Phonon-like and single-particle dynamics in liquid lithium

The dynamic structure factor, S(Q, E), of liquid lithium (T = 415 K) has be en remeasured by inelastic X-ray scattering (IXS) in an extended momentum t ransfer region (Q = 1.4-110 nm(-1)) and with improved energy resolution (do wn to 1.5 meV). These new data on such prototypical simple liquid allow to observe the full-evolution with Q from a phonon-like collective mode toward s 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 in termediate 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 rea ched.