Detection of Third Sound in Liquid Helium Films

A third sound was detected in thin films of liquid helium II. It appears as an oscillation in the thickness of the film, and the predicted velocity is about 50 cm/sec. Velocity variations with film height were measured. Temperature effects were measured at up to 2 deg K. The velocity and amplitude of third-sound decrease with incrcasing temperatures is in qualitative agreement with theory. Since the velocity is comparable with the film transfer critical velocity, it is suggested that the sudden onset of dissipation at the critical velocity arises from third-sound quanta creation. (L.N.N.)