Phonon-like and single-particle dynamics in liquid lithium

The dynamic structure factor, S ( Q , E ), of liquid lithium ( T = 475 K) has been remeasured by inelastic X-ray scattering (IXS) in an extended momentum transfer region ( Q = 1.4–110 nm −1 ) and with improved energy resolution (down 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 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.