Rydberg level energies for ${}^7\mathrm{Li}$ were measured using triple-resonance laser excitation, followed by drifted field ionization. In addition to the principal $n{}^{2}P$ 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 $E_I\left({}^7\mathrm{Li}\right)=43487.15940\left(18\right){\mathrm{cm}}^{-1}$. The ${}^{6,7}\mathrm{Li}$ isotope shift (IS) was measured in selected $n{}^{2}P$ Rydberg levels and extrapolation to the series limit yields $\mathrm{IS}{\left(E_I\right)}^{7,6}=18067.54\left(21\right)\mathrm{MHz}$. Results are compared with recent theoretical calculations: $E_I$ values from experiment and theory agree to within $0.0011{\mathrm{cm}}^{-1}$, with the remaining discrepancy comparable to uncertainty in QED corrections of order ${\alpha }^{4}Ry$. The difference between experiment and calculated mass-based $\mathrm{IS}\left(E_I\right)$ yields a change in nuclear charge radii between the two isotopes $\delta {⟨r^2⟩}^{7,6}=-0.60\left(10\right){\mathrm{fm}}^2$.

• Received 6 February 2007

DOI:https://doi.org/10.1103/PhysRevA.75.052503