Quasi-elastic neutron scattering diffusion

The application of neutron spin-echo spectroscopy to the analysis of the slow dynamics of biomolecules is still in its infancy, but developing fast. The few published investigations either pertain to the diffusion of globular proteins in solution [332-334] or focus on the internal subnanosecond dynamics on the length scale, <10 A as measured on wet powders [335,336]. The latter regime overlaps with other quasi-elastic neutron scattering methods as backscattering and TOE spectrometry [337-339]. 

Estimates of the rotational diffusivity may be made from MD calculations by fitting an exponential function to Legendre polynomials that express the decorrelation of a unit vector that is fixed in the methane coordinate frame (11). The rotational diffusivity was found to increase with concentration (as a result of sorbate-sorbate collisions which act to decorrelate the molecular orientation). The values are of the same order as those for liquid methane and are 2 orders of magnitude larger than those found by Jobic et al. (73) from a quasi-elastic neutron scattering study of methane in NaZSM-5. 

A. Comparison of Self-Diffusion Coefficients Determined by Pulsed-Field Gradient NMR and Quasi-elastic Neutron Scattering... 

Previous studies about dynamics of water near interfaces by quasi-elastic neutron scattering involved measurements of the mobility of water on the surface of Nafion membranes [63,64], the diffusive motions and the density of states of water in silica gels [65], and the interfacial melting of ice in graphite and talc pow-... 

This is the consequence of the confined diffusion theory, which has been used to analyze the quasi-elastic neutron scattering data. This simple theory gives... 

T. Springer, Quasi-elastic Neutron Scattering for the Investigation of Diffusive Motions in Solids and Liquids, Vol. 64, Springer-Verlag, New York, 1972. 

Trantham, E. C., Rorschach, H. E., Clegg, J. S., Hazlewood, C. F., Nicklow, R. M., Wakabayashi, N. (1984). Diffusive properties of water in Anemia cysts as determined from quasi-elastic neutron scattering spectra. Biophys J. 45,927-938. 

Incoherent quasi-elastic neutron scattering from diffusing hydrogen atoms allows us to investigate the elementary jumps involved in the tracer diffusion process. 

Bar N-K, Ernst H, Jobic H and Karger J (1999), Combined quasi-elastic neutron scattering and NMR study of hydrogen diffusion in zeolites , Magn Reson Chem, 37, S79. 

My views (as experimentalist) are rather qualitative mostly based simply in many experimental results and in evaluations of the orders of magnitude of several physical properties. In the paper of 1985 (Teixeira, Bellissent-Funel, Chen, Dianoux) we deduced from the analysis of the quasi-elastic neutron scattering by pure water, that self diffusion of water molecules is mainly due to rotations of the molecule. In my view, at least at low temperatures, when, instantaneously, most of the pairs of molecules are bonded, the molecule can rotate around one bond if the other possible bonds are broken simultaneously. 

To correlate the adsorption and diffusion properties to the cation distribution in the structure of CaNaA zeolites, temperature programmed desorption, anomalous X-ray powder diffraction and quasi elastic neutron scattering experiments were performed. It is shown that water adsorption and diffusion behaviour differs with the calcium content. Fourier maps obtained from anomalous diffraction experiments near the Ca K-edge allow to identify the calcium crystallographic sites unambiguously. 

Similar to the PPG NMR method, neutron scattering techniques are successfully employed for the determination of diffusivities under equihbrium conditions. These techniques and their apphcation are discussed in Chapter 5 by H. Jobic. Particularly efficient is a novel combination of quasi-elastic neutron scattering (QENS) and a neutron spin-echo technique (NSE), which considerably expands the range of accessible diffusivities, viz. down to 10... 

Quasi-elastic neutron scattering (QENS) is related to stochastic particle motions. Because the displacements are random, the diffusive motion of particles in liquids caimot be quantized, and the energies are continuously distributed. Unlike the case of cooperative motions like phonons in solids or molecular vibrational excitations, in the dynamic scattering function S(Q,(o), there are no 5-functions at finite momentum and energy transfers. Instead, the dynamic scattering function is centered at zero-energy transfer with a characteristic quasielastic line width proportional to the diffusivity of the particles. 

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