Phonon-roton excitations in liquid He4 at negative pressures

We present neutron scattering measurements of the phonon-roton excitations of superfluid He4 held at negative pressures from zero to -5 bar. The liquid was stretched to negative pressures by immersing it in the porous medium MCM-41. In the wave vector range 0.35≤Q≤1.55 Angstrom-1 and temperature T=0.4 K investigated, the phonon and maxon energies decrease systematically below bulk values as the negative pressure is increased. The energies are consistent with extrapolation of positive pressure values from which the negative internal pressure can be estimated. The maximum negative pressure realized is consistent with surface tension arguments and the MCM-41 pore diameter of 47 Angstrom