Lattice dynamics of ammonia

The frequencies of selected intermolecular modes of vibration and libration in a single crystal of deuterated ammonia (ND3) have been measured by the technique of coherent inelastic neutron scattering, at temperatures of 20 and 95K. The results are compared with the previous optical measurements at the Γ point, and with calculations based on two different models for the intermolecular potential function. A detailed assessment of these data leads to a set of mode frequencies for the Γ, R, and M points. The elastic constants are calculated from the measured acoustic mode dispersion curves propagating along the three major symmetry directions. The existing intermolecular force models are in good qualitative agreement with experiment, but significant discrepancies remain to be resolved by future theoretical refinements. (author)