Search

Water continues to be the focus of a considerable number of experimental and theoretical investigations because of the important role it plays in many phenomena in fundamental and applied science, as well as its biological and technological importance. The ability of water to create a three-dimensional network of hydrogen bonds makes this molecule uniquely versatile. Atoms in water display characteristic mean kinetic energies well above the thermodynamic temperature of the system, as opposed to the celebrated case of a perfect gas of non-interacting particles. Nuclear quantum effects have a profound influence on the structure and dynamics of hydrogen-bonded systems, such as water. These effects are reflected in the underlying nuclear momentum distributions, experimentally accessible using electron-volt neutron spectroscopy at pulsed spallation neutron sources, with a technique known as deep inelastic neutron scattering. In this topical review, the technique is discussed along with the conceptual framework and several case studies that bring to the fore the role of nuclear quantum effects in describing water. These state-of-the-art measurements are also benchmarked against theoretical predictions using path-integral molecular dynamics. We close by discussing and highlighting current challenges and future opportunities in this thriving area of condensed-matter research.

Quantum effects on the water proton dynamics over the surface of a hydrated protein are measured by means of broadband dielectric spectroscopy and deep inelastic neutron scattering. Dielectric spectroscopy indicates a reduced energy barrier for a hydrogenated protein sample compared to a deuterated one, along with a large and temperature-dependent isotopic ratio, in good agreement with theoretical studies. Recent deep inelastic neutron scattering data have been reanalyzed, and now show that the momentum distribution of water protons reflects a characteristic delocalization at ambient temperatures. These experimental findings might have far-reaching implications for enzymatic catalysis involving proton transfer processes, as in the case of the lysozyme protein studied in this report.

The static structure factors S(Q) of liquid normal H2and D2have been measured in the wave-vector region 5 nm−1

The recent experimental determination of the 3He single-particle mean kinetic energies in solid hcp, bcc and dense liquid phases has allowed to perform an accurate test of the single-particle dynamical properties derived from the existing theoretical models. A Monte Carlo simulation of a Filter-Difference neutron spectrometer for Deep Inelastic Neutron Scattering with enhanced resolution performance is presented. It aims to study the feasibility of an accurate experimental determination of the 3He momentum distributions in pure liquid and in 3He-4He liquid mixtures.

AbstractWe have prepared and characterized PMMA films doped with three new Pt-containing acetylides for second-order optical nonlinearity. The noncentrosymmetric order of chromophores in the matrix was achieved through corona poling and monitored in situwith measurements of second-harmonic generation. The values of second-harmonic coefficient are of the same order as in DR1-doped films with the advantage of a lower absorption in the visible region. The data of the orientational decay confirm the relaxation dynamics of doped PMMA.

New perspectives for epithermal neutron spectroscopy are opened up by the recent development of new instrumentation for inverse geometry time of flight spectrometers at pulsed neutron sources. One example is the Very Low Angle Detector (VLAD) bank which will be installed as an upgrade of the VESUVIO neutron spectrometer, at the ISIS pulsed neutron source. VLAD is developed for detecting epithermal neutrons in the 1-100 eV energy range at very low scattering angles (l°-5°). VLAD will extend the kinematical region covered by today''s neutron scattering experiments to the region of low wave vector transfers (<10 Å- 1) and high energy transfers (>1 eV). Accessing such kinematical region will allow new experimental studies in condensed matter systems. The neutron detection is based on Resonance Detectors (RD), which consist of the combination of a resonance foil used as neutron-to-gamma converter and a photon detector. The results obtained with a prototype VLAD detector confirm the potential of this kind of experiments at scattering angles as low as 2°- 5°. GEANT4 simulations are used to address issues, such as detector cross talk, which arise with the construction of compact RD arrays. [Copyright &y& Elsevier]