R. Senesi

Measurement of momentum distribution of light atoms and molecules in condensed matter systems using inelastic neutron scattering

Studies of single-particle momentum distributions in light atoms and molecules are reviewed with specific emphasis on experimental measurements using the deep inelastic neutron scattering technique at eV energies. The technique has undergone a remarkable development since the mid-1980s, when intense fluxes of epithermal neutrons were made available from pulsed neutron sources. These types of measurements provide a probe of the short-time dynamics of the recoiling atoms or molecules as well as information on the local structure of the materials. The paper introduces both the theoretical framework for the interpretation of deep inelastic neutron scattering experiments and thoroughly illustrates the physical principles underlying the impulse approximation from light atoms and molecules. The most relevant experimental studies performed on a variety of condensed matter systems in the last 20 years are reviewed. The experimental technique is critically presented in the context of a full list of published work. It is shown how, in some cases, these measurements can be used to extract directly the effective Born–Oppenheimer potential. A summary of the progress made to date in instrument development is also provided. Current data analysis and the interpretation of the results for a variety of physical systems is chosen to illustrate the scope and power of the method. The review ends with a brief consideration of likely developments in the foreseeable future. Particular discussion is given to the use of the VESUVIO spectrometer at ISIS. Contents PAGE 1. Introduction 378 2. Theoretical basis of measurements 381   2.1. The impulse approximation and the neutron Compton profile 381   2.2. Validity of the impulse approximation and corrections at finite q 384   2.3. Properties of the dynamic structure factor SIA (q ω) in the impulse approximation 389   2.4. Extracting the atomic momentum distribution from the neutron Compton profile 390   2.5. Determination of effective Born–Oppenheimer potentials 394 3. Theoretical momentum distributions of atoms 395   3.1. Maxwellian regime and atoms in harmonic solids 395   3.2. Quantum systems and weakly quantum systems 397   3.3. Fermi and Bose systems 398   3.4. Molecular systems 399   3.5. Polyatomic molecules 401 4. An exact calculation: liquid H2 and D2 403 5. Experimental technique 408   5.1. Direct and inverse geometry spectrometers for DINS measurements 408   5.2. The VESUVIO spectrometer 409   5.3. The resonance filter configuration 411   5.4. The resonance detector configuration 415   5.5. Extracting the neutron Compton profile from observations 417 6. Review of existing measurements 420   6.1. Liquid and solid 4He 420   6.2. Liquid and solid 3He 428   6.3. Liquid 4He–3He mixtures 431   6.4. Liquid para-H2, ortho-D2 and N2 436   6.5. Hydrogen sulphide 444   6.6. Water and ice 447   6.7. Single crystal measurements: the example of KDP (KH2PO4) 453 7. Conclusions and perspectives 457   7.1. Applications in physics 459   7.2. Applications in chemistry 460   7.3. Applications in biology 460   7.4. Technological applications 461 Acknowledgments 461 Appendix A: The intensity deficit problem 462 References 463

VESUVIO: a novel instrument for performing spectroscopic studies in condensed matter with eV neutrons at the ISIS facility

The VESUVIO project aims to provide unique prototype instrumentation at the ISIS-pulsed neutron source and to establish a routine experimental and theoretical program in neutron scattering spectroscopy at eV energies. This instrumentation will be specifically designed for high momentum, , and energy transfer inelastic neutron scattering studies of microscopic dynamical processes in materials and will represent a unique facility for EU researchers. It will allow to derive single-particle kinetic energies and single-particle momentum distributions, n(p), providing additional and/or complementary information to other neutron inelastic spectroscopic techniques.

Rigid-cage effects on the optical properties of the dye 3,3′-diethyloxadicarbocyanine incorporated in silica-gel glasses

We investigated the optical properties of the saturable absorber 3,3′-diethyloxadicarbocyanine iodide incorporated in a matrix of silica-gel glass by the sol-gel technique. Absorption, emission, and fluorescence lifetime were studied as functions of the densification procedures. With respect to the liquid solutions, we observed a strong quenching of the P-isomer luminescence and an increase of the N-isomer fluorescence lifetime. These effects are ascribed to a restriction on the molecular photoisomerization rearrangement caused by the rigidity of the silica cage. Moreover, entrapping reduces intermolecular concentration effects. Finally, different from other glass-embedded dyes, both the absorption and the emission are blue-shifted by about 20 nm.

Membrane thickness and the mechanism of action of the short peptaibol trichogin GA IV

Trichogin GA IV (GAIV) is an antimicrobial peptide of the peptaibol family, like the extensively studied alamethicin (Alm). GAIV acts by perturbing membrane permeability. Previous data have shown that pore formation is related to GAIV aggregation and insertion in the hydrophobic core of the membrane. This behavior is similar to that of Alm and in agreement with a barrel-stave mechanism, in which transmembrane oriented peptides aggregate to form a channel. However, while the 19-amino acid long Alm has a length comparable to the membrane thickness, GAIV comprises only 10 amino acids, and its helix is about half the normal bilayer thickness. Here, we report the results of neutron reflectivity measurements, showing that GAIV inserts in the hydrophobic region of the membrane, causing a significant thinning of the bilayer. Molecular dynamics simulations of GAIV/membrane systems were also performed. For these studies we developed a novel approach for constructing the initial configuration, by embedding the short peptide in the hydrophobic core of the bilayer. These calculations indicated that in the transmembrane orientation GAIV interacts strongly with the polar phospholipid headgroups, drawing them towards its N- and C-termini, inducing membrane thinning and becoming able to span the bilayer. Finally, vesicle leakage experiments demonstrated that GAIV activity is significantly higher with thinner membranes, becoming similar to that of Alm when the bilayer thickness is comparable to its size. Overall, these data indicate that a barrel-stave mechanism of pore formation might be possible for GAIV and for similarly short peptaibols despite their relatively small size.

