ADSORPTION OF HE-3 ON HE-4 CRYSTAL-SURFACES

We have compared the surface properties of crystals respectively grown from normal He-4 containing 130 ppb of He-3 and from ultrapure He-4 ( 0.4 ppb). Below 0.4 K, He-3 impurities are found to decrease both the surface tension and the step energy. Our results are consistent with t he existence of two-dimensional bound states for He-3 atoms at the sol id-liquid interface. Quantitative agreement with the data is found wit h a binding energy epsilon(s) approximate to 4.3 K and a He-3 density saturating around 0.4 monolayer. The presence of steps is found to inc rease the binding energy by about 10 mK.