Frequency metrology on theMg3s2S1→3s4pP1line for comparison with quasar data

Abstract

We report a frequency metrology study on the $\mathrm{Mg}3s^2{}^{1}S\to 3s4p{}^{1}P$ transition near $202.5\mathrm{nm}$. For this purpose, the fourth harmonic of the output from an injection-seeded $\mathrm{Ti}:\mathrm{sapphire}$ pulsed laser is employed in a Mg atomic beam experiment with laser-induced fluorescence detection. Absolute frequency calibration with a frequency comb laser is performed on the cw seeding radiation, while the chirp-induced frequency shift between the pulsed output and the seed light is monitored on line. The resulting transition frequency for the main isotope ${}^{24}\mathrm{Mg}$ is determined at $49346.756809\left(35\right){\mathrm{m}}^{-1}$. This value is three orders of magnitude more precise than the best value in the literature. The line positions of the other isotopes ${}^{25}\mathrm{Mg}$ and ${}^{26}\mathrm{Mg}$ are also measured at comparable accuracy, giving rise to very exact values for the isotopic shifts. The achieved precision for the transition frequency at the $7\times {10}^{-10}$ level makes this second resonance line of $\mathrm{Mg}\mathrm{\text{I}}$ an additional candidate for inclusion in many-multiplet methods, aimed at detecting a possible temporal variation of the fine-structure constant $\alpha$ from comparison with quasar spectra. The isotopic shifts obtained are also important to correct for possible systematic shifts due to evolution of isotopic abundances, which may mimic $\alpha$-variation effects.