PNNL

Abstract

Rydberg level energies for 7Li were measured using triple-resonance laser excitation, followed by drifted field ionization. In addition to the principal 2P series, weak Stark mixing from residual electric fields allowed observation of 2S and hydrogenic Stark manifold series at higher n. Limit analyses for the series yield the spectroscopic ionization energy $EI (7Li) = 43487.15940(18) cm-1. The 6,7Li isotope shift (IS) was measured in selected 2P Rydberg levels and extrapolation to the series limit yields IS(EI)7,6 = 18067.54(21) MHz. Results are compared with recent theoretical calculations: EI values from experiment and theory agree to within 0.0011 cm-1, with the remaining discrepancy comparable to uncertainty in QED corrections of order alpha ^4 Ry. The difference between experiment and calculated mass-based IS$(E_I)$ yields a change in nuclear charge radii delta7,6 = -0.60(10) fm2.