Excitations produced in liquidHe4by aHe4atomic beam

Abstract

Phonons and rotons produced at the surface of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ by a ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atomic beam are found to propagate ballistically in the liquid with an angular distribution which is displaced from that expected if each atom were converted into a single excitation. The data are qualitatively consistent with the idea that incident atoms lose some of their energy in the production of low-energy ripplons as they approach the surface, as suggested by Echenique and Pendry.