Reference wavelengths of Si ii, C ii, Fe i, and Ni ii for quasar absorption spectroscopy

Wavelengths of absorption lines in the spectra of galaxies along the line of sight to distant quasars can be used to probe the variablility of the fine structure constant, α, at high redshifts, provided that the laboratory wavelengths are known to better than 6 parts in 10 ⁸ , corresponding to a radial velocity of ≈20 ms ⁻¹ . For several lines of Si ii, C ii, Fe i, and Ni ii, previously published wavelengths are inadequate for this purpose. Improved wavelengths for these lines were derived by re-analysing archival Fourier transform (FT) spectra of iron hollow cathode lamps (HCL), a silicon carbide Penning discharge lamp, and with new spectra of nickel HCLs. By reoptimizing the energy levels of Fe i, the absolute uncertainty of 13 resonance lines has been reduced by over a factor of 2. A similar analysis for Si ii gives improved values for 45 lines with wavelength uncertainties over an order of magnitude smaller than previous measurements. Improved wavelengths for eight lines of Ni ii were measured and Ritz wavelengths from optimized energy levels determined for an additional three lines at shorter wavelengths. Three lines of C ii near 135 nm were observed using FT spectroscopy and the wavelengths confirm previous measurements.