Neutron inelastic scattering localized motions

Inelastic neutron scattering gives access to the low frequency internal vibrations which are not easily observed in optical spectroscopies such as infrared and Raman as a matter of fact the vibrational bands of the zeolites lay below 1000 cm-l. In a similar manner the external vibrations of the whole molecule with respect to the internal surfaces, and the rotations of a given group of the molecule have been assigned on the basis of INS measurements. Insofar as these low frequency motions are very sensitive to the local structure and to the dynamics of the sorbed phase, their study provides useful information as to the location and orientation of the sorbed molecule progressive localisation as the temperature decreases, rotational motion around a given molecular axis, etc. 

Nuclear magnetic resonance and inelastic neutron scattering have both been used to study the local dynamical response of the ring motions in PANl [57,58] and in PPV films [59,60]. The NMR experiments are able to detect the two aforementioned types of motion, ring flips and librations. At lower temperatures the ring motion is almost exclusively librational with modest torsional excursions about an equilibrium position. At elevated temperatures there are pronounced flips, approaching a full 180° of rotation. 

In neutron scattering, the elastic part reflects the static correlations (Bragg peaks reveal the unit cell structure and the diffuse pattern shows the non-periodic, local disorder). The inelastic scattering is due to the periodic motions of atoms or ions (phonons) and the quasi-elastic scattering is caused by any kind of non-periodic motion (or magnetic disorder). The transition between low frequency periodic motions (translational and rotational oscillation) and diffusive motion (non-periodic) is not well understood. 

---