Angular correlation effects in neutron diffraction from molecular fluids

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Abstract
The structure factor S(q) for a fluid of rigid molecules is related to the angular pair correlation function, g(R 12ω1ω2). Several limiting cases of this relation are considered, and its use as a means of obtaining experimental knowledge of orientation correlations is briefly discussed. A first-order perturbation treatment of S(q) is presented, using as a reference a system of molecules with isotropic forces. The theory is used to predict the effect on S(q) of anisotropic dispersion, overlap and multipole (dipole, quadrupole, octopole, and hexadecapole) forces for linear AA, AB and BAB molecules, and also for tetrahedral AB4 molecules and octahedral AB6 molecules. Comparison with experiment is made for liquid bromine and liquid carbon tetrachloride; for both liquids orientation correlations make a substantial contribution to S(q). The theoretical calculations suggest that the most important anisotropic force contribution to S(q) is probably the quadrupolar force in the case of bromine, and the octopolar force in the case of carbon tetrachloride. The models used in this work for the anisotropic dispersion and overlap forces were not able to account for the observed behaviour of S(q) for these two liquids.