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The experimental elastic reflectivity of 4He atoms from liquid 4He surfaces behaves as if the 4He atoms are reflected from a very smooth elastic potential. The authors explain this anomalous result in terms of Van der Waals forces and coupling to the ripplons which swamps all other processes. Interpretation of the experiments confirms that the surface of liquid 4He is diffuse on a scale of 5 AA.
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Echenique et al. (1976) studied this question.
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