We report on ${\mathrm{C}}^{3+}(1s2\ell 2{\ell }^{\prime }{}^{2S+1}\mathrm{L})$-resolved cross sections of electron capture in collisions of swift ${\mathrm{C}}^{4+}(1s2s{}^3\mathrm{S})$ ions with helium and hydrogen. The study focuses on the formation of doubly excited triply open-shell ${\mathrm{C}}^{3+}(1s2s2p){}^4\mathrm{P}$ and ${{}^2\mathrm{P}}_{\pm }$ states with emphasis on the ratio $R$ of their cross sections as a measure of spin statistics. Using zero-degree Auger projectile spectroscopy and a three-electron close-coupling semiclassical approach, we resolve a long-standing puzzle and controversy on the value of $R$ and on the effect of cascades, to clarify the underlying physics. The present results invalidate the frozen core approximation generally used in the past when considering electron capture in multielectron multi-open-shell quantum systems. A distinctive screening effect due to the Pauli exclusion principle (Pauli shielding) is proposed to account for the value of $R$, consistent with our findings.

• Received 23 December 2019
• Revised 10 February 2020
• Accepted 22 February 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.113401