Scattering of electron-volt neutrons by light and heavy water

In a recent experiment to determine the structure of liquid water the time-of-flight neutron diffraction cross sections for light and heavy water were measured at neutron energies up to 100eV, and at three scattering angles, i.e. 400, 900 and 1500. At energies below 1 eV the curves for heavy water show the usual series of interference oscillations arising from inter-atomic correlations. The same oscillations are not apparent in light water because of the large proton incoherent cross section for neutrons. Plotted versus energy data reveal other features which have not been reported before. In particular for neutron energies in the range 0.2-1.0 eV the cross sections show a distinct bulge, particularly at the larger scattering angles. The neutron energy for the onset of this feature is the same at all angles, although the size of the bulge is angle dependent. There is a similar feature in the neutron energy region of 1-approx.50 eV. Subsequently the data fall with increasing energy. Although the behavior of the cross sections with neutron energy is not fully understood at present, detailed computer simulations with a simple harmonic oscillator model scattering law indicate that the bulge starting at approx.0.2eV is caused in part by the onset of vibrational excitation of the molecule - a consequence of the complicated kinematic relationships involved in a time-of-flight experiment