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Journal of the Optical Society of America B

Journal of the Optical Society of America B

| OPTICAL PHYSICS

  • Editor: Henry van Driel
  • Vol. 28, Iss. 4 — Apr. 1, 2011
  • pp: 737–745
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Wavelengths of the 3 d 6 ( D 5 ) 4 s a D 6 3 d 5 ( S 6 ) 4 s 4 p y P 6 multiplet of Fe II (UV 8)

Gillian Nave and Craig J. Sansonetti  »View Author Affiliations


JOSA B, Vol. 28, Issue 4, pp. 737-745 (2011)
http://dx.doi.org/10.1364/JOSAB.28.000737


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Abstract

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 a D 6 y P 6 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 a D 6 9 / 2 y P 6 7 / 2 transition.

1. INTRODUCTION

The universality and constancy of the laws of nature rely on the invariance of the fundamental constants. However, some recent measurements of quasar [quasi-stellar objects (QSOs)] absorption-line spectra suggest that the fine-structure constant α [1

1. P. J. Mohr, B. N. Taylor, and D. B. Newell, “The 2006 CODATA Recommended Values of the Fundamental Physical Constants” (Web Version 5.2). Available: http://physics.nist.gov/constants [2010, May 11]. National Institute of Standards and Technology, Gaithersburg, MD 20899 (2007).

] may have had a different value during the early universe [2

2. M. T. Murphy, J. K. Webb, and V. V. Flambaum, “Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra,” Mon. Not. R. Astron. Soc. 345, 609–638 (2003). [CrossRef]

]. Other measurements (e.g., [3

3. H. Chand, R. Srianand, P. Petitjean, B. Aracil, R. Quast, and D. Reimers, “Variation of the fine-structure constant: very high resolution spectrum of QSO HE 0515-4414,” Astron. Astrophys. 451, 45–56 (2006). [CrossRef]

]) do not show any change. The attempt to resolve these discrepancies can probe deviations from the standard model of particle physics and thus provide tests of modern theories of fundamental interactions that are hard to attain in other ways.

2. PREVIOUS MEASUREMENTS OF THE aD6yP6 MULTIPLET

The spectra in [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

] were calibrated with respect to the Ge standards of Kaufman and Andrew [17

17. V. Kaufman and K. L. Andrew, “Germanium vacuum ultraviolet Ritz standards,” J. Opt. Soc. Am. 52, 1223–1237 (1962). [CrossRef]

]. In addition to the spectra used in that paper, we recorded a spectrum using FT spectroscopy with a pure iron cathode that covers the wavelength region of the aD6yP6 multiplet (fe1115 in Table 2). It was calibrated with iron lines measured in one of the spectra used for [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

] (lp0301 in Table 2). The resulting value for the wave length of the aD69/2yP67/2 line was 1608.45050±0.00004Å, 1.5×107 times smaller than the wavelength obtained from the archival spectra and outside their joint uncertainty. This inconsistency is also larger than the uncertainty required for measurements of possible changes in α.

3. SUMMARY OF CURRENT EXPERIMENTAL DATA

4. CALIBRATION OF FT SPECTRA

All of the spectra were calibrated assuming a linear FT wavenumber scale, so that in principle only one reference line is required to put the measurements on an absolute scale. In practice, many lines are used. To obtain the absolute wavenumbers, a multiplicative correction factor, keff, is derived from the reference lines and applied to each observed wavenumber σobs so that
σcorr=(1+keff)σobs,
(1)
where σcorr is the corrected wavenumber.

All the spectra in [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

] (3830 to 5760Å) and [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] (1830 to 3850Å) trace their calibration to 28 Ar II lines in the visible region. The original calibration in [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

, 19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] used the wavenumbers by Norlén [24

24. G. Norlén, “Wavelengths and energy levels of Ar I and Ar II based on new interferometric measurements in the region 34009800Å,” Phys. Scr. 8, 249–268 (1973). [CrossRef]

] for these lines. Norlén calibrated these Ar II lines with respect to Kr86 I lines emitted from an electrodeless microwave discharge lamp that had in turn been calibrated with respect to an Engelhard lamp, which was the prescribed source for the primary wavelength standard at the time of his measurements. The estimated standard uncertainty of Norlén’s Ar II wavenumbers varies from 0.0007cm1 at 19,429cm1 to 0.001cm1 at 22,992cm1. The Ar II lines were used to calibrate a “master spectrum” (spectrum k19 in Table 2). Additional spectra of both Fe–Ne and Fe–Ar hollow cathode lamps covering wavelengths from 2778 to 7387Å were calibrated from this master spectrum.

