Electron Momentum Density of He andH2; Compton X-Ray Scattering

The electron momentum density (EMD) of the electrons in the helium atom and hydrogen molecule were measured by Compton x-ray scattering. The results for helium, in both the liquid and gaseous state, are in agreement within the statistical accuracy of the experiment with Hartree-Fock calculations of the EMD. For the hydrogen molecule the effects of binding on the EMD are clearly observed. There is disagreement between Hartree-Fock calculations of the EMD for the hydrogen molecule and the measured EMD. This disagreement is attributed to non-Hartree-Fock electron-electron interaction effects, which are sizable in the ${\mathrm{H}}_2$ molecule. In addition, for these systems, further proof of the validity of the impulse approximation is obtained by an accurate measurement of the Compton shift.