Viscous Waves in 4 He Films

A quartz crystal resonator was used to generate viscous shear waves, penetration depth ∼ 20 nm, in HeI and HeII films. The complex transverse acoustic impedance was measured and interpreted using transmission line theory. Normal films of thickness 1.5< d <60 nm were studied. Below T λ , measurements were made on saturated films with 14< d <23 nm. An unexpected feature may be due to a standing wave resonance on vortices. The efficiency for the evaporation of atoms from the film surface by rotons was determined.