Inelastic X-ray scattering study of the collective dynamics in simple liquid metals T. Scopigno a, * , U. Balucani b , G. Ruocco a ,F.Sette c a Dipartimento di Fisica and INFM, Universit  a di Roma ‘La Sapienza’, I-00185 Roma, Italy b Istituto di Elettronica Quantistica CNR, I-50127 Firenze, Italy c European Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble cedex, France Abstract The collective dynamics of the so-called ‘simple’ liquids (in particular, liquid metals) are a good paradigm to il- lustrate the capabilities of inelastic X-ray scattering (IXS). In particular, it is shown how the present accuracy of the data can be exploited to provide not only a precise determination of the main spectral features (dispersion relations), but also a clue for the physical assessment of the microscopic relaxation mechanisms underlying the dynamics of the liquid state. Starting from new IXS data on liquid sodium as an example, we apply a generalized hydrodynamic framework in order to describe the details of the experimental lineshapes. A comparison with recent data on liquid lithium and aluminium is also reported: although quantitative differences indicate that the principle of corresponding states seems not to be applicable, in all these systems a simple viscoelastic approach turns out to be inadequate to reproduce the experimental data. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.10.Eq; 61.25.Mv; 63.50.þx 1. Introduction As is well known, liquid metals exhibit re- markably pronounced inelastic peaks in their density fluctuations spectra up to very high Q values (well above the hydrodynamic limit), a feature that makes them ideal candidates to test different models for collective properties at finite wavevectors. For this main reason, in the last 30 years,manyexperimentalinvestigationshavebeen performed on these systems by means of inelastic neutronscattering(INS)technique,whichwas,up to a few years ago, the only tool adequate to in- vestigate the dynamics of condensed matter in the mesoscopic wavevector region [1–6]. Exploiting energy conservation arguments, indeed, one infers thatinthescatteredspectrumaninelasticpeakdue to one of these excitations is well separated from the location of the incident neutron beam, and consequentlyitsdetectiondoesnotaprioriinvolve dramatic resolution problems. This is definitely notthecaseforX-rays(IXS):heretheenergiesof theincidentphotonsaretypicallyoforderkeV,i.e. 10 6 –10 7 times higher than those of the excitations Journal of Non-Crystalline Solids 312–314 (2002) 121–127 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +33-4 76 88 26 03; fax: +33-4 76882160. E-mail address: tullio.scopigno@phys.uniroma1.it (T.Scop- igno). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(02)01660-5