The structure factor of liquid bromine by neutron diffraction

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Abstract
High precision neutron-diffraction measurements for liquid bromine at 21°C have been made for incident wavelengths of 0·35, 0·50 and 0·70 Å. The results are compared with previous neutron data and show a systematic variation in oscillation amplitude. At high Q-values the periodicity in the oscillatory pattern corresponds to an effective bond length of 2·28 ± 0·01 Å for the 0·7 Å data and 2·32 ± 0·01 Å for the 0·5 Å data. These observed differences cannot be accounted for in terms of the currently established procedures for inelasticity corrections. Although the evaluated structure factors Sm (Q) for each measurement exhibit significant differences, the inter-molecular contribution Dm (Q) has the same shape for all cases and the pair distribution function dL (r) derived from the data is in good agreement with that previously obtained. The predictions from RISM are unable to reproduce the detailed behaviour of the curve. The inclusion of a strong quadrupole-quadrupole term in the interaction potential for molecular dynamics simulation confirms that the double peak in the distribution function is due to a preference for T-shaped orientational configurations but the experimental data give a more pronounced effect than current computational predictions. Additional measurements at -4°C show that there are negligible changes in orientational correlation as a function of temperature.