New evidence for the idea of timescale invariance of relaxation processes in simple liquids: the case of molten sodium

The idea of the timescale invariance of relaxation processes in liquids (Yulmetyevetal 2001Phys. Rev. E64 057101; 2002JETPLett.76 147) is used to analyse the short-wave collectiv ee xcitation in liquid sodium, as recently measured by means of very-high-energy-resolution inelastic x-ray scattering (Scopigno et al 2002 Phys. Rev. E 65 031205). The dynamic structure factor, S(Q ,ω ), calculated on the basis of this idea is in very good agreement with the experimental data in the wavevector range from 1.5 to 14. 6n m −1 ,w here pronounced collective excitations exist. The frequency dependence of the non∗