Phonon Interference in Thin Films of Liquid Helium

Acoustic wave interference has been observed at frequencies between 18 and 58 GHz in thin films of liquid helium absorbed on surfaces of Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$. The thickness of the films, ranging from 10 to 200 Å, could be measured to within a few percent in most cases. The dependence of the van der Waals potential on the film thickness is also discussed.