Exchange interference in the angular correlation between magnesium photo- and Coster-Kronig electrons

For the well defined process of 2s photoionization in atomic magnesium with subsequent L-1-L2,3M1 P-1 or P-3 Coster-Kronig decay, exchange interferenc e effects are studied for coincident two-electron emission. Both electrons are observed for back-to-back emission in a plane perpendicular to the phot on-beam direction, changing the photon energy so that the photoelectron lin e is moved, from below to above, through the state-selected Coster-Kronig l ine. The coincidence signal of the singlet Coster-Kronig channel shows the expected constructive interference effect leading to an increase by a facto r of 4. However, in the coincidence signal of the triplet Coster-Kronig cha nnel the expected destructive interference appears to be hidden due to (P-1 (1)-P-3(1)) spin-orbit induced J-mixing. Implementing this complication int o the theory which includes electron exchange and the influence of post-col lision interaction, very good agreement is achieved between theoretical and experimental data. This gives quantitative confirmation of the selected pr ocess of sequential two-electron emission described in a one-step formulati on.