title: Determination of the short-wavelength propagation threshold in the collective excitations of liquid ammonia
abstract: The dynamics structure factor S(Q,E) of liquid ammonia l-NH3 at T = 200 K and at its vapor pressure has been measured by inelastic x-ray scattering (IXS) in the 1-15 nm(-1) momentum transfer ( Q) range. Contrary to previous IXS studies on other associated liquids and glasses, in l-NH3 a large inelastic signal is observed up to Q = 15 nm(-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).
authors.affiliation: European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France.
authors.initials: F
authors.lastname: Sette
authors.initials: G
authors.lastname: Ruocco
authors.initials: A
authors.lastname: Cunsolo
authors.initials: C
authors.lastname: Masciovecchio
authors.initials: G
authors.lastname: Monaco
authors.initials: R
authors.lastname: Verbeni