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 (Yulmetyev et al 2001 Phys. Rev. E 64 057101; 2002 JETP Lett. 76 147) is used to analyse the short-wave collective excitation 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 14.6 nm −1 , where pronounced collective excitations exist. The frequency dependence of the non-Markovity parameter i ( Q , ω) ( i = 1, 2, 3, 4) allows us to reveal the alternation of the amplification and the decay of non-Markovity effects for the observed region of wavevector Q .