Scattering of high-energy phonons at irregular surfaces without and with liquid He

The scattering of high-energy phonons at irregular surfaces without and with liquid He is discussed theoretically by taking account of all eigenmodes of phonons in a solid. It is found that the diffuse component for irregular surfaces without liquid He increases with increasing frequency proportional to ${\nu }^4$. The mode-converted surface phonons at irregular surfaces play a role in the diffuse scattering observed in the time-of-flight phonon-reflection experiments. In this connection, the causes of the anomalous behavior of high-energy phonons scattered at the solid–liquid-He interface are discussed. It is pointed out that the mode-converted surface phonons are of considerable importance for the effective phonon transmission across the solid–liquid-He interface. This is concluded from the analysis of the scattering processes of mode-converted surface phonons. The shape of phonon-reflection signals is obtained as a function of time.