Neutron diffraction determination of the thermodynamic derivatives of the microscopic structure of liquid parahydrogen

We have measured, using time-of-flight neutron diffraction, the thermodynamic derivatives of the microscopic structure factor of liquid parahydrogen in the vicinity of the triple point. Experiments have been carried out on the T=17.1 K isotherm and on the n=22.2 nm-3 isochore so that the thermodynamic derivatives at constant temperature and density could be evaluated. The hydrogen data have been compared with the corresponding quantities for deuterium and quantitatively significant differences have been observed. Assuming that hydrogen and deuterium experience the same intermolecular potential, these differences can only be attributed to the different quantum behaviors of the two molecules.