Techniques in neutron scattering studies of molecular systems

TECHNIQUES IN NEUTRON SCATTERING STUDIES OF MOLECULAR SYSTEMS... 

The radial distribution function plays an important role in the study of liquid systems. In the first place, g(r) is a physical quantity that can be determined experimentally by a number of techniques, for instance X-ray and neutron scattering (for atomic and molecular fluids), light scattering and imaging techniques (in the case of colloidal liquids and other complex fluids). Second, g(r) can also be determined from theoretical approximations and from computer simulations if the pair interparticle potential is known. Third, from the knowledge of g(r) and of the interparticle interactions, the thermodynamic properties of the system can be obtained. These three aspects are discussed in more detail in the following sections. In addition, let us mention that the static structure is also important in determining physical quantities such as the dynamic an other transport properties. Some theoretical approaches for those quantities use as an input precisely this structural information of the system [15-17,30,31]. 

Note that the study of molecular aspects in confined systems has become possible only in recent years. These studies require the development of time domain techniques such as quasi-electron neutron scattering (QNS) and laser spectroscopy, on one hand, and the potential of computer simulation on the other. As we articulated in this chapter, the synergy between theories and experiments played a key role in the development of our understanding of this area. 

Among the tools that permit one to obtain molecular information about interfaces [e.g., x-ray and neutron scattering, solid state nmr (2)], fluorescence quenching methods (3) offer some important advantages. They are sensitive. The equipment is readily available and relatively inexpensive. There is scope and versatility to those methods. There are many sources in the literature one can turn to for ideas for new experiments to study systems composed of synthetic polymers, because of the wide-spread applications of fluorescence techniques in the biological sciences (4). This chapter provides a brief Introduction to some applications of fluorescence quenching to study interfaces in polymer systems. 

Neutron inelastic scattering techniques have been widely applied to the study of vibrational and rotational dynamics in hydrogenous molecular systems.1 The bulk of this research has been concerned with the study of intermolecular and interionic motions in solids, but a limited yet significant amount of effort has been directed toward the study of large-amplitude intramolecular vibrations, most notably torsional vibrations and hydrogen-bond modes.2 The present paper is restricted primarily to a discussion of the application of neutron scattering to the study of torsional vibrations and barriers to rotation of methyl groups in molecules. We will present several examples in which neutron spectra have provided information complementary to that obtained by the more widely available and applicable infrared and Raman techniques. We will also discuss in simple terms some limitations and pitfalls of the neutron technique and the interpretation of neutron spectral results. 

Before the introduction of measuring techniques such as pulsed field gradient (PEG) NMR ([14,16,45], pp. 168-206) and quasielastic neutron scattering (QENS) [49,50], which are able to trace the diffusion path of the individual molecules, molecular diffusion in adsorbate-adsorbent systems has mainly been studied by adsorption/desorption techniques [ 16]. In the case of singlefile systems, adsorption/desorption techniques cannot be expected to provide new features in comparison to the case of normal diffusion [51,52]. In adsorption/desorption measurements it is irrelevant whether or not two adjacent molecules have exchanged their positions. But it is this effect which makes the difference between normal and single-file diffusion. 

In this chapter we describe a new microscopic method of analyzing the dynamical properties of classical fluids. Our primary interest lies in the study of those time correlation functions which describe the fluctuations in fluids and which can be directly measured by neutron and light scattering or obtained from computer molecular dynamics experiments. While the discussions here deal specifically with simple fluids, such as argon, the techniques developed are also applicable to more complicated systems (see Section 7). 

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