Phonons in liquid4He from a heated metal film. II. The angular distribution

For part I see ibid, vol.6, p.2813 (1994). We have measured the angular distribution of phonons emitted by an Au film heater in liquid 4 He. For long heater pulses (10 mu s), the distribution is almost Lambertian ( alpha cos theta ), but for short pulses (0.1 mu s), the distribution is very anisotropic. The HWHM Of the beam of high-frequency (h(cross) omega /k B >10 K) phonons is approximately 3.5 degrees , and the HWHM Of the low-frequency (h(cross) omega /k B <1 K) phonon beam is approximately 10.5 degrees . We suggest that the angular distribution of the low-frequency phonons is due initially to classical phonon transmission at the heater/ 4 He interface, but is then broadened by decays and interactions in the liquid. These interactions in the liquid create the high-frequency phonons. The high-frequency phonon beam is narrower than the low-frequency phonon beam due to momentum conservation in the four-phonon process (4PP) interactions that create them. Narrow beams of phonon emission from cleaved crystal surfaces have been observed before, but we believe that this is the first time that similarly narrow beams from evaporated metal surfaces have been observed.