We use saturated absorption spectroscopy to study neutral ${}^9\mathrm{Be}$ atoms produced in a dc discharge lamp. Previous measurements of the $2s2p{}^{1}P^o$ state energy were limited in precision due to overlapping hyperfine structure. In this work, we apply a 400 G magnetic field, which splits the magnetic sublevels creating a spectral feature near the center-of-gravity energy of the state. We determine the transition frequency from the ground state to be $1276080105.5\left(1.2\right)\mathrm{MHz}$, corresponding to a center-of-gravity energy of $42565.45058\left(4\right){\text{cm}}^{-1}$. This result is a factor of 30 more precise than the previous best measurement and constitutes the most precise determination of an excited-state energy in a four-electron system to date. As theoretical methods advance, this result will enable stringent tests of quantum electrodynamics in four-electron systems.

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• Laser spectroscopy
• Atoms
• Precision measurements
• Pump-probe spectroscopy