Saturation Effects in the Phonon Reflection at a Quartz-Liquid Helium Interface*

The reflection coefficient of longitudinal 25-GHz phonons at a quartz-liquid 4He interface was measured for various phonon intensities. For thoroughly cleaned quartz surfaces a reflection coefficient of I independent of the incident intensity was found. When the sample was covered with a thin hydrocarbon film, however, the reflection coefficient dropped by typically 5-10% when the phonon intensity was low. At high intensities this extra phonon loss disappeared, suggesting the saturation of the inelastic processes at the quartzhelium interface.