D. Colognesi

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

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...

High Resolution Raman and Neutron Investigation of Mg(BH4)2 in an Extensive Temperature Range

Raman spectra of Mg(BH(4))(2) have been measured in an extensive temperature range, from 15 to 473 K. Taking into account the high temperature conversion from the alpha to the beta phase, we have observed evident signatures of this phase transition and determined the Raman vibrational spectrum of each phase. The neutron scattering spectra of the beta phase sample were also recorded. The present experimental results have been compared to the density functional theory calculations available in the literature, and a substantial agreement has been found.

Neutron Scattering Measurements and Computation of the Quantum Dynamics of Hydrogen Molecules Trapped in the Small and Large Cages of Clathrate Hydrates

We report inelastic neutron scattering (INS) measurements on molecular hydrogen trapped in simple (D2O) and binary (D2O plus perdeuterated tetrahydrofuran) clathrate hydrates, performed at a low temperature using two different neutron spectrometers to probe both energy and momentum transfer. The INS spectra of binary clathrate samples exhibit a rich structure containing sharp bands arising from both the rotational transitions and the rattling modes of the guest H2 molecule. They agree well with the rigorous fully quantum simulations, which account for the subtle effects of the anisotropy, angular and radial, of the host cage on the H2 microscopic dynamics and the resulting spectra. The simple clathrate samples present a much greater challenge, due to the multiple H2 occupancy of the large cages, which makes the quantum calculations an extremely difficult task. In addition, we discuss in detail various physical aspects of the experimental and simulated INS spectra, such as their temperature dependence, the effects of the cage geometry, and the different features associated with the ortho-hydrogen and para-hydrogen species.

Vibrational spectra of augelites Al2(OH)3(XO4) (X=P, As, V)

Abstract The solid-state vibrational spectra (IR, Raman, inelastic neutron scattering (INS)) of synthetic augelites Al 2 (OH) 3 (XO 4 ) (X=P, As, V) (space group C 2/ m , Z =4) are discussed on the basis of a unit-cell group analysis. The spectral effects of deuteration, especially on the deformation modes involving the two structurally different OH-groups, and of substitution in the anionic constituents in the framework are dealt with.

Hydrogen dynamics in heavy alkali metal hydrides obtained through inelastic neutron scattering

Inelastic neutron scattering spectra from polycrystalline NaH, KH, RbH and CsH, measured at low temperature in the energy transfer range 3 meV<E< 500 meV, are reported. From the medium-energy regions, coinciding with the optical phonon bands, accurate hydrogen-projected densities of phonon states are extracted and compared to ab initio lattice dynamics results. The overall agreement is very good. Further lattice dynamics calculations, based on a pairwise Born–Mayer semi-empirical potential scheme, were also performed, providing only limited and qualitative agreement with the experimental data. In conclusion, incoherent inelastic neutron spectroscopy proves to be a stringent validation tool for lattice dynamics simulations of H-containing materials.

Vibrational Spectroscopy of Superconducting MgB2 by Neutron Inelastic Scattering

Neutron inelastic scattering measurements have been performed on superconducting MgB 2 above and below T c . The temperature dependence of the generalized phonon density-of-states showed clear anomalous behaviour near 24 meV in the acoustic phonon region, which may be interpreted as evidence of a substantial contribution to the total electron-phonon coupling strength deriving from these phonons. Weaker evidence for a corresponding response in the high-energy B bond stretching phonons was also encountered.

Experimental inelastic neutron scattering spectrum of hydrogen hexagonal clathrate-hydrate compared with rigorous quantum simulations

We have performed high-resolution inelastic neutron scattering (INS) measurements on binary hydrogen clathrate hydrates exhibiting the hexagonal structure (sH). Two samples, differing only in the ortho/para fraction of hydrogen, were prepared using heavy water and methyl tert-butyl ether as the promoter in its perdeuterated form. The INS spectrum of the translation-rotation (TR) excitations of the guest H2 molecule was obtained by subtracting the very weak signal due to the D2O lattice modes. By means of a subtraction procedure, it has been possible to obtain separately the spectra of caged p-H2 and o-H2. sH clathrates are comprised of three distinct types of cages, two of which, differing in shape and size, are each occupied by one H2 molecule only. Both contribute to the measured INS spectrum which is, therefore, rather complex and challenging to assign unambiguously. To assist with the interpretation, the INS spectra are calculated accurately utilizing the quantum methodology which incorporates the coupled five-dimensional TR energy levels and wave functions of the H2 molecule confined in each type of nanocage. The computed INS spectra are highly realistic and reflect the complexity of the coupled TR dynamics of the guest H2 in the anisotropic confining environment. The simulated INS spectra of p-H2 and o-H2 in the small and medium cages are compared with the experimental data, and are indispensable for their interpretation.

Deep inelastic neutron scattering from freely rotating molecules

The incoherent structure factor for deep inelastic neutron scattering from freely rotating molecules is determined analytically in the framework of the impulse approximation. The single nucleus momentum distributions of the molecules constituents are then derived and related to the normal mode decomposition of the internal vibrations. A practical example of deep inelastic neutron scattering response function and single-particle momentum distribution for a symmetric top molecule is provided in the case of a freely rotating ammonia molecule.

The HD molecule in small and medium cages of clathrate hydrates: Quantum dynamics studied by neutron scattering measurements and computation

We report inelastic neutron scattering (INS) measurements on molecular hydrogen deuteride (HD) trapped in binary cubic (sII) and hexagonal (sH) clathrate hydrates, performed at low temperature using two different neutron spectrometers in order to probe both energy and momentum transfer. The INS spectra of binary clathrate samples exhibit a rich structure containing sharp bands arising from both the rotational transitions and the rattling modes of the guest molecule. For the clathrates with sII structure, there is a very good agreement with the rigorous fully quantum simulations which account for the subtle effects of the anisotropy, angular and radial, of the host cage on the HD microscopic dynamics. The sH clathrate sample presents a much greater challenge, due to the uncertainties regarding the crystal structure, which is known only for similar crystals with different promoter, but nor for HD (or H2) plus methyl tert-butyl ether (MTBE-d12).