Isotopically resolved calibration of the 285-nm MgI resonance line for comparison with quasar absorptions

We have performed high-precision spectroscopy on the 3s(2) S-1 -> 3s3p P-1 first resonance line of MgI at 285 nm. This measurement is motivated by the recent studies comparing observed quasar absorption lines with laboratory measurements in the search for a possible variation of the fine structure constant alpha. The Mg transition is excited using the up-converted radiation of an injection-seeded Ti : sapphire pulsed laser system, while the frequency of the continuous wave seed light is measured using a femtosecond frequency comb laser. The line positions of the isotopes are fully resolved and have accuracies better than 8 x 10(-9). Using terrestrial abundances for the Mg isotopes Mg-24: Mg-25: Mg-26 of 78.99 : 10.00 : 11.01, we determine the composite line wavenumber to be 35 051.2808(2) cm(-1), which can be compared to the astronomical observations where the isotopic shifts are unresolved.