Neutron accessible time scale

In this review we consider large-scale polymer motions which naturally occur on mesoscopic time scales. In order to access such times by neutron scattering a very high resolution technique is needed in order to obtain times of several tens of nanoseconds. Such a technique is neutron spin echo (NSE), which can directly measure energy changes in the neutron during scattering [32,33]. 

The space and time scales accessible by the neutron scattering techniques are comparable to the ones covered by molecular simulations so that comparisons between experiment and predictions can be made not only for the diffusivities, but also for the jumps between adsorption sites and for the distribution of adsorbed molecules. 

Atomic motions cause the energies of scattered neutrons to be changed. Depending on the time scale of motion, one needs to cover an energy range from 10 neV-1 eV and to span wave vectors from 0.01-10 A . This wide time scale of analysis is covered by different classes of instruments. The phase space region ( ,Q) accessible to the various types of neutron spectrometers is schematized in Fig. 1. 

The macroscopic structure of matter can be assessed, for example, by optical microscopy and can then be linked to its microscopic origin through X-ray, neutron, or electron diffraction experiments and the various forms of electron and atomic-force microscopy. A factor of 10 -10 separates the atomic, nanometer scale from the macroscopic, micrometer scale. Macroscopic dynamic techniques ultimately linked to molecular motion are, for example, dynamic mechanical and dielectric analyses and calorimetry. In order to have direct access to the details of the underlying microscopic motion, one must, however, use computational methods. A realistic microscopic description of motion has recently become possible through accurate molecular dynamics simulations and will be described in this review. It will be shown that the basic large-ampHtude molecular motion exists on a picosecond time scale (1 ps = 10 s), a ffictor at... 

Figure 7 This figure maps out the length and energy scales that are accessible with the different inelastic neutron scattering techniques. In the text, we have limited our discussion to the time-of-flight (TOF) spectrometers, backscattering (BS) spectrometers, and spin echo spectrometers.

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