Probing electron correlation through radiative lifetime measurements upon inner-valence photoionization of Ne and Ar

This work demonstrates that electron correlation can have a strong effect on the radiative lifetime of atoms. We report measurements of the radiative lifetimes of inner-valence hole states, the 3s3p^{6 2}S_1/2 state of Ar⁺ and the 2s2p^{6 2}S_1/2 state of Ne⁺ by using the time-correlated single photon counting technique combined with photoionization by synchrotron radiation. Theoretical calculations utilizing the multi-configuration Dirac-Fock method agreed well with the experimental results. In particular, the radiative lifetime was found to depend very sensitively on the mixing of valence excited state configurations. While the Ne⁺ 2s2p^{6 2}S_1/2 state only has relatively weak inter-shell correlation, Ar⁺ 3s3p^{6 2}S_1/2 state has strong intra-shell correlation within the M-shell. This intra-shell correlation enhances configuration mixing and causes the radiative lifetime of the Ar⁺ 3s3p^{6 2}S_1/2 state to become very much longer than that of the Ne⁺ 2s2p^{6 2}S_1/2 state.