Ionization energy of 6,7Li determined by triple-resonance laser spectroscopy

Rydberg level energies for 7Li were measured using triple-resonance laser excitation, followed by drifted field ionization. In addition to the principal n 2P series, weak Stark mixing from residual electric fields allowed observation of n 2S and hydrogenic Stark manifold series at higher n. Limit analyses for the series yield the spectroscopic ionization energy EI(7Li)=43 487.159 40(18) cm-1. The 6,7Li isotope shift (IS) was measured in selected n 2P Rydberg levels and extrapolation to the series limit yields IS(EI)7,6=18 067.54(21) MHz. Results are compared with recent theoretical calculations: EI values from experiment and theory agree to within 0.0011 cm-1, with the remaining discrepancy comparable to uncertainty in QED corrections of order α4Ry. The difference between experiment and calculated mass-based IS(EI) yields a change in nuclear charge radii between the two isotopes δ7,6=-0.60(10) fm2