While the ab initio calculations using plane-wave description of the ejected electron do not agree quantitatively with the experiments, the β() profile has the correct shape; the calculations just overestimate the energy of the β() minimum. The value of β is dependent on the contributions of partial spherical waves with different angular momenta to 𝜓 . When 𝜓 is dominated by ℓ = 0 waves (near the ionization threshold), β  0. The mixing in of higher angular momentum spherical partial waves gives rise to cross-terms that result in a negative β. As higher angular momentum waves become dominant at higher energies, β increases and becomes positive. The energy where this occurs is determined by the integral 𝜓 𝑟𝑌 , Φ . The disagreement between the ab initio calculations and experiment, therefore, must be because partial waves with ℓ 0 do not overlap early enough with 𝑟𝑌 , Φ ⟩ as the energy increases. There are two possible explanations for this: (1) The true Dyson orbital is more diffuse than the computed one, and therefore starts overlapping with ℓ 0 waves at lower energies.