YAP scintillators for resonant detection of epithermal neutrons at pulsed neutron sources

Recent studies indicate the resonance detector (RD) technique as an interesting approach for neutron spectroscopy in the electron volt energy region. This work summarizes the results of a series of experiments where RD consisting of YAlO3 (YAP) scintillators were used to detect scattered neutrons with energy in the range 1–200 eV. The response of YAP scintillators to radiative capture γ emission from a 238U analyzer foil was characterized in a series of experiments performed on the VESUVIO spectrometer at the ISIS pulsed neutron source. In these experiments a biparametric data acquisition allowed the simultaneous measurements of both neutron time-of-flight and γ pulse height (energy) spectra. The analysis of the γ pulse height and neutron time of flight spectra permitted to identify and distinguish the signal and background components. These measurements showed that a significant improvement in the signal-to-background ratio can be achieved by setting a lower level discrimination on the pulse height at ab...

Direct Measurement of Competing Quantum Effects on the Kinetic Energy of Heavy Water upon Melting

Even at room temperature, quantum mechanics plays a major role in determining the quantitative behavior of light nuclei, changing significantly the values of physical properties such as the heat capacity. However, other observables appear to be only weakly affected by nuclear quantum effects (NQEs); for instance, the melting temperatures of light and heavy water differ by less than 4 K. Recent theoretical work has attributed this to a competition between intra- and intermolecular NQEs, which can be separated by computing the anisotropy of the quantum kinetic energy tensor. The principal values of this tensor change in opposite directions when ice melts, leading to a very small net quantum mechanical effect on the melting point. This Letter presents thefirst direct experimental observation of this phenomenon, achieved by measuring the deuterium momentum distributionsn(p) in heavy water and ice using deep inelastic neutron scattering (DINS) and resolving their anisotropy. Results from the experiments, supplemented by a theoretical analysis, show that the anisotropy of the quantum kinetic energy tensor can also be captured for heavier atoms such as oxygen. (The iceberg image in the Table of Contents and Abstract graphics was used with permission of the NOAAs National Ocean Service, 2012 (http://commons.wikimedia.org/wiki/ File:Iceberg_-_NOAA.jpg).)

A resonant detector for high-energy inelastic neutron scattering experiments

Results on the application of the resonant detector (RD) for epithermal neutron scattering in an unexplored kinematical region are presented. The RD is based on resonance radiative neutron capture for energy analysis of the scattered neutrons in an inverse geometry time of flight spectrometer. Application of the RD to detection of epithermal neutrons at very low scattering angles allows access to an unexplored scattering kinematical region, the High-energy Inelastic Neutron Scattering (HINS) region, of low wave vector (3A−1<q<10A−1) and high energy transfers (0.1eV<ω<10eV). Results of HINS measurements from polycrystalline ice are presented.

Single particle dynamics in fluid and solid hydrogen sulphide: An inelastic neutron scattering study

Inelastic neutron scattering experiments were performed at intermediate and high momentum transfer, up to 88.2 A−1, to study the temperature dependence of single hydrogen mean kinetic energy in polycrystalline and liquid hydrogen sulphide (H2S), in the temperature range 16–206 K. Values of the hydrogen mean kinetic energy were extracted, within the impulse approximation, by fitting to the high momentum transfer data a model response function, obtained from a momentum distribution which is the orientational average of a multivariate Gaussian function. The extracted kinetic energies are compared with a harmonic model for the vibrational and roto-translational dynamics. The model makes use of the hydrogen-projected density of states worked out from intermediate momentum transfer data, as well as of optical frequencies determined from Raman and infrared (IR) spectroscopy. A fairly good agreement is obtained in the whole temperature range, while noticeably lower values for the kinetic energy are found if a sin...

DINS measurements on VESUVIO in the Resonance Detector configuration: proton mean kinetic energy in water

Deep Inelastic Neutron Scattering (DINS) measurements have been performed on a liquid water sample at two different temperatures and pressures. The experiments were carried out using the VESUVIO spectrometer at the ISIS spallation neutron source. This experiment represents the first DINS measurement from water using the Resonance Detector configuration, employing yttrium-aluminum-perovskite scintillator and a 238U analyzer foil. The maximum energy of the scattered neutrons was about 70 eV, allowing to access an extended kinematic space with energy and wave vector transfers at the proton recoil peak in the range 1 eV ≤ ω ≤ 20 eV and 25 A−1 ≤ q ≤ 90 A−1, respectively. Comparison with DINS measurements on water performed in the standard Resonance Filter configuration indicates the potential advantages offered by the use of Resonance Detector approach for DINS measurements at forward scattering angles.

Dye-doped zirconia-based Ormosil planar waveguides: optical properties and surface morphology

Abstract Zirconia-based Ormosil planar waveguides with different ZrO 2 contents were produced by a low temperature sol–gel process and subsequent spin coating on microscope glass slides. Propagation losses as low as 0.5 dB/cm or less were found at 488 nm for films with low ZrO 2 content; in contrast, worse waveguide quality was observed with increasing ZrO 2 doping level. A morphological investigation by atomic force microscopy confirmed such result, giving evidence of an increase of surface roughness with ZrO 2 percentage. The feasibility of organic dye doping of the films for active waveguides was also demonstrated by incorporation of DODCI saturable absorber: fluorescence spectra were registered both in bulk and guided-wave configuration.