Alessandro Cunsolo INFM Operative Group in Grenoble and CRS-Soft, c/o Institut Laue-Langevin, Grenoble, France On the new opportunities opened by the development of next generation IXS spectrometers

-The construction of IXS spectrometers with.1 meV resolution would allow to partially bridge the dynamic gap existing with low Q spectroscopies…… No man’s land In principle the access to lower Q = 4  sin(  /2) can be achieved, but…… Potassium The “bottle-neck” is the instrumental resolution

1) relaxation processes in the dynamics of disordered systems Various interesting subjects can be investigated inside such “dynamic gap”

The propagation of a density fluctuation perturbs the local equilibrium of the fluid.The propagation of a density fluctuation perturbs the local equilibrium of the fluid. Such equilibrium is then restored through energy rearrangements affecting the density wave towards some internal degree of freedom (relaxation processes).Such equilibrium is then restored through energy rearrangements affecting the density wave towards some internal degree of freedom (relaxation processes). The propagation of the density wave depends on how its period (T) compares with the relaxation time-scale (  ).The propagation of the density wave depends on how its period (T) compares with the relaxation time-scale (  ).

Q=Q * ~1/d * Visco-elastic crossover  s (Q) q cSQcSQ viscous regime c∞Qc∞Q elastic regime  s (Q*)=1/  The dispersion curve

Resolution limited range Dispersion curve of water: first evidence of a viscoelastic behavior ħsħs -ħs-ħs A typical DHO best fit lineshape An improved resolution would allow to study viscoelastic effects also in the overcooled phase  s,1 ~1/   q1q1  s,2 ~1/   q2q2  s,3 ~1/   q1q1 1/  increases with increasing T

The viscoelastic behavior of the lineshape Possible effects of an improved instrumental resolution   Convolution for an hypothetical.1 meV (lorentzian) resolution function   (meV) Raw spectraHypothetical higher resolution spectra Non convoluted (model) line-shapes   (meV) Q=2 nm -1 S(Q,  )/S(Q) (meV -1 ) Elastic regime Visco-elastic regime Viscous regime

Evidence of an additional low frequency mode in water

C T (Q,  C L (Q,  ) IXS results versus MD simulations Crossover from viscoelasticity to transverse dynamics: the experimental observation of the gradual L-T mode-splitting would require a much better resolution …… M. Sampoli et al. Phys. Rev. Lett. 79, 1678 (1997) C L (Q,E)= (E/q) 2 S (Q,E)

Experimental evidence of a modes bifurcation in the dynamic of mixtures He Ne He 77 -Ne 23 at T=60 K, c mix ~1000 m/s dynamic decoupling of the two components Bafile et al., PRL , (2002) at higher Q’s, the heavy (“slow “) component cannot support the propagation of the fast acoustic mode, which therefore propagates only through the light component. In the hydrodynamic limit the two components of a mixture “cooperate” in the sound propagation → only one acoustic mode.

Dynamics of materials under confined geometries Models of critical dynamics debated, with nearly no existing experimental verification Schoen and Porcheron, EPJ 2003 Simulation results for a confined simple fluid Collective modes in water confined in Nafion Sazcchetti et al. Submutted to PRL The origin of this mode seems a bit controversial Normal modes of confined water droplet? Transverse mode in confined water?

Life Science Examples Collective dynamics in lipid membranes  Collective modes affect significantly the physical properties of phospholipids membranes  Correlated molecular motions of lipid acyl chains and the density fluctuations in the plane of bilayer are important for the transport of small molecules through the bilayer Gel-fluid phase transition M.C. Rheinstadter et al., PRL 93, 2004

Conclusions a) the study of relaxation processes b) the observation of a transverse dynamic in the THz response of fluids e) the study of nearly-critical systems d) the dynamical characterization of liquids under geometrical confinement c) the evidence of a mode splitting in the dynamics of liquid mixture f) the study of the dynamics of biological systems 1) The development of an IXS spectrometer with.1 meV would reduce the gap between low and high frequency spectroscopies 2) Various scientific topics would benefit from such upgrade. Some mentioned examples: