Rotational Molecular Motion and the Transient Nature of Liquids as Seen by Slow-Neutron Scattering

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Abstract
The analysis of neutron spectra scattered from molecular solids and liquids presents a formidable problem which so far has been only slightly touched upon. For a period of ten years, the translational motions of atoms in monatomic liquids have been investigated by neutron spectroscopists; their theoretical interpretation has reached a certain degree of refinement. An understanding of the effects of rotational motions on the neutron spectra scattered from molecular liquids, however, has not been developed to the same degree. The aim of the present work is to use some simple but reasonable model for the translational motions and to investigate in more detail the effect of the various possible rotational motions on the observed neutron spectrum. The general principle used is that translational and rotational motions are statistically uncorrelated. Thus, when the molecule as a whole vibrates and diffuses, it is assumed that it can also perform librations, free rotations, or rotational diffusion. Phonon expansions are done for the molecular vibrations and librations. Sears's basic theory is used to describe the free rotations and rotational diffusion. A cross section is obtained which contains all the basic components in the scattered-neutron spectrum. A discussion of "good"- and "bad"-resolution measurements is carried through. The value of older neutron-linewidth determinations is discussed. It is concluded that the resolution width ought to be ≤ 0.1 × (the width of the rotational spectrum) if finer details of the rotational motion should be observed as resolved structure. With the aid of good-resolution data it is shown that the nature of the rotational motion changes drastically over the liquid range, from librations to almost free rotations. Comparison is made to changes of rotational motion of ions in the phase transition from the ferro- to the para-electric phase in the solid state. It is shown that from such observations of molecular motion the liquid appears as a phase transition or as a transient phenomenon.