The UV spectra reported in [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] were calibrated with respect to the results by Learner and Thorne [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

] by using a bridging spectrum. This bridging spectrum used two different detectors, one on each output of the FT spectrometer. The first overlapped with the visible wavenumbers in [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

] in order to obtain a wavenumber calibration and the second covered the UV wavenumbers being measured. Since the two outputs of the FT spectrometer are not exactly in antiphase, the resulting phase correction has a discontinuity in the region around 35,000cm1 where the two detectors overlap, as shown in Fig. 1 of [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

]. The full procedure is described in detail in [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

].

The 28 Ar II lines used as wavenumber standards in [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

, 19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] were subsequently remeasured by Whaling et al. [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

] using FT spectroscopy with molecular CO lines as standards. The uncertainty of these measurements is 0.0002cm1. The molecular CO standards used in [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

] were measured using heterodyne frequency spectroscopy with an uncertainty of around 1109 and are ultimately traceable to the cesium primary standard [26

26. A. G. Maki and J. S. Wells, “New wavenumber calibration tables from heterodyne frequency measurements,” J. Res. Natl. Inst. Stand. Technol. 97, 409–470 (1992).

]. The wavenumbers by Whaling et al. [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

] are systematically higher than those by Norlén [24

24. G. Norlén, “Wavelengths and energy levels of Ar I and Ar II based on new interferometric measurements in the region 34009800Å,” Phys. Scr. 8, 249–268 (1973). [CrossRef]

] by 6.7±0.8 parts in 108, corresponding to a wavenumber discrepancy of about 0.0014cm1 at 21,000cm1. Since the results by Whaling et al. [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

] are more accurate and precise than those by Norlén [24

24. G. Norlén, “Wavelengths and energy levels of Ar I and Ar II based on new interferometric measurements in the region 34009800Å,” Phys. Scr. 8, 249–268 (1973). [CrossRef]

], all the wavenumbers in [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

, 19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] and Table 3 of [14

14. G. Nave, S. Johansson, and A. P. Thorne, “Precision vacuum- ultraviolet wavelengths of Fe II measured by Fourier-transform and grating spectrometry,” J. Opt. Soc. Am. B 14, 1035–1042 (1997). [CrossRef]

] have been increased by 6.7 parts in 108 wherever they are used in the current work.

To present accurate wavenumbers for Fe II lines around 1600Å, it is necessary first to confirm the accuracy of the iron lines in the visible region that were calibrated with respect to selected lines of Ar II lines [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

] in order to investigate the accuracy with which this calibration is transferred to the VUV and to resolve the discrepancy between iron and germanium standard wavelengths identified in [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

].

4A. Calibration of the Visible-Region Spectra

In order to confirm the calibration of the master spectrum, k19, used in [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

, 19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

], we took additional spectra using the NIST 2m FT spectrometer [23

23. G. Nave, C. J. Sansonetti, and U. Griesmann, “Progress on the NIST IR-vis-UV Fourier transform spectrometer,” in Fourier Transform Spectroscopy: Methods and Applications, Vol. 3 of OSA Technical Digest Series (Optical Society of America, 1997), pp. 38–40.

]. The source was a water-cooled high-current hollow cathode lamp with a current of 1.5A and argon at pressures of 130 to 330Pa (1 to 2.5Torr). The spectra covered the region 8500 to 37,000cm1 with resolutions of either 0.02 or 0.03cm1. A 1mm aperture was used in order to minimize possible illumination effects. The detector was a silicon photodiode detector with a 2mm×2mm active area.

The spectrometer was aligned optimally using a diffused, expanded beam from a helium–neon laser, ensuring that the modulation of the laser fringes was maximized throughout the 2m scan. Before recording some of the spectra, the spectrometer was deliberately misaligned and realigned in order to test whether small misalignments that could not be detected using our alignment procedure affected the wavenumber scale.

