To determined the $3$s energy levels of lithium isotopes, a $735$ nm Ti:Sapphire laser is used to
excite the ground state atomic lithium from a atomic beam system. The laser frequency is measured
simultaneously by optical frquency comb, which gives the accuracy to $10^{-12}$ level. The
excitation laser is stabilized by phase-locking to a specific comb line. The induced fluorescences
spectra of $2$p-$2$s decay channel are fitted to a Voigt function with a Gaussian background to
decide the transition line centers. The best signal-to-noise ratio of the transition spectrum is
about $200$. The measured transition centers are $815\,618\,181.26(48)$ MHz and $815\,606
\,729.8(24)$ MHz for $^7$Li and $^6$Li $2$s-$3$s transitions respectively, which is improved by an
order of magnitude for \,$^7$Li. However, The hyperfine structure constants, $93.73(48)$ MHz for
\,$^7$Li and $36.0(35)$ MHz for \,$^6$Li, show the discrepancy with previous measurements
\cite{Bushaw2003,Ewald2004}.

To determined the $3$s energy levels of lithium isotopes, a $735$ nm Ti:Sapphire laser is used to
excite the ground state atomic lithium from a atomic beam system. The laser frequency is measured
simultaneously by optical frquency comb, which gives the accuracy to $10^{-12}$ level. The
excitation laser is stabilized by phase-locking to a specific comb line. The induced fluorescences
spectra of $2$p-$2$s decay channel are fitted to a Voigt function with a Gaussian background to
decide the transition line centers. The best signal-to-noise ratio of the transition spectrum is
about $200$. The measured transition centers are $815\,618\,181.26(48)$ MHz and $815\,606
\,729.8(24)$ MHz for $^7$Li and $^6$Li $2$s-$3$s transitions respectively, which is improved by an
order of magnitude for \,$^7$Li. However, The hyperfine structure constants, $93.73(48)$ MHz for
\,$^7$Li and $36.0(35)$ MHz for \,$^6$Li, show the discrepancy with previous measurements
\cite{Bushaw2003,Ewald2004}.

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