Short-Wavelength Collective Excitations in Liquid Rubidium Observed by Coherent Neutron Scattering

The short-wavelength collective excitations in liquid rubidium at 320 K have been studied by coherent neutron scattering. For values of $\kappa =\frac{2\pi }{\lambda }$ up to 1.0 ${\mathrm{\AA }}^{-1\mathrm{}}$, clear evidence of propagating modes was found from the shape of the scattering function $S(\kappa , \omega )$ at constant values of $\kappa$. This result shows that the existence of such modes does not depend upon either quantum effects or low thermal population of the modes.