Frequency measurement of theP13−P03fine-structure transition ofMg24

A frequency measurement of the fine-structure transition ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$, $\Delta m_j=0\mathrm{}$ in the first metastable triplet of ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ is reported. The experiment has been performed on a metastable atomic beam interacting with the submillimetric radiation ($\lambda \simeq 0.5$ mm) of a phase-locked backward wave oscillator. The transition is observed via the fluorescence emission at 4571 Å of the intercombination line ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}-{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$. The center frequency was evaluated to be $\nu =601\mathrm{}277.16\mp 0.05$ MHz with precision limited by first-order Doppler effect.