VIBRATIONAL DIAGNOSTICS OF THE EPITAXIAL-GROWTH OF ULTRATHIN FILMS OF SODIUM ON CU(001)

We have grown layer-by-layer thin epitaxial films of sodium on a Cu(001) substrate with thickness varying from 1 to 30 monolayers and determined their elastic properties by means of inelastic helium atom scattering. These data provide the first evidence for longitudinal acoustic resonances confined in the film. The substrate-induced strain leads to a 20% stiffening of the longitudinal mode frequency for propagation normal to the surface and an even larger stiffening of the Rayleigh wave with respect to the expected Na ideal surface. This technique is shown to provide detailed information on the elastic properties of ultrathin films in a domain presently inaccessible to Brillouin scattering or ordinary transducers.