Theory of the neon $1s$ correlation-peak intensities

The correlation-state spectrum accompanying $1s$ photoemission in atomic neon was calculated by several methods, using the sudden approximation and focusing on states that are approximately described by single-electron excitation of the form $2p\to np$. All the calculations gave satisfactory energy values, but the predicted intensities differed widely. Multiconfiguration Hartree-Fock (MCHF) orbitals and orthogonalized MCHF orbitals gave intensities in poor agreement with experiment. A final-state configuration-interaction calculation gave accurate energies for seven $2p\to np$ correlation states, but when combined with a single-determinant initial state, yielded intensities low by about a factor of 2. Initial-state configuration interaction (ISCI), including double-electron excitation of the form $2p^6\to 2p^{4}n{p}^2$, etc., brought the intensities of the $2p^5\mathrm{np}$-type states into agreement with experiment. It was thus shown that ISCI is of equal importance to final-state CI in determining correlation-peak intensities. Correlation-state (or "shakeup") spectra therefore contain unique information about electron correlation in the ground state.