density variation at the liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ surface is essential.