Three-phonon interactions and initial stage of phonon pulse evolution in He=II

An expression for the characteristic rate of three-phonon processes in superfluid (4)He, which is valid in the entire range of phonon energies where three-phonon processes are allowed is derived proceeding from the hydrodynamic Landau Hamiltonian. Possible limiting cases are analyzed and compared with the results of previous investigations. It is found that three-phonon processes completely govern the initial relaxation of a phonon pulse injected into He II by a heated solid. As a result, the equilibrium form of phonon distribution is established in the anomalous region of phonon dispersion over a time interval of the order of 10(-10) s. (C) 1999 American Institute of Physics. [S1063-777X(99)00307-2].