Neutron scattering spectroscopy of liquids

Abstract

This paper reports the use of atomic substitution techiques in some preliminary studies by neutron scattering spectroscopy of the microscopic transport properties of acetic acid and methanol. By making selective substitution of deuterium and fluorine for the hydrogens in the CH₃ and OH groups of these molecules, the separate contributions to the quantized and diffusive motions of each have been analyzed. This is possible because the substitution of fluorine or deuterium for hydrogen in a particular group reduces the scattering cross-section of that group by at least an order of magnitude. The contributions of the group motions to the neutron scattering spectrum are then correspondingly reduced. In the inelastic scattering region of the spectrum we have been able to make vibrational assignments as a result. In the quasi-elastic region the intensity and angular dependence of the scattering show a marked dependency on the nature of the protons substituted. In this region the results have been analyzed by determining an effective molecular diffusion coefficient and by comparing this with values from bulk-phase studies. Particular attention has been devoted to the question of the extent to which intra-molecular hindered rotation contributes to the quasi-elastic scattering.