X-Ray Determination of Electron Momenta in Li, Be, B, Na, Mg, Al, and LiF

Details of the ground-state electron momentum density in Li, Be, B, LiF, Na, Mg, and Al have been investigated by means of high-resolution Compton line-shape measurements. A sharp discontinuity is observed in the momentum density near the Fermi Surface in Li, Na, Mg, and Al, whereas in Be the discontinuity (if any) is too small to be discerned. Above the Fermi momentum, these metals all exhibit high-momentum tails in their valence-electron momentum densities which can be attributed largely to electron-electron correlations. The momentum density in Be and Al is sensitive to crystallographic orientation, while the density measured in Li and Na is essentially spherically symmetric. In both Be and B the measured valence-electron densities are more $2p$-like than $2s$-like. The measured momentum density of LiF resembles a superposition of calculated free-atom Li⁺ and F^-, rather than Li⁰ and F⁰ momentum densities. Agreement between the data, analyzed in the impulse approximation, and Hartree-Fock calculations for the core-electron momentum densities is good in Li, B, Na, and Mg, but unsatisfactory in Be and Al.