Figure 2 shows the calibration of one of our spectra using Ar II and iron lines from [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

, 19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

, 25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

] as standards. The calibration constant keff does not depend on wavenumber and is the same for all three sets of standards to within 1108 when the iron lines from [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

, 19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] are adjusted to the wavenumber scale of [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

]. The possibility of shifts due to nonuniform illumination of the aperture were investigated by taking a spectrum with the 5mm diameter image of the hollow cathode lamp offset from the 1mm aperture by about 2mm. This spectrum also shows good agreement between the Ar II and iron calibrations.

Many of the early interferograms from the NSO FT spectrometer were asymmetrically sampled, with a much larger number of points on one side of zero optical path difference than the other. An FT of an asymmetrically-sampled interferogram gives a spectrum with a large, antisymmetric imaginary part [27

27. R. C. M. Learner, A. P. Thorne, I. Wynne-Jones, J. W. Brault, and M. C. Abrams, “Phase correction of emission line Fourier transform spectra,” J. Opt. Soc. Am. A 12, 2165–2171 (1995). [CrossRef]

]. A small error in the phase correction causes a small part of this antisymmetric imaginary part to be rotated into the real part of the spectrum, distorting the line profiles and causing a wavenumber shift. The zero optical path difference in spectrum k19 is roughly one fifth of the way through the interferogram. A Gaussian profile with a FWHM of w produces a wavenumber shift of roughly 0.3w per radian of phase error, as shown in Fig. 3 of [27

27. R. C. M. Learner, A. P. Thorne, I. Wynne-Jones, J. W. Brault, and M. C. Abrams, “Phase correction of emission line Fourier transform spectra,” J. Opt. Soc. Am. A 12, 2165–2171 (1995). [CrossRef]

].

We conclude that the wavenumbers measured in the master spectrum, k19, are accurate. Although results from this spectrum were used in [18

18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

] and Table 3 in [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

], it did not dominate the weighted average values reported in these papers.

4B. Calibration of the UV Spectra

Tables 4 and 5 in [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] cover wavenumbers from 33,695 to 54,637cm1 in Fe I and Fe II, respectively. The wavenumbers in these tables were measured using the VUV FT spectrometer at IC. The calibration of these spectra was transferred from the master spectrum (k19 in Table 2) using a bridging spectrum (i56 in Table 2), as described in Section 4. The principal spectrum covering wavenumbers below 35,000cm1 in Table 4 of [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] is i6 in Table 2. It overlaps with the master spectrum between 33,000 and 34,000cm1. Figure 5 shows a comparison of wavenumbers in i6 with the master spectrum k19. The wavenumbers in spectrum i6 are systematically smaller than in k19 by 3.9±0.5 parts in 108. Although the region of overlap of i6 with k19 is small and thus insensitive to nonlinearities in the wavenumber scale, this result supports our earlier speculation in [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

] that the calibration of the UV data using the bridging spectrum may be incorrect. Based on the comparison of Fig. 5, we conclude the wavenumbers in Tables 4 and 5 in [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] should be increased by 10.6 parts in 108, consisting of 3.9 parts in 108 to correct the transfer of the calibration to the UV, and an additional 6.7 parts in 108 to put all the spectra on the wavenumber scale by Whaling et al. [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

].

We compared our corrected values for iron lines in the UV to the results by Aldenius et al. [7

7. M. Aldenius, S. Johansson, and M. T. Murphy, “Accurate laboratory ultraviolet wavelengths for quasar absorption-line constraints on varying fundamental constants,” Mon. Not. R. Astron. Soc. 370, 444–452 (2006). [CrossRef]

, 8

8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

], who present wavenumbers of iron lines measured in a high-current hollow cathode lamp using a UV FT spectrometer similar to the one used in [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

]. Instead of recording a pure iron spectrum, they included small pieces of Mg, Ti, Cr, Mn, and Zn in their Fe cathode. This ensured that spectral lines due to all of these species were placed on the same wavenumber scale, which was calibrated using the Ar II lines by Whaling et al. [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

]. Table 3 compares the wavenumbers of [8

8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

] with the corrected values of [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

]. Although the wavenumbers in [8

8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

] agree with our revised val ues within their joint uncertainties, they are systematically smaller by 3.7 parts in 108. Although this might suggest that it is incorrect to increase the wavenumbers of [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

], it might also indicate that the wavenumbers in [8

8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

] need to be increased.

