Microscopic quasielastic neutron scattering

Diffusion measurements fall into two broad classes. Under macroscopic equilibrium, i.e. if the overall concentration within the sample remains constant, molecular diffusion can only be studied by following the diffusion path of the individual molecules ( microscopic measurement by quasielastic neutron scattering (QENS) [48,183,184], nuclear magnetic relaxation and line-shape analysis, PFG NMR) or by introducing differently labelled (but otherwise identical) molecules into the sample and monitoring their equilibration over the sample ( macroscopic measurements by tracer techniques) [185,186]. The process of molecular movement studied under such conditions is called self-diffusion. 

Kanaya, T., Tsukushi, T., Kaji, K., Bartos, J., and Kristiak, J., Microscopic basis of free-volume concept as studied by quasielastic neutron scattering and positron annihilation lifetime spectroscopy, Phys. Rev. E, 60,1906-1912 (1999). 

Microscopic, such as NMR spectroscopy, pulsed field gradient NMR (PFG-NMR), quasielastic neutron scattering (QENS), or isotope exchange technique... 

For the investigation of the spatial and temporal development of the microscopic hydrogen diffusion mechanism in metals, quasielastic neutron scattering (QNS) is a unique tool. It has been applied first to simple systems 1) 2)... 

Recent experimental advances reinforce the idea that the microscopic motions and the elastic properties can be usefully interrelated. For example, quasielastic neutron scattering measurements probe the motions of the network junctions [53,54]. Molecular dynamics simulations have also added new insight, for example, by demonstrating the existence of local constraints on the network chains at strand lengths less than the molecular weight necessary for chain entanglements [55]. Recently P NMR spin-lattice relaxation experiments on crosslinked rubber have been used to monitor specifically the dynamics of the network junctions [52,56]. 

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