RESONANT SCATTERING OF SURFACE ACOUSTIC-WAVES BY HYDROGEN CRYSTALLITES

Surface acoustic (Rayleigh) waves have been used to study the annealing behavior of quench-condensed hydrogen films. At temperatures far below the triple point such films undergo drastic structural changes, forming larger crystallites of typically 1 mu m diameter. The rearrangement of the film structure could be traced in situ, by changes of sound velocity and attenuation arising from scattering of the Rayleigh waves by inhomogeneities. The dominant scattering mechanism appears to be well described by a resonant interaction between surface acoustic waves and elastic eigenmodes of the hydrogen crystallites (''surface shape resonances''). We propose a very simple model for this scattering process, where the resonant modes are essentially treated as harmonic oscillators, and find wide agreement with our experiments.