  We obtain the memory kernel of the generalized Langevin equation, describing
a particle interacting with longitudinal phonons in a liquid. The kernel is
obtained analytically at T=0 Kelvin and numerically at T>0 Kelvin. We find that
it shows some non-trivial structural features like negative correlations for
some range of time separations. The system is shown to have three
characteristic time scales, that control the shape of the kernel, and the
transition between quadratic and linear behavior of the mean squared distance
(MSD). Although the derivation of the structure in the memory kernel is
obtained within a specific dynamical model, the phenomenon is shown to be quite
generic.
