Frequency metrology on the Mg 3s2 1S→3s4p 1P line for comparison with quasar data

We report a frequency metrology study on the Mg 3s(2) S-1 -> 3s4p P-1 transition near 202.5 nm. For this purpose, the fourth harmonic of the output from an injection-seeded Ti: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 Mg-24 is determined at 49 346.756 809(35) 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 Mg-25 and Mg-26 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 7x10(-10) level makes this second resonance line of Mg 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.