ANOMALOUS ELASTIC PROPERTIES OF QUENCH-CONDENSED HYDROGEN FILMS

Quench-condensed films of molecular hydrogen (H-2, HD, and D-2), prepa red around 1 K, undergo a strong structural rearrangement upon anneali ng at 3-4 K. Caused by dewetting the film transforms into crystallites with sizes of the order of mu m. Surface acoustic waves couple resona ntly to the eigenmodes of these crystallites and are strongly scattere d. The systematic variation with time of sound velocity and attenuatio n allows to trace the evolution of the crystallites. Moreover, at any stage of evolution a drastic temperature dependence of sound velocity and attenuation is observed between 0.1 K and 3 K. This has to be attr ibuted to a strong change of the effective eigenfrequencies of the cry stallites which cannot be readily explained by the bulk properties of hydrogen.