Precision Test of Many-Body QED in the Be⁺ 2p Fine Structure Doublet Using Short-Lived Isotopes

Absolute transition frequencies of the 2s2 S1/2→2p2 P1/2,3/2 transitions in Be+ were measured for the isotopes 7,9–12Be. The fine structure splitting of the 2p state and its isotope dependence are extracted and compared to results of ab initio calculations using explicitly correlated basis functions, including relativistic and quantum electrodynamics effects at the order of mα6 and mα7 ln α. Accuracy has been improved in both the theory and experiment by 2 orders of magnitude, and good agreement is observed. This represents one of the most accurate tests of quantum electrodynamics for many-electron systems, being insensitive to nuclear uncertainties