Triplet-singlet interaction in the 1s 2s3S1 hyperfine splitting of he-like Li+

The hyperfine structure (hfs) interaction between the metastable 23S1 and 21S0 states in6, 7Li+ causes very small equal and opposite energy level shifts,δ, in the two levels having the same total angular momentum quantum numberF=I. This amounts to 12ppm with respect to the total hfs splitting of 30 GHz for the7Li+ 23S1 state and has been measured for the first time directly for the case of a core-free electronic system by combined laser-microwave spectroscopy. A new calculation ofδ with accurate integral-transform 21S0 and 23S1 wave functions is reported. Configuration interactions and electron exchange and correlation effects, which differ for parallel (3S1) and antiparallel (1S0) spins, are treated in detail in order to obtain very good agreement between measured and theoretical values for this tiny perturbation. A complete understanding ofδ is indispensable for extracting fundamental constants from two-electron spectroscopic data.