Phonon-like and single particle dynamics in liquid lithium

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.