Abstract
The second peak of neutron spectra of liquid He II, which is observed over a wide range of wave vectors, 0.5\ensuremath{\Vert}k\ensuremath{\Vert}52.0 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$, is identified as the resonance of a roton-plus-maxon pair with total momentum k. With this identification and from the observed energy of the peak, one can determine the sign and the absolute value of the zeroth moment of the effective roton-maxon interaction. The effective interaction thus defined turns out to be of the same order of magnitude as the roton-roton interaction, so that it might play a significant role for thermostatic properties and transport phenomena of liquid He II near the \ensuremath{\lambda} point. The formal expressions for the one- and two-quasiparticle excitation terms of the dynamical structure function for a Bose-condensed system are derived, and simplified versions made available for a quantitative comparison with observed neutron spectra.
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