Dynamic form factor of liquid4He at intermediate momentum transfer

Measurements of the inelastic neutron scattering from liquid4He atT=1.2 K in the range of wavevector transfer 3≤Q≤12 Å−1 have been performed to verify the oscillations in the peak position and the width of the scattering function withQ observed by Cowley and Woods and by Martelet al. We confirm these oscillations and extend the measurements to show that the oscillations in the width continue up toQ≈12 Å−1 while oscillations in the peak position, which have a much smaller amplitude, could be observed with confidence up toQ≈8 Å−1 only. We also present a straightforward RPA calculation of the dynamic form factorS(Q, ω), beginning from the pair interatomic potential, to see how well the observedS(Q, ω) can be described and to investigate the origin of the oscillations. The observedS(Q, ω) is quite well reproduced and the oscillations in the width and peak position ofS(Q, ω) are seen to originate from oscillations in the interaction in the RPA which we have approximated as the4He-4He scattering amplitude calculated fromv(r). While the present calculation is quite different from the model proposed previously by Martelet al., the physical origin of the oscillations inS(Q, ω) is the same, namely oscillations in the4He-4He atom scattering amplitude.