Neutron scattering and molecular correlations in a supercooled liquid

We show that the intermediate scattering function S-n(q,t) for neutron scat tering (ns) can be expanded naturally with respect to a set of molecular co rrelation functions that give a complete description of the translational a nd orientational two-point correlations in the liquid. The general properti es of this expansion are discussed with special focus on the q dependence, and hints for a (partial) determination of the molecular correlation functi ons from neutron scattering results are given. The resulting representation of the static structure factor S-n(q) is studied in detail for a model sys tem using data from a molecular dynamics simulation of a supercooled liquid of rigid diatomic molecules. The comparison between the exact result for S -n(q) and different approximations that result from a truncation of the ser ies representation demonstrates its good convergence for the given model sy stem. On the other hand it shows explicitly that the coupling between trans lational and orientational degrees of freedom of each molecule and rotation al motion of different molecules cannot be neglected in the supercooled reg ime. Further we report the existence of a prepeak in the ns static structur e factor of the examined fragile glass former, demonstrating that prepeaks can occur even in the most simple molecular liquids. Besides examining the dependence of the prepeak on the scattering length and the temperature we u se the expansion of S-n(q) into molecular correlation functions to paint ou t the intermediate range orientational order as its principle origin. [S106 3-651X(99)05407-0].