Structure factor of liquid deuterium: a neutron diffraction experiment

The structure factor of liquid deuterium has been measured, for the first time, by means of a nuetron diffraction experiment. The experiment has been carried out at the pulsed neutron source ISIS (UK) on the SANDALS diffractometer. Five thermodynamic points were selected, in the liquid phase, near the triple point, three of them on the isotherm at 20.7 K, and the other two on the same isochore of the lowest density (25.4 nm3). Owing to the SANDALS design, the low scattering angle geometry greatly reduced the effects of inelastic scattering. The high Q region of the structure factor is dominated by the single molecule term which is well approximated with a rigid rotor model, corrected by a Debye--Waller factor. As expected, this region of the diffraction pattern is independent of both density and temperature. The lower Q region contains information on the intermolecular structure factor which is modulated by the molecular form factor.