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Alessandro Cunsolo

On the new opportunities opened by the development of next generation IXS spectrometers. Alessandro Cunsolo INFM Operative Group in Grenoble and CRS-Soft, c/o Institut Laue-Langevin, Grenoble, France. Potassium. No man’s land. The “bottle-neck” is the instrumental resolution.

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Alessandro Cunsolo

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

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

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

  4. 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). • The propagation of the density wave depends on how its period (T) compares with the relaxation time-scale (t).

  5. The dispersion curve c∞Q Visco-elastic crossover elastic regime Q=Q*~1/d* ws(Q) ws(Q*)=1/t cSQ viscous regime q

  6. A typical DHO best fit lineshape ws,1~1/t1 ws,2~1/t2 ħws -ħws ws,3~1/t3 Resolution limited range q1 q2 q1 An improved resolution would allow to study viscoelastic effects also in the overcooled phase Dispersion curve of water: first evidence of a viscoelastic behavior 1/t increases with increasing T

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

  8. Evidence of an additional low frequency mode in water

  9. IXS results versus MD simulations CT(Q,E) CL (Q,E)= (E/q)2S (Q,E) CL(Q,E) 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)

  10. He77-Ne23atT=60 K, cmix~1000 m/s dynamic decoupling of the two components Bafile et al., PRL 88 085502,(2002) He Ne 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. Experimental evidence of a modes bifurcation in the dynamic of mixtures

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

  12. 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

  13. Conclusions 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: a) the study of relaxation processes b) the observation of a transverse dynamic in the THz response of fluids c) the evidence of a mode splitting in the dynamics of liquid mixture d) the dynamical characterization of liquids under geometrical confinement e) the study of nearly-critical systems f) the study of the dynamics of biological systems

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