Determination of the short-wavelength propagation threshold in the collective excitations of liquid ammonia

The dynamics structure factor S(Q,E) of liquid ammonia f-NH3 at T = 200 K a nd at its vapor pressure has been measured by inelastic x-ray scattering (I XS) in the 1-15 nm(-1) momentum transfer (Q) range. Contrary to previous IX S studies on other associated liquids and glasses, in E-NH3 a large inelast ic signal is observed up to Q = 15 nm(-1). This, enabling S(Q, E) measureme nts as a function of Q at constant E transfer, allows us to demonstrate exp erimentally the transition from a propagating dynamics regime, where the ac oustic excitation energy linearly disperses with Q, to a high-a regime, whe re it is no longer possible to observe a dominant excitation in the S(Q, E) .