Creation of High-Energy Phonons from Low-Energy Phonons in Liquid Helium

We present a theory for the remarkable observation that high-energy phonons are created from a moving cloud of low-energy phonons in liquid ⁴He. This phenomenon depends on the loss of high-energy $(h)$ phonons from a moving phonon cloud. The h phonons are created from the low-energy $(l)$ phonons by four-phonon processes and escape from the back of the l-phonon cloud because they have a lower velocity than the l phonons. This theory accounts for the high efficiency of the conversion process; a major part of the energy of the low-energy phonons can be converted to high-energy phonons in a time of order 10μs.