Fortunately, there are data that allow us to test these alternatives. In addition to iron lines, the spectra in [8

8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

] contained four lines due to Mg I and Mg II that have since been measured using frequency comb spectroscopy [9

9. E. J. Salumbides, S. Hannemann, K. S. E. Eikema, and W. Ubachs, “Isotopically resolved calibration of the 285nm Mg I resonance line for comparison with quasar absorptions,” Mon. Not. R. Astron. Soc. 373, L41–L44 (2006). [CrossRef]

, 10

10. S. Hannemann, E. J. Salumbides, S. Witte, R. T. Zinkstok, E.-J. van Duijn, K. S. E. Eikema, and W. Ubachs, “Frequency metrology on the Mg3s2S13s4pP1 line for comparison with quasar data,” Phys. Rev. A 74, 012505 (2006). [CrossRef]

, 11

11. V. Batteiger, S. Knünz, M. Herrmann, G. Saathoff, H. A. Schüssler, B. Bernhardt, T. Wilken, R. Holzwarth, T. W. Hänsch, and T. Udem, “Precision spectroscopy of the 3s-3p fine-structure doublet in Mg+,” Phys. Rev. A 80, 022503 (2009). [CrossRef]

] with much higher accuracy than achievable using FT spectroscopy. Table 4 compares the wavenumbers of these four magnesium lines from [8

8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

] with those derived from frequency comb measurements of isotopically pure values. For this comparison, the results of [8

8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

] have been increased by 3.7 parts in 108, as suggested by the comparison of Fe II lines in Table 3. With this adjustment, the results by Aldenius agree with the frequency comb values within their joint uncertainties, having a mean deviation of 0.7±3 parts in 108. Without the adjustment, the mean deviation would be (4±3)×108.

We conclude that the wavenumbers in Tables 4 and 5 in [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] should be increased by 10.6 parts in 1083.9 parts in 108 to correct for the incorrect transfer of the calibration from the master spectrum to the UV and 6.7 parts in 108 to put all of the spectra on the scale by Whaling et al. [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

]. We have performed this correction in the following sections of this paper. The wavenumbers by Aldenius [8

8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

] should be increased by 3.7 parts in 108 to put them on the same scale. This adjustment of scale brings the measurements of lines of Mg I and Mg II in [8

8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

] into agreement with the more accurate frequency comb values [9

9. E. J. Salumbides, S. Hannemann, K. S. E. Eikema, and W. Ubachs, “Isotopically resolved calibration of the 285nm Mg I resonance line for comparison with quasar absorptions,” Mon. Not. R. Astron. Soc. 373, L41–L44 (2006). [CrossRef]

, 10

10. S. Hannemann, E. J. Salumbides, S. Witte, R. T. Zinkstok, E.-J. van Duijn, K. S. E. Eikema, and W. Ubachs, “Frequency metrology on the Mg3s2S13s4pP1 line for comparison with quasar data,” Phys. Rev. A 74, 012505 (2006). [CrossRef]

, 11

11. V. Batteiger, S. Knünz, M. Herrmann, G. Saathoff, H. A. Schüssler, B. Bernhardt, T. Wilken, R. Holzwarth, T. W. Hänsch, and T. Udem, “Precision spectroscopy of the 3s-3p fine-structure doublet in Mg+,” Phys. Rev. A 80, 022503 (2009). [CrossRef]

].

5. WAVENUMBERS OF a6Dy6P TRANSITIONS

The wavenumbers of the aD6yP6 transitions can be obtained either from direct measurements or from energy levels derived from a larger set of experimental data (Ritz wavenumbers). Direct measurements will have larger uncertainties due to the cumulative addition of the uncertainties in the transfer of the calibration from the visible to the UV. Ritz wave numbers are more accurate due to the increased redundancy, but use of a large set of experimental data to derive the energy levels makes it less clear exactly how the Ritz wavenumbers are derived. We illustrate this process by using a small subset of the strongest transitions that determine the yP6 levels that are present in the visible and UV regions of the spectrum where we have corrected the wavenumber calibration.

