Ionization energy ofLi6,7determined by triple-resonance laser spectroscopy

Abstract

Rydberg level energies for $^{7}\mathrm{Li}$ were measured using triple-resonance laser excitation, followed by drifted field ionization. In addition to the principal $n\phantom{\rule{0.2em}{0ex}}^{2}P$ series, weak Stark mixing from residual electric fields allowed observation of $n\phantom{\rule{0.2em}{0ex}}^{2}S$ and hydrogenic Stark manifold series at higher $n$. Limit analyses for the series yield the spectroscopic ionization energy ${E}_{I}(^{7}\mathrm{Li})=43\phantom{\rule{0.2em}{0ex}}487.159\phantom{\rule{0.2em}{0ex}}40(18)\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The $^{6,7}\mathrm{Li}$ isotope shift (IS) was measured in selected $n\phantom{\rule{0.2em}{0ex}}^{2}P$ Rydberg levels and extrapolation to the series limit yields $\mathrm{IS}{({E}_{I})}^{7,6}=18\phantom{\rule{0.2em}{0ex}}067.54(21)\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. Results are compared with recent theoretical calculations: ${E}_{I}$ values from experiment and theory agree to within $0.0011\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, with the remaining discrepancy comparable to uncertainty in QED corrections of order ${\ensuremath{\alpha}}^{4}Ry$. The difference between experiment and calculated mass-based $\mathrm{IS}({E}_{I})$ yields a change in nuclear charge radii between the two isotopes $\ensuremath{\delta}{⟨{r}^{2}⟩}^{7,6}=\ensuremath{-}0.60(10)\phantom{\rule{0.3em}{0ex}}{\mathrm{fm}}^{2}$.