  We report on the observation of an anomalously high attenuation of high
frequency surface acoustic waves by thin films of liquid $^{4}$He. The
piezoelectric acoustic waves propagate along the surface of a lithium niobate
substrate, which is coated with varying amounts of liquid helium. When the
thickness of the helium layer is much larger than the wavelength of the surface
acoustic wave on the substrate its attenuation is dominated by the excitation
of compressional waves into the liquid, in good agreement with theory and
previous measurements. However, for sufficiently thin helium coverage, we find
that the acoustic wave attenuation is significantly increased beyond that
measured with the substrate submerged in bulk liquid. Possible mechanisms for
this enhanced attenuation are discussed.