The yP6 levels can be determined from three sets of lines in the UV and visible regions, as shown in Fig. 6. The first set of nine lines near 2350Å determines the three 3d6(D6)4pzP6 levels. All nine lines are present in archival spectra from IC, which we have recalibrated using the results of Subsection 4B. Two of the nine lines are blended with other lines and a third, between aD9/26 and zP7/26, is self-absorbed in the IC spectra. These lines are unsuitable for determining the zP6 levels. The recalibrated values of the remaining six lines are shown in the fourth column of Table 5. Each line is observed with a signal-to-noise ratio of more than 100 in at least eight spectra, all of which agree within 0.006cm1. The wavenumbers in Table 5 are weighted mean values of the individual measurements and the standard deviation in the last decimal place is given in parentheses following the wavenumber. The lower levels in the third column are determined from 10 to 20 different transitions to upper levels and have been optimized to the archival spectra with the program LOPT [29

29. A. E. Kramida, “The program LOPT for least-squares optimization of energy levels,” Comput. Phys. Commun. 182, 419–434 (2011). [CrossRef]

] (described later in this section). The total standard uncertainty in the upper levels includes the calibration uncertainty of 2.3×108 times the level value.

The second set of three transitions around 5000Å determines the 3d54s2aS5/26 level from the three zP6 levels. These lines are present in k19 and other archival spectra taken at the NSO that we have recalibrated to correspond to the wavenumber scale by Whaling et al. [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

]. Each line is present in five spectra, all of which agree within 0.0035cm1. Wavenumbers for these transitions are shown in Table 6 and give a mean value of (23,317.6344±0.0010)cm1 for the 3d54s2aS5/26 level.

Finally, the yP6 levels can be determined from the aS5/26 level from three lines around 2580Å, present in the IC spectra. The recalibrated wavenumbers are shown in Table 7 with the resulting yP6 level values. These values were used to calculate Ritz wavenumbers for the aD6yP6 transitions, as shown in the third column of Table 8.

Alternate values for the Ritz wavenumbers of the aD6yP6 transitions can be obtained from energy levels optimized using wavenumbers from the archival Fe II spectra from the NSO and IC corrected according to Subsections 4A, 4B. The program LOPT [29

29. A. E. Kramida, “The program LOPT for least-squares optimization of energy levels,” Comput. Phys. Commun. 182, 419–434 (2011). [CrossRef]

] was used to derive optimized values for 939 energy levels from 9567 transitions. Weights were assigned proportional to the inverse of the estimated variance of the wavenumber. Lines with more than one possible classification, lines that were blended, or lines for which the identification was questionable were assigned a low weight. Two iterations were made. In the first, lines connecting the lowest aD6 term to higher 3d6(D5)4p levels were assigned a weight proportional to the inverse of the statistical variance of the wavenumber, omitting the calibration uncertainty. This was done to obtain accurate values and uncertainties for the aD6 intervals. These intervals were determined from differences between lines close to one another in the same spectrum sharing the same calibration factor. Hence, the calibration uncertainty does not contribute to the uncertainty in the relative values of these energy levels. The values of the aD6 levels obtained in this iteration are given in the third column of Table 5. In the second iteration, the aD6 levels were fixed to the values and uncertainties determined from the first iteration. The weights of the aD63d6(D5)4p transitions were assigned by combining in quadrature the statistical uncertainty in the measurement of the line position and the calibration uncertainty in order to obtain accurate uncertainties for the 3d6(D5)4p and higher levels. The values of the yP6 levels are given in the fourth column of Table 7. Ritz wavenumbers for the aD63d6(D5)4p transitions based on these globally optimized level values are presented in the fifth column of Table 8.

The corrected experimental wavenumbers from the archival spectra are given in the fourth column of Table 8. The main contribution to the uncertainty in the experimental wavenumbers is from the calibration and consists of two components—the uncertainty in the standards and the uncertainty in calibrating the spectrum. The calibration uncertainty is common to all lines in the calibrated spectrum and must be added to the uncertainties of wavenumbers measured using transfer standards, rather than added in quadrature as would be the case for random errors. Hence the uncertainty in the wavenumbers increases with each calibration step, resulting in larger uncertainties at the shortest wavenumbers, which are furthest from the calibration standards. The experimental standard uncertainties in Table 8 are determined by combining in quadrature the statistical uncertainty in determining the line position and the calibration uncertainty of 4×108 times the wavenumber. The experimental wavenumber and both Ritz wavenumbers agree within their joint uncertainties. The Ritz wavenumbers determined from optimized energy levels have the smallest uncertainties. Wavelengths corresponding to these wavenumbers are given in the seventh column.

