Accurate laboratory ultraviolet wavelengths for quasar absorption‐line constraints on varying fundamental constants

The most precise method of investigating possible space–time variations of the fine‐structure constant, α≡ (1/ħ c )( e 2 /4πε 0 ) , using high‐redshift quasar absorption lines is the many‐multiplet (MM) method. For reliable results this method requires very accurate relative laboratory wavelengths for a number of UV resonance transitions from several different ionic species. For this purpose laboratory wavelengths and wavenumbers of 23 UV lines from Mg  i , Mg  ii , Ti  ii , Cr  ii , Mn  ii , Fe  ii and Zn  ii have been measured using high‐resolution Fourier transform (FT) spectrometry. The spectra of the different ions (except for one Fe  ii line, one Mg  i line and the Ti  ii lines) are all measured simultaneously in the same FT spectrometry recording by using a composite hollow cathode as a light source. This decreases the relative uncertainties of all the wavelengths. In addition to any measurement uncertainty, the wavelength uncertainty is determined by that of the Ar  ii calibration lines, by possible pressure shifts and by illumination effects. The absolute wavenumbers have uncertainties of typically ±0.001–±0.002 cm −1 ( Δλ≈ 0.06–0.1 mÅ at 2500 Å), while the relative wavenumbers for strong, symmetric lines in the same spectral recording have uncertainties of ± 0.0005  cm −1 ( Δλ≈ 0.03 mÅ at 2500 Å) or better, depending mostly on uncertainties in the line‐fitting procedure. This high relative precision greatly reduces the potential for systematic effects in the MM method, while the new Ti  ii measurements now allow these transitions to be used in MM analyses.