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Timestamp: 2019-04-22 15:06:39+00:00

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We investigate the wavenumber scale of Fe I and Fe II lines using new spectra recorded with Fourier transform spectroscopy and a reanalysis of archival spectra. We find that standards in Ar II, Mg I, Mg II, and Ge I give a consistent wavenumber calibration. We use the recalibrated spectra to derive accurate wavelengths for the aD6−yP6 multiplet of Fe II (UV 8) using both directly measured lines and Ritz wavelengths. Lines from this multiplet are important for astronomical tests of the invariance of the fine-structure constant on a cosmological time scale. We recommend a wavelength of 1608.45081 Å with one standard deviation uncertainty of 0.00007 Å for the aD69/2−yP67/2 transition.
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Fig. 1 Region of the Fe II a D 6 − y P 6 transitions. The labeled lines show the J values of the lower and upper energy levels, respectively.
Fig. 2 Calibration of wavenumbers in spectrum fe0409.002 in Table 2 using Ar II standards from  and iron standards from [18, 19]. The error bars represent the statistical uncertainty in the measurement of the wavenumber.
Fig. 3 Phase in the master spectrum, k19, used in . The insert shows the residual phase after fitting the points to an eleventh-order polynomial.
Fig. 4 Comparison of wavenumbers in the master spectrum, k19, calibrated from Ar II standards from  and iron standards from [18, 19] adjusted to the scale of . The error bars represent the statistical uncertainty in the measurement of the wavenumber.
Fig. 5 Comparison of wavenumbers in the master spectrum, k19, with those in i6, the main spectrum contributing to Table 4 of  in this wavelength region.
Fig. 6 Partial term diagram of Fe II showing the determination of the y P 6 levels using transitions in the UV and visible regions.
Fig. 7 Figure 3 from , with all of the Ge I wavenumbers reduced by 1.4 parts in 10 8 and the Fe I and Fe II wavenumbers increased by 3.9 parts in 10 8 . The mean value of k eff for the Ge I wavenumbers is ( 1.221 ± 0.020 ) × 10 − 6 , in agreement within the joint uncertainties with the value of ( 1.206 ± 0.020 ) × 10 − 6 from the Fe I and Fe II lines.

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