Structural relaxation and surface diffusion of quench-condensed hydrogen films

We have studied the structural relaxation of quench-condensed films of molecular hydrogen with ultrasonic surface acoustic waves (SAW). Upon annealing, frequency-dependent scattering effects occur indicating a growth of hydrogen crystallites of sizes around 1 mu m. The dominant scattering mechanism appears to be a resonant coupling of the SAW to some elastic eigenmodes of the crystallites. During deposition at 1.4K and subsequent annealing we measured the change of attenuation and sound velocity of the SAW and are able to extract activation energies for the underlying surface diffusion. A strong temperature dependence below 2.5 K of the resonance frequency is reported in the case of H-2- and D-2-crystallites.