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Reference wavelengths of Si ii, Cii, Fe i, and Ni ii for quasarabsorption spectroscopy



Gillian Nave, Christian Clear


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, alpha, at high redshifts, provided that the laboratory wavelengths are known to better than 6 parts in 108, 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-analyzing archival Fourier transform (FT) spectra of iron hollow cathode lamps (HCL) and a silicon carbide Penning discharge lamp, and with new spectra of nickel HCLs. By re-optimizing 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 32 lines with wavelength uncertainties over an order of magnitude smaller than previous measurements. Additional Ritz wavelengths for 6 lines of Si II were derived using adjusted wavelengths of Kaufman & Edlen, and for 7 lines that are at shorter wavelengths than our FT spectra. Improved wavelengths for 8 lines of Ni II were measured and Ritz wavelengths determined for an additional 3 lines at shorter wavelengths. Three lines of C II near 135 nm were observed using FT spectroscopy and the wavelengths confirm previous measurements.
Monthly Notices of the Royal Astronomical Society


atomic data, laboratory atomic, ultraviolet


Nave, G. and Clear, C. (2020), Reference wavelengths of Si ii, Cii, Fe i, and Ni ii for quasarabsorption spectroscopy, Monthly Notices of the Royal Astronomical Society, [online],, (Accessed June 21, 2024)


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Created December 26, 2020, Updated May 3, 2021