Roton propagation and phonon-roton scattering in He II

The propagation characteristics of high-energy excitations in He II are studied as a function of pressure, temperature, and frequency by means of a superconducting Sn fluorescent generator and a Sn tunnel detector and time-of-flight techniques. At saturated vapor pressure and low temperatures ($T\approx 0.1$ K) and with the detector energy gap possessing its full value (∼ 14 K), a single well-defined pulse is observed to arrive at a time corresponding to that expected for the first echo (three traverses of the cell) of a low-energy phonon. This "echo pulse" disappears as the pressure is raised to 10 ± 2 bar and one observes a driven roton second sound at higher pressures. The echo pulse is interpreted as arising from a collinear interaction near the detector of a low-energy phonon with "fast" rotons (group velocity approximately equals the sound velocity $c_0$) by means of a three-particle interaction (phonon + roton → roton) first suggested by Pitaevskii. The data indicate that this Pitaevskii process turns off at high pressures and only the four-particle process (phonon + roton → phonon + roton) remains. With increasing magnetic field on the detector, ballistic phonons are observed as expected. The scattering of these ballistic phonons and a cloud of rotons are probed by means of a novel double-pulse technique and provides strong evidence for the existence of this collinear three-particle interaction at low pressures.