Inelastic x-ray scattering in liquid helium

A detailed experiment and theoretical study of the electronic dynamic structure factor $S(k,\omega )$ for $1.6 {\AA }^{-1\mathrm{}}\le k\le 4.5 {\AA }^{-1\mathrm{}}$ in liquid helium is reported. A simple hydrogenic theory for the one-electron part of $S(\overrightarrow{\mathrm{k}},\omega )$ is constructed. The theory accounts surprisingly well for the measured $S(\overrightarrow{\mathrm{k}},\omega )$ over the complete range of wave vectors. Two-electron excitations and electron correlation effects thence have at most a minor effect on the dynamic response function.