Seminars Archive

Collective dynamics in disordered systems by inelastic photon scattering spectroscopy: results and perspectives.

Abstract
The development of synchrotron-based inelastic photon scattering techniques have provided powerful tools for investigating collective dynamics in disordered systems without the kinematic constraints typical of thermal neutron scattering. Examples of applications of inelastic x-ray (IXS) and ultraviolet (IUVS) scattering are discussed. Specifically, we report on: i) a comparative IXS study on four prototype systems (i.e. Neon, Nitrogen, Ammonia and Water) in their respective liquid and supercritical phase. The main findings are the observation of the occurrence of an adiabatic-to-isothermal transition of sound velocity on going from the liquid to the supercritical phase [1], and a change in the microscopic mechanism responsible for the structural relaxation [2]. ii) an IUVS study on water in the thermodynamic region where, according to diffraction data, a structural transition between two distinct arrangements of local structure is expected [3]. We observed that the density behavior of the structural relaxation time is driven by two competing effects, which we associated to the roles played by the activation energy and entropy on relaxation dynamics [4]. Finally, we present the project of a free electron laser based time resolved instrument (TIMER) able to extend the standard transient grating technique in the VUV/soft x-ray spectral region [5]. This instrument will allow probing the kinematic region (Q=0.02÷1.2 nm-1 and E=0.1÷10000 μeV) that, at present, cannot be explored by available instruments. This (Q,E)-range would be of the greatest relevance for the understanding of the physics of disordered systems, and it would also provide a sensitive probing of heat transport and electron correlations in nanostructured materials. Within the TIMER project we have also installed a research laboratory equipped with conventional laser sources. This facility is dedicated to carry out time resolved transient grating experiments and feasibility tests related to the development of the Elastic and Inelastic Scattering beamline. [1] F. Bencivenga et al., Europhys. Lett. 75, 70 (2006). [2] F. Bencivenga et al., Phys. Rev. Lett. 98, 085501 (2007). [3] M.-C. Bellissent-Funel and L. Bosio, J. Chem. Phys. 102, 3727 (1995). [4] F. Bencivenga et al., J. Chem. Phys. 131, 194502 (2009). [5] F. Bencivenga and C. Masciovecchio, Nucl. Instrum. Meth. A 606, 785 (2009).