Molecular motions in liquid and solid hydrogen and deuterium

Egelstaff, P. A.; Haywood, B. C.; Webb, F. J.

doi:10.1088/0370-1328/90/3/314

Cited by 46 publications

(11 citation statements)

References 20 publications

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“…Our results expand upon previous measurements of the dynamics of water at ambient temperature obtained by neutron scattering and NMR. Previous quasi-elastic neutron scattering (QENS) measurements considering water diffusion used a random-jump diffusion model, which applies well to the small q values used in this experiment, but also to larger q (Teixeira et al, 1985;Cavatorta et al, 1994;Egelstaff et al, 1967). A diffusion constant of 2.36 Â 10 À9 m 2 s À1 was found (Cavatorta et al, 1994).…”

Section: Discussionsupporting

confidence: 58%

Time-of-flight modulated intensity small-angle neutron scattering measurement of the self-diffusion constant of water

Kuhn¹,

Geerits²,

Plomp³

et al. 2021

J Appl Cryst

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The modulated intensity by zero effort small-angle neutron scattering (MI-SANS) technique is used to measure scattering with a high energy resolution on samples normally ill-suited for neutron resonance spin echo. The self-diffusion constant of water is measured over a q–t range of 0.01–0.2 Å−1 and 70–500 ps. In addition to demonstrating the methodology of using time-of-flight MI-SANS instruments to observe diffusion in liquids, the results support previous measurements on water performed with different methods. This polarized neutron technique simultaneously measures the intermediate scattering function for a wide range of time and length scales. Two radio frequency flippers were used in a spin-echo setup with a 100 kHz frequency difference in order to create a high-resolution time measurement. The results are compared with self-diffusion measurements made by other techniques and the general applicability of MI-SANS at a pulsed source is assessed.

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Section: Discussionsupporting

confidence: 58%

Time-of-flight modulated intensity small-angle neutron scattering measurement of the self-diffusion constant of water

Kuhn¹,

Geerits²,

Plomp³

et al. 2021

J Appl Cryst

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“…The deuterium's rotational state is described as a nearly free quantum-mechanical rotator by a Gaussian shaped scattering law Srot with a half width at half maximum (HWHM) of 0.45 meV for the J=1→0 relaxation 28 . Since the rotational relaxation J=1→0 upon impact of a neutron is an incoherent scattering process 29 , we again use the incoherent Lovesey model for the translational part of the scattering law.…”

Section: Rotational Statesmentioning

confidence: 99%

Scattering cross sections of liquid deuterium for ultracold neutrons: Experimental results and a calculation model

et al. 2015

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We present scattering cross sections σscatt of ultracold neutrons (UCN) in liquid deuterium at T = 20.6 K, as recently measured by means of a transmission experiment. The indispensable thorough raw data treatment procedure is explained. A calculation model for coherent and incoherent scattering in liquid deuterium in the hydrodynamic limit based on appropriate physical concepts is provided and shown to fit the data well. The applicability of the incoherent approximation for UCN scattering in liquid deuterium was tested and found to deliver acceptable results.

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“…[15]). In some cases, TCS measurements have been fairly reproduced by considering the effect of intermolecular vibrations on the VACF spectrum and by hypothesizing the presence of huge molecular clusters diffusing in liquid hydrogen, leading to a ∼ 35-40 times augmented molecular mass 16,17,19,29 to be used in the diffusive part (ES) of the single-molecule dynamics. However, such a high mass value, usually explained in terms of translational hindering and cluster formation, is not easily justified, and might reveal instead some inadequacy of the ES lines-hape.…”

Section: The Ddcs Of Hmentioning

confidence: 99%

“…However, the above mentioned quantum effects were mostly neglected, or accounted for only approximately, or finally included in an effective way by adjustments to pioneering experimental data. [11][12][13]19,20 In order to investigate their role in the neutron response of these liquids, we focus here on the most significant case of H 2 at liquid temperatures (i.e., para-H 2 ), which combines a stronger quantum behavior with a full predominance (in most kinematic conditions) of the self component, the latter being quite reliably accessed by quantum simulation methods (see Sect. III) and sensitively probed by neutrons in a unique way.…”

Section: Introductionmentioning

confidence: 99%

Velocity autocorrelation in liquid parahydrogen by quantum simulations for direct parameter-free computations of neutron cross sections

et al. 2015

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Accurate knowledge of the single-molecule (self) translational dynamics of liquid para-H 2 is an essential requirement for the calculation of the neutron scattering properties of this important quantum liquid. We show that, by using Centroid Molecular Dynamics (CMD) quantum simulations of the velocity autocorrelation function, calculations of the total neutron cross section (TCS) remarkably agree with experimental data at the thermal and epithermal incident neutron energies where para-H 2 dynamics is actually dominated by the self contributions. This result shows that a proper account of the quantum nature of the fluid, as provided by CMD, is a necessary and very effective condition to obtain the correct absolute-scale cross section values directly from firstprinciple computations of the double differential cross section, and without the need of introducing any empirically adjusted quantity. At subthermal incident energies, appropriate modeling of the para-H 2 intermolecular (distinct) dynamics also becomes crucial, but quantum simulations are not yet able to cope with it. Existing simple models which account for the distinct part provide an appropriate correction of self-only calculations and bring the computed results in reasonable accord with TCS experimental data available until very recently. However, if just published cross section measurements in the cold range are considered, the agreement turns out to be by far superior and very satisfactory. The possible origin of slight residual differences will be commented and suggest further computational and experimental efforts. Nonetheless, the ability to reproduce the total cross section in the wide range between 1 and 900 meV represents an encouraging and important validation step of the CMD method and of the present simple algorithm.

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Molecular motions in liquid and solid hydrogen and deuterium

Cited by 46 publications

References 20 publications

Time-of-flight modulated intensity small-angle neutron scattering measurement of the self-diffusion constant of water

Time-of-flight modulated intensity small-angle neutron scattering measurement of the self-diffusion constant of water

Scattering cross sections of liquid deuterium for ultracold neutrons: Experimental results and a calculation model

Velocity autocorrelation in liquid parahydrogen by quantum simulations for direct parameter-free computations of neutron cross sections

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