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

Rydberg level energies for Li-7 were measured using triple-resonance laser excitation, followed by drifted field ionization. In addition to the principal n P-2 series, weak Stark mixing from residual electric fields allowed observation of n(2)S and hydrogenic Stark manifold series at higher n. Limit analyses for the series yield the spectroscopic ionization energy E-I(Li-7)= 43 487.159 40 (18) cm(-1). The Li-6,Li-7 isotope shift ( IS) was measured in selected n(2)P Rydberg levels and extrapolation to the series limit yields IS (E-I)(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 alpha(4)Ry. The difference between experiment and calculated mass-based IS (E-I) yields a change in nuclear charge radii between the two isotopes delta < r2 >(7,6)=- 0.60 (10)fm(2).