Observation of ballistic rotors in liquid 4He

The excitations produced when a pulsed 4He atomic beam strikes the surface of liquid 4He held near T approximately OK have been measured as a function of direction in the liquid and time elapsed since the beam was generated. Comparison of the data with a theory which assumes that a single atom is converted into a single excitation indicates that the observed signals are due to ballistic phonons and rotons, although the experimental velocity and angular distributions of the excitations are displaced from the theoretical ones. The direction of the displacements suggests that multiple ripplon production occurs as an atom approaches the liquid surface, as predicted by Echenique and Pendry