Determination of the Short-Wavelength Propagation Threshold in the Collective Excitations of Liquid Ammonia

The dynamics structure factor $S(Q,E)$ of liquid ammonia $\ell -{\mathrm{NH}}_3$ at $T=200\mathrm{}\mathrm{K}$ and at its vapor pressure has been measured by inelastic x-ray scattering (IXS) in the $1–15{\mathrm{nm}}^{-1\mathrm{}}$ momentum transfer ( $Q$) range. Contrary to previous IXS studies on other associated liquids and glasses, in $\ell -{\mathrm{NH}}_3$ a large inelastic signal is observed up to $Q=15\mathrm{}{\mathrm{nm}}^{-1\mathrm{}}$. This, enabling $S(Q,E)$ measurements as a function of $Q$ at constant $E$ transfer, allows us to demonstrate experimentally the transition from a propagating dynamics regime, where the acoustic excitation energy linearly disperses with $Q$, to a $\mathrm{high}-Q$ regime, where it is no longer possible to observe a dominant excitation in the $S(Q,E)$.