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Inelastic helium-atom scattering from epitaxial films (2--20 monolayers) of sodium on Cu(001) reveals a large number of nearly dispersionless phonon modes whose frequencies depend on the thickness and scale as the lower harmonics of an open-ended organ pipe. These data provide evidence for confined acoustic resonances, corresponding to longitudinal standing waves normal to the surface with frequencies about 20% larger than in the bulk.
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Benedek et al. (1992) studied this question.
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