1,721,032 research outputs found
On the nitrogen-induced far-infrared absorption spectra
No full textThe rototranslational absorption spectrum of gaseous N2 is analyzed, considering quadrupolar and hexadecapolar induction mechanisms. The available experimental data are accounted for by using a line-shape analysis in which empirical profiles describe the single-line translational profiles. We thus derive the simple procedure that allows one to predict the N2 spectrum at any temperature. On the basis of the results obtained for the pure gas, we also propose a procedure to compute the far-infrared spectrum of the N2–Ar gaseous mixture. The good agreement between computed and experimental N2–Ar data indicates that it is possible to predict the far-infrared absorption induced by N2 on the isotropic polarizability of any interacting partner
Far infrared absorption of the gaseous CH4–Ar mixture
No full textWe present the first measurements of the rototranslational absorption spectrum of the CH4cm –Ar gaseous system in the far infrared region 40–600−1K at densities as low as possible; the binary absorption spectrum of the CH . Measurements were made at 296, 243, and 1954–Ar pair was then obtained. We performed a line-shape analysis of these spectra following the procedure previously introduced for pure methane. We find that the experimental data can be described at high frequencies by using ab initio values of the octopole and hexadecapole multipole moments and anisotropic overlap intensities very close to those used for pure methane. At low frequencies, the computed spectrum is significantly lower than the experimental data at any temperature. We thus include in our analysis a low-frequency spectral contribution associated with the isotropic overlap induction mechanism. Good fits of the experimental data are then obtain at each temperature over all the spectral range
Temperature dependence of the dynamics of superoxide dismutase by quasi-elastic neutron scattering
No full textThe atomic dynamics of the enzyme Cu, Zn superoxide dismutase has been investigated by means of quasi-elastic neutron scattering in the temperature range 4-320 K and in an extended momentum range up to 8.5 Angstrom(-1). Below 200 K the integrated elastic scattering intensity can be well described by a quasi-harmonic model, while above this temperature a pronounced decrease of the elastic intensity is observed, together with the onset of a small quasi-elastic component. This behaviour, which is similar to that already observed in other globular proteins, can be attributed to the onset of torsional degrees of freedom which give rise to transitions between slightly different conformational substates of the protein tertiary structure
Quantum effects in the single particle kinetic energy of high density fluid <sup>4</sup>He
No full textDeep inelastic neutron-scattering measurements have been performed in dense fluid 4He along three isochores at number densities 33, 38, and 45 nm-3 and temperatures ranging from 4.35 to 56 K. Data have been analyzed in the plane-wave impulse-approximation framework to obtain the root-mean-square values of the single-particle momentum along a given direction px. The density and temperature behavior of the derived mean kinetic energies is discussed in comparison with a simple harmonic model for the fluid and with quantum simulations performed using a path-integral Monte Carlo code. The comparison with the harmonic models sheds light on the density dependence of the zero-point kinetic energy of the particles and points out the inadequacy of a single-frequency Einstein-oscillator model in describing its temperature dependence. The quantum simulation results are in substantial agreement with the experimental data although systematic deviations are found as the density is increased
Enhancing the performances of a resonance detector spectrometer for deep inelastic neutron scattering measurements
Full text linkThe possibility is explored to sum up neutron Compton profiles at different scattering angles in deep
inelastic neutron scattering measurements within the Resonance Detector (RD) configuration to
enhance the statistics for a more reliable extraction of the momentum distribution of the constituents
in the target. The RD configuration allows to select the energy of the scattered neutrons up to several
tens of electron Volt, thus accessing energy and wave vector transfers well above 1 eV and 30 ˚A 1,
respectively. In the high-q/o regime, the final state effects could be considered as negligible, as shown
in a series of simulations using a Monte Carlo method with different inverse geometry instrument
setups. The simulations show that it could be possible to conceive an instrument set up where the RD
configuration allows the proper summation of several spectra at different scattering angles, providing a
good separation of the proton recoil signal from that of the heavier atoms, thus avoiding the cell
subtraction by fitting procedure
Exploring ultra-fast proton dynamics in water under a static electric field
No full textWe present an experimental investigation of the single-particle dynamics of hydrogen
in liquid water and ice subject to static electric field using deep inelastic neutron scattering. The
nuclear mean kinetic energy, EK, of hydrogen in liquid water at room temperature does not
show sensible changes when an electric field of magnitude 105 V/m is turned on. On the contrary,
the value of EK in ice at 263 K and subject to the same electric field is found to be substantially
lower than the reference value for ice Ih at the similar temperature of 271 K and without electric
field. This is true both if the electric field is kept on or not while the sample cools from 300 K to
263 K. Concurrent diffraction measurements performed on ice subjected to an electric field show
no sizeable structural changes with respect to the expected powder-averaged ice-Ih diffraction
pattern
Collision induced fundamental band of N2and N2-Ar mixtures
No full textThe roto-vibrational absorption band of N2 and N2-Ar mixtures at low density have been measured and the induced spectrum of the N2-Ar pairs has been obtained. The detailed analysis of this spectrum shows that the isotropic overlap contribution is negligible, while the quadrupolar one accounts for 80 per cent of the total intensity; the remaining 20 per cent of the intensity is attributed to anisotropic induction mechanisms with a shorter range than the quadrupolar one. In order to fit the experimental line-shape it has been necessary to take the dependence of the quadrupole transition moment on the rotational quantum numbers in account
Deep inelastic neutron scattering off D<sub>2</sub> and H<sub>2</sub> and momentum distributions of nuclei in diatomic molecules
Full text linkA quanto-mechanicla calculation for the description of neutron scattering at very high momentum transfer off both homonuclear and heteronuclear diatomic molecules
Structure of the Ne-Xe mixture near the 26-MPa demixing curve at T=275 K
No full textWe report neutron diffraction measurements for two Ne-Xe mixtures at T = 275 K and P =26 MPa, having a Xe molar fraction of 0.81 and 0.45, respectively. These thermodynamic points are chosen to be in the single phase region, close to the gas-gas demixing surface. Total structure factors and neutron weighted pair distribution functions have been extracted. Partial pair distribution functions gij(r) have been calculated by means of molecular dynamics simulation using Lennard-Jones interaction potentials and the Lorentz-Berthelot mixing rules, obtaining a good agreement with the experimental data. These partial pair distribution functions have been analyzed in terms of density expansion and also compared with those derived in a previous experiment on a He-Xe mixture. The main difference found between the structural properties of the two mixtures is that, in the Ne containing mixture, a definitely larger fraction of atoms of the lighter species occupies the Xe-Xe first neighbor shell. The model already used in interpreting the thermodynamic behavior of the He-Xe mixture in terms of microscopic properties suggests that this structural difference is related to the different behavior of the pressure-temperature projection of the critical line in the two mixtures
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