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

The dynamics structure factor $S(Q,E)$ of liquid ammonia -NH₃ at T0ex0ex=0ex0ex200K and at its vapor pressure has been measured by inelastic x-ray scattering (IXS) in the 1--15nm^-1 momentum transfer ( Q) range. Contrary to previous IXS studies on other associated liquids and glasses, in -NH₃ a large inelastic signal is observed up to Q0ex0ex=0ex0ex15nm^-1. 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 high-Q regime, where it is no longer possible to observe a dominant excitation in the $S(Q,E)$.