  The dynamic structure factor, S(Q,E), of liquid lithium (T=475 K) has been
determined by inelastic x-ray scattering (IXS) in the momentum transfer region
(Q = 1.4-110 nm-1). These data allow to observe how, in a simple liquid, a
phonon-like collective mode evolves towards the single particle dynamics. As a
function of Q, one finds: i) at low Q's, a sound mode with a positive
dispersion of the sound velocity, ii) at intermediate Q's, excitations whose
energy oscillates similarly to phonons in the crystal Brillouin zones, and iii)
at high Q's, the S(Q,E) approaches a Gaussian shape, indicating that the single
particle dynamics has been reached.
