PRECISE LABORATORY WAVELENGTHS OF THE MG-I AND MG-II RESONANCE TRANSITIONS AT 2853, 2803 AND 2796 ANGSTROM

Strong ultraviolet resonance transitions are observed routinely both i n the Galactic interstellar medium and in quasar absorption systems. T he quality of the astronomical spectroscopic data now available demand s more precise laboratory rest wavelengths. Of particular interest is the accuracy with which one can constrain space-time variations in fun damental constants using quasar spectra. A recent analysis by Webb et al. of 25 quasar spectra using Mg and Fe transitions tentatively sugge sts that the fine-structure constant was smaller at earlier epochs. To permit a check on this result, and to allow further more extensive in vestigations, we have carried out a new determination of the laborator y wavelengths of Mg I 2853 Angstrom, Mg rr 2796 Angstrom and Mg II 280 3 Angstrom by high-resolution Fourier transform spectroscopy. Our resu lts for Mg II 2796 Angstrom are consistent with the value measured ind ependently by two other groups. To our knowledge, no previous measurem ents of comparable precision exist for Mg I 2853 Angstrom and Mg II 28 03 Angstrom.