Frequency analysis of phonons backscattered from interfaces between solids and helium

Monochromatic phonons were generated using superconducting tunneling junctions. These phonons were scattered at a solid surface. The backscattered phonons were detected by two different methods. A tunneling junction was used as a detector with a frequency threshold corresponding to its energy gap and a bolometer was used as a detector which is sensitive to all phonons regardless of frequency. When the scattering surface was exposed to helium gas or bulk liquid the two detector signals changed differently. This qualitative frequency analysis yields evidence for frequency conversion of phonons in the two atomic layers of helium at the surface.