6. RE-EXAMINATION OF FE II WAVENUMBERS FROM [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

]

The Fe I lines in [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

] were calibrated with respect to lines of Ge I. Figure 3 of that paper showed that the calibration factor keff derived from Fe I and Fe II lines is smaller than that derived from Ge I by 6.5 parts in 108. We attributed this to a possible problem in the transfer of the wavenumber calibration of the Fe I and Fe II lines from the region of the Ar II wavenumber standards to the VUV, thus suggesting that the wavenumber standards in [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] are too small. In Subsection 4B, we confirmed that the wavenumbers in Tables 4 and 5 of [19

19. G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

] should be increased by 3.9 parts in 108 due to the transfer of the calibration. This reduces, but does not fully explain, the calibration discrepancy in [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

].

The Ge I lines used to calibrate the spectra in [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

] were measured by Kaufman and Andrew [17

17. V. Kaufman and K. L. Andrew, “Germanium vacuum ultraviolet Ritz standards,” J. Opt. Soc. Am. 52, 1223–1237 (1962). [CrossRef]

]. The wavenumber standard they used was the 5462Å line of Hg198 emitted by an electrodeless discharge lamp maintained at a temperature of 19°C, containing Ar at a pressure of 400Pa (3Torr). The vacuum wavelength of this line was assumed to be 5462.27075Å. This value was based on a vacuum wavelength of 5462.27063Å measured in the same lamp at 7°C [30

30. V. Kaufman, “Wavelengths, energy levels, and pressure shifts in mercury 198,” J. Opt. Soc. Am. 52, 866–870 (1962). [CrossRef]

], with an adjustment for the different temperature using the measurements of Emara [31

31. S. H. Emara, “Wavelength shifts in Hg198 as a function of temperature,” J. Res. Natl. Bur. Stand. 65A, 473–474 (1961).

]. The 5462Å line was remeasured by Salit et al. [32

32. M. L. Salit, C. J. Sansonetti, D. Veza, and J. C. Travis, “Investigation of single-factor calibration of the wave-number scale in Fourier-transform spectroscopy,” J. Opt. Soc. Am. B 21, 1543–1550 (2004). [CrossRef]

] using a temperature of 8°C. A value of (5462.27085±0.00007)Å was obtained. More recent work by Sansonetti and Veza [33

33. C. J. Sansonetti and D. Veza, “Doppler-free measurement of the 546nm line of mercury,” J. Phys. B 43, 205003 (2010). [CrossRef]

] gives the wavelength of this line as 5462.270825(11)Å, in agreement with [32

32. M. L. Salit, C. J. Sansonetti, D. Veza, and J. C. Travis, “Investigation of single-factor calibration of the wave-number scale in Fourier-transform spectroscopy,” J. Opt. Soc. Am. B 21, 1543–1550 (2004). [CrossRef]

], but more precise. Adoption of this value for the wavelength of the Hg198 line implies that all of the Ge I wavenumbers in [17

17. V. Kaufman and K. L. Andrew, “Germanium vacuum ultraviolet Ritz standards,” J. Opt. Soc. Am. 52, 1223–1237 (1962). [CrossRef]

] should be decreased by 1.4 parts in 108. Figure 7 shows how Fig. 3 in [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

] (Spectrum lp0301 in Table 2) changes with the adjustment of both the iron and germanium wavenumbers. The calibrations based on Ge and Fe lines now differ by only 1.5 parts in 108, which is within the joint uncertainties. We thus conclude that the calibration derived from Fe I and Fe II lines is in agreement with that derived from Ge I when both sets of standards are adjusted to correspond with the most recent measurements.

Spectrum lp0301 in Table 2 can be used to calibrate spectrum fe1115 in Table 2, referred to in the last paragraph of Section 2. A value of (62,171.634±0.006)cm1 is obtained for the wavenumber of the aD9/26yP7/26 line, corresponding to a wavelength of (1608.45057±0.00016)Å. This disagrees with the Ritz value by 1.7 times the joint uncertainty and marginally disagrees with the experimental values of Table 8. The mean difference in the experimental values for all nine aD6yP6 lines is (0.008±0.004)cm1. We believe this difference is due to a small slope in the calibration of lp0301, but we have been unable to confirm this with our data. The principal contributors to the uncertainty are the uncertainty in the iron and germanium standards, the uncertainty in calibrating the spectrum in [13

13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

] from these standards, and the uncertainty in calibrating spectrum fe1115 from spectrum lp0301.

7. CONCLUSIONS

We investigated the wavenumber scale of published Fe I and Fe II lines using new spectra recorded with the NIST 2m FT spectrometer and a reanalysis of archival spectra. Our new spectra confirm the wavenumber scale of visible-region iron lines calibrated using the Ar II wavenumber standards by Whaling et al. [25

25. W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

].

Having confirmed the wavenumber scale of iron lines in the visible and UV regions, we have used lines from these spectra to derive Ritz values for the wavenumbers and wavelengths of lines in the aD6yP6 multiplet of Fe II (UV 8). Ritz wavenumbers derived using two different methods agree with one another and with directly measured wavenumbers within the joint uncertainties. We recommend a value of 1608.45081±0.00007Å for the wavelength of the aD9/26yP7/26 line of Fe II, which is an important line for detection of changes in the fine-structure constant during the history of the universe using quasar absorption-line spectra.

ACKNOWLEDGMENTS

We thank Michael T. Murphy for alerting us to the importance of the Fe II line at 1608Å. We also thank Anne P. Thorne and Juliet C. Pickering for helpful discussions on the calibration of FT spectrometers and the linearity of the FT wavenumber scale. This work was partially supported by the National Aeronautics and Space Administration (NASA) interagency agreement NNH10AN38I.

Table 1. Proposed Corrections to Previous Papers

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Table 2. Summary of Spectra

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Table 3. Comparison of Wavenumbers of Fe Lines in [8] and Adjusted Wavenumbers in [19]a

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Table 4. Comparison of Adjusted Wavenumbers of Mg Lines in [8] with Frequency Comb Measurements Taken from [9, 10, 11]a

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Table 5. Determination of the zP6 Levels of Fe II from Transitions to the Ground Term around 2350Å a

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Table 6. Determination of the aS5/26 Level of Fe II Using Transitions from the zP6 Levelsa

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Table 7. Determination of the yP6 Levels of Fe II from Transitions to the aS6 Levela

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Table 8. Experimental and Ritz Wavenumbers for the aD6yP6 Multipleta

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Fig. 1 Region of the Fe II aD6yP6 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 [25] 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 [18]. 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 [25] and iron standards from [18, 19] adjusted to the scale of [25]. 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 [19] in this wavelength region.
Fig. 6 Partial term diagram of Fe II showing the determination of the yP6 levels using transitions in the UV and visible regions.
Fig. 7 Figure 3 from [13], with all of the Ge I wavenumbers reduced by 1.4 parts in 108 and the Fe I and Fe II wavenumbers increased by 3.9 parts in 108. The mean value of keff for the Ge I wavenumbers is (1.221±0.020)×106, in agreement within the joint uncertainties with the value of (1.206±0.020)×106 from the Fe I and Fe II lines.
1.

P. J. Mohr, B. N. Taylor, and D. B. Newell, “The 2006 CODATA Recommended Values of the Fundamental Physical Constants” (Web Version 5.2). Available: http://physics.nist.gov/constants [2010, May 11]. National Institute of Standards and Technology, Gaithersburg, MD 20899 (2007).

2.

M. T. Murphy, J. K. Webb, and V. V. Flambaum, “Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra,” Mon. Not. R. Astron. Soc. 345, 609–638 (2003). [CrossRef]

3.

H. Chand, R. Srianand, P. Petitjean, B. Aracil, R. Quast, and D. Reimers, “Variation of the fine-structure constant: very high resolution spectrum of QSO HE 0515-4414,” Astron. Astrophys. 451, 45–56 (2006). [CrossRef]

4.

V. A. Dzuba, V. V. Flambaum, and J. K. Webb, “Space-time variation of physical constants and relativistic corrections in atoms,” Phys. Rev. Lett. 82, 888–891 (1999). [CrossRef]

5.

J. N. Bahcall, W. L. W. Sargent, and M. Schmidt, “An analysis of the absorption spectrum of 3c 191,” Astrophys. J. 149, L11–L15 (1967). [CrossRef]

6.

J. C. Pickering, A. P. Thorne, and J. K. Webb, “Precise laboratory wavelengths of the Mg I and Mg II resonance transitions at 2853, 2803 and 2796 Angstroms,” Mon. Not. R. Astron. Soc. 300, 131–134 (1998). [CrossRef]

7.

M. Aldenius, S. Johansson, and M. T. Murphy, “Accurate laboratory ultraviolet wavelengths for quasar absorption-line constraints on varying fundamental constants,” Mon. Not. R. Astron. Soc. 370, 444–452 (2006). [CrossRef]

8.

M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]

9.

E. J. Salumbides, S. Hannemann, K. S. E. Eikema, and W. Ubachs, “Isotopically resolved calibration of the 285nm Mg I resonance line for comparison with quasar absorptions,” Mon. Not. R. Astron. Soc. 373, L41–L44 (2006). [CrossRef]

10.

S. Hannemann, E. J. Salumbides, S. Witte, R. T. Zinkstok, E.-J. van Duijn, K. S. E. Eikema, and W. Ubachs, “Frequency metrology on the Mg3s2S13s4pP1 line for comparison with quasar data,” Phys. Rev. A 74, 012505 (2006). [CrossRef]

11.

V. Batteiger, S. Knünz, M. Herrmann, G. Saathoff, H. A. Schüssler, B. Bernhardt, T. Wilken, R. Holzwarth, T. W. Hänsch, and T. Udem, “Precision spectroscopy of the 3s-3p fine-structure doublet in Mg+,” Phys. Rev. A 80, 022503 (2009). [CrossRef]

12.

M. T. Murphy, J. K. Webb, and V. V. Flambaum, “Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra,” Mon. Not. R. Astron. Soc. 345, 609–638 (2003). [CrossRef]

13.

G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935Å,” J. Opt. Soc. Am. B 21, 442–453 (2004). [CrossRef]

14.

G. Nave, S. Johansson, and A. P. Thorne, “Precision vacuum- ultraviolet wavelengths of Fe II measured by Fourier-transform and grating spectrometry,” J. Opt. Soc. Am. B 14, 1035–1042 (1997). [CrossRef]

15.

S. Johansson, “The spectrum and term system of Fe II,” Phys. Scr. 18, 217–265 (1978). [CrossRef]

16.

J. C. Pickering, M. P. Donnelly, H. Nilsson, A. Hibbert, and S. Johansson, “The FERRUM project: experimental oscillator strengths of the UV 8 multiplet and other UV transitions from the yP6 levels of Fe II,” Astron. Astrophys. 396, 715–722 (2002). [CrossRef]

17.

V. Kaufman and K. L. Andrew, “Germanium vacuum ultraviolet Ritz standards,” J. Opt. Soc. Am. 52, 1223–1237 (1962). [CrossRef]

18.

R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045–2059 (1988). [CrossRef]

19.

G. Nave, R. C. M. Learner, A. P. Thorne, and C. J. Harris, “Precision Fe I and Fe II wavelengths in the ultraviolet spectrum of the iron-neon hollow-cathode lamp,” J. Opt. Soc. Am. B 8, 2028–2041 (1991). [CrossRef]

20.

A. P. Thorne, C. J. Harris, I. Wynne-Jones, R. C. M. Learner, and G. Cox, “A Fourier transform spectrometer for the vacuum ultraviolet: design and performance,” J. Phys. E 20, 54–60 (1987). [CrossRef]

21.

U. Griesmann, R. Kling, J. H. Burnett, and L. Bratasz, “NIST FT700 vacuum ultraviolet Fourier transform spectrometer: applications in ultraviolet spectrometry and radiometry,” Proc. SPIE 3818, 180–188 (1999). [CrossRef]

22.

J. W. Brault and M. C. Abrams, “DECOMP: a Fourier transform spectra decomposition program,” in High-Resolution Fourier Transform Spectroscopy, Vol. 6 of OSA Technical Digest Series (Optical Society of America, 1989), pp. 110–112.

23.

G. Nave, C. J. Sansonetti, and U. Griesmann, “Progress on the NIST IR-vis-UV Fourier transform spectrometer,” in Fourier Transform Spectroscopy: Methods and Applications, Vol. 3 of OSA Technical Digest Series (Optical Society of America, 1997), pp. 38–40.

24.

G. Norlén, “Wavelengths and energy levels of Ar I and Ar II based on new interferometric measurements in the region 34009800Å,” Phys. Scr. 8, 249–268 (1973). [CrossRef]

25.

W. Whaling, W. H. C. Anderson, M. T. Carle, J. W. Brault, and H. A. Zarem, “Argon ion linelist and level energies in the hollow-cathode discharge,” J. Quant. Spectrosc. Radