Electron-pair momentum densities of group 14, 15, and 16 atoms in their low-lying electronic states

Abstract

Electron-pair intracule (relative motion) h̄(υ) and extracule (center-of-mass motion) d̄(P) densities in momentum space are studied for the 3P, 1D, and 1S terms of five group 14 atoms with p2 configurations, the 4S, 2D, and 2P terms of five group 15 atoms with p3 configurations, and the 3P, 1D, and 1S terms of five group 16 atoms with p4 configurations. Common to all fifteen atoms, the intracule densities show that a low energy term has a greater probability of finding a pair of electrons with a large relative momentum υ than a high energy term. The Fermi hole effect in a high spin term appears naively in momentum space, and the average relative momentum 〈υ〉 is larger in a high angular momentum term. For the terms arising from the pm electronic configurations (m=2–4), the differences in the radial extracule densities are found to be almost isomorphic with the corresponding intracule ones. In a term with a high angular momentum, the average center-of-mass momentum 〈P〉 of an electron pair is always larger, and two electrons are less likely to have opposite momenta. The major origin of these differences in the electron-pair densities lies in the valence np orbitals, and the contribution of different electron-pair motions to the relative stability of terms can be explained by a rigorous relation between the kinetic energy Tnp and the second intracule 〈υ2〉np and extracule 〈P2〉np moments of the valence np subshell.