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We report direct measurements of two-electron contributions to the ground-state energy of high-Z heliumlike ions. The difference in radiative-recombination x-ray energy (i.e., ionization potential of the recombined ion) was measured for bare and hydrogenlike target ions trapped in an electron-beam ion trap for six elements ranging from Z=32 to Z=83. The achieved uncertainties (as small as 1.6 eV) test the second-order many-body contributions to the energy and approach the size of the two-electron (screened) Lamb shift. We also report a measurement of the ground-state ionization potential of heliumlike bismuth relative to heliumlike xenon. All results are in agreement with available theories.
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Marrs et al. (1995) studied this question.
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