Inelasticity Correction of a Time-of-Flight Neutron Diffraction Data -Intramolecular and Intermolecular Structure of a liquid D₂0-

Abstract

The analysis of a time-of-flight neutron diffraction data for a liquid D₂0 was carried out, for the first time, by applying the dynamical correction. The effect of the inelasticity on the data was expressed in terms of the logarithmic derivatives of the neutron-flux spectrum and the detector sensitivity with respect to the wave numbers of the incident and scattered neutrons, and the ratio of the flight distance of the scattered neutron to that of the incident neutron. The inelasticity correction in the self-scattering term reproduced the increase of the differential cross-section for neutron scattering with increasing Q above ca.10A-1 and with decreasing Q for Q≤10Å^-1, at 2θ=90° and 150°. The interference differential scattering cross-section, which gives all the structural information in molecular liquids, was divided into the intramolecular and iintermolecular terms. The initial stage in the analysis determined the molecular geometry through the intramolecularinterference term which involves the inelasticity contribution the internuclear distances in a D₂0 molecule in liquid D₂0, r_a(OD)and r_a(DD) are equal to 0.971±0.03Å and 1.55±0.02Å, respectively. Under the elastic approximation r_a(OD) and r_a(DD) were small by 1-3% and 5-10%, respectively. The second. stage in the analysis determined the intermolecular structure: the intermolecular pair distribution function goD(r) was obtained with sufficient accuracy with the aid of the other intermolecular pair distribution functions goD(r)and goo(r), the data of which were taken from the TOF neutron diffraction study due to Soper-Silver and the X-ray study due to NartenLevy, respectively. The form of intermolecular pair distribution function goD(r) indicates: (i) The first and second peaks locate at 1.92Å and ca.3 4Å (ii) The coordination number n(O…D) is equal to 2.3±0.3. (iii) The short range correlations between D and, O pairs in the nearest and second nearest water molecules are highly coordinated within the range of ca.4Å in the liquid D₂0, since the Q dependence of goD(r) does not show any significant oscillation for Q>4Å.