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/ħc)(e2/4πε0), 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.