Inelastic Neutron Scattering from Lattices, Molecular Crystals and Powders
New neutron sources provide epithermal neutrons well suited in energy to investigate the internal motions of molecules. With these higher energies also come high momenta. Do extreme limits of these two factors yield apparent quasi-free behaviour for the scattering centres? Asymptotic analysis for model systems of harmonic lattices, isolated oscillators and oscillators in lattices answer this qualitative question. Exact numerical results reveal how good approximate methods actually are. A lot of structure in oscillator lines can result from kinematic effects of the lattice environment and is not revealed by approximate treatments. This is a possible artifact to be first removed from experimental interpretation before more subtle explanations of splittings and shifts are invoked.
The advantages offered by high momentum transfer (neutron) spectroscopy over usual spectroscopic techniques are explained, as are the problems thereby encountered when then dealing with powders. A systematic technique involving the use of displacement tensors in spherical coordinates yields simple, exact results rather than the usual expansions or numerics.
KeywordsSaddle Point Wave Packet Momentum Transfer Neutron Source Gaussian Approximation
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