Momentum distribution and final-state effects in liquid He-4

Neutron time-of-flight measurements of the dynamic structure factor S(Q,omega) of both normal liquid and superfluid He-4 are presented. Using the MARI spectrometer at the ISIS pulsed neutron source (U.K.), S(Q,omega) was measured over continuous ranges in wave vector Q up to 10 Angstrom(-1) at a temperature of 1.42 K, while at T=2.5 K the range extended up to Q=17 Angstrom(-1). The quality of the data represents a significant increase in statistical precision over previous measurements. The width and peak position of S(Q,omega) at both temperatures is seen to oscillate as a function of Q; this is due to coherence effects and has been observed in previous measurements. These oscillations art seen to continue up to a wave vector in excess of 10 Angstrom(-1). For the T = 2.5 K data, the measured S(Q,omega) spectra are analyzed using the technique developed by Glyde. Three free parameters are fitted at each Q and very good agreement is obtained. The Q scaling of these parameters is used to extract information about the shape of the normal fluid momentum distribution n(p), which is found to be distinctly non-Gaussian and in good agreement with Monte Carlo calculations. The mean kinetic energy per atom is obtained from the data and is in good agreement with previous neutron and x-ray work.