Soft Report - Dipartimento di Fisica - Sapienza

Neutron inelastic scattering on liquid CD 4 : a fruitful probe ofthe dynamics in simple molecular liquidsThe limited experimental work devoted in theseyears to the determination of the dynamic propertiesof simple molecular liquids has nonetheless producedextremely interesting results concerning thedispersion law of collective excitations and the quitedifferent role of damping with changing the system[1]. Moreover, comparison with monatomic fluids canfurther enrich the accessible information on thegeneral dynamic behavior of these liquids. Ourrecent neutron investigation on liquiddeuteromethane (CD 4) allowed, in particular, forstimulating steps forward in the individuation of thebasic phenomena ruling the actual extension of thepropagation Q-range of collective excitations indisparate mono- and polyatomic fluids [2].Remarkable differences between our results for CD 4and those for other molecular liquids as ammonia,carbon tetrachloride, and sulphur dioxide [1] werefound, pointing in particular at the strikingly differentvalues found for Q t*, i.e. the reduced wavevectortransfer threshold value at which modes wereobserved to cease propagation in these liquids(Q*=Q l, with l the mean free path). For instance,Q t* ~ 2 for the liquids of Ref. [1]. Differently, in CD 4Q t* ~ 6, as it can be deduced (with a mean free pathl ~ 0.4 nm for the CD 4 sample under consideration)from comparison of the Q-dependences of excitationfrequency and width (HWHM), both shown in Fig. 1.The results for CD 4, fairly supported also by asatisfactory agreement with MD simulations based onrealistic site-site interactions [2], show instead moremarked similarities with monatomic liquids as Ar andNe. Such resemblances are suggested both by thecomparisons wih Ar data shown in Fig. 1, and by thetypical Q t* values (of the order of 6) of liquid Ar andNe [3].This overall picture led us to individuate an empiricalrelation, approximately valid for all the liquidsmentioned here, between the Q t* values at whichmodes overdamping is observed to occur, and thebasic interaction and transport properties of thefluid. Such a relation is Q t* Γ* ≈ const., with Γ* thereduced sound damping coefficient defined byΓ* = (m/ε) 1/2 Γ / σ. Here ε and σ can be taken as theeffective Lennard-Jones parameters for the variousfluids, and Γ is defined, as usual, in terms of thermaldiffusivity, bulk and shear viscosities, and specificheats ratio [2,3]. For all the liquids we could findacceptable experimental dynamical data, we findQ t* Γ* ≈ 19 within 11%. Even smaller fluctuationsare obtained by omitting the bulk viscositycontribution, unavoidably deriving from approximateestimates. In such a case, thus referring only toshear viscosity experimental data, we findQ t* Γ shear* ≈ 15 within 7%.Thus, apparently contrasting dynamic behaviors turnall out to follow, as far as the propagation range ofexcitations is concerned, the above approximateempirical relation, which provides a first evidence ofthe important interplay between attractive forces anddamping mechanisms in determining the transitionto the non-propagating regime in a liquid. Such asemi-quantitative relation, if confirmed by more thanthe few test-cases available, can indeed provide areasonable tool to predict the range whereexcitations in a liquid are expected to propagate.Nonetheless, a unified picture of the dynamics ofmono- and polyatomic fluids is still far from hand,especially when the Q-dependence of the modeswidth is analysed in different systems.References[1] F. J. Bermejo et al., J. Chem. Phys. 95, 5387(1991); M. García-Hernández et al., J. Chem. Phys.96, 8477 (1992) ; F. Sette et al., Phys. Rev. Lett.84, 4136 (2000).[2] E. Guarini et al., Europhys. Lett. 72, 969 (2005).[3] A. A. van Well and L. A. de Graaf, Phys. Rev. A32, 2396 (1985).[4] U. Bafile et al., Phys. Rev. Lett. 65, 2394 (1990).Fig. 1 Q-dependence of the frequency (top) andHWHM (bottom) of collective modes in liquid CD 4:experimental (circles) and MD (stars) results. Fullsquares are Argon data read off from Fig. 12(a) ofref. [3]. In both frames, the dotted and dash-dottedcurves are the full hydrodynamic solutions [4]calculated, respectively, for CD 4 and Ar.Authors:E. Guarini (a), F. Barocchi (a), G. Venturi (a), U.Bafile (b), F. Formisano (c), M. Sampoli (d).(a) CNR-INFM, Dipartimento di Fisica Firenze, CRS-Soft; (b) CNR-ISC Firenze; (c) CNR-INFM, OGGGrenoble, CRS Soft; (d) Dipartimento di EnergeticaFirenze, CRS-Soft103SOFT Scientific Report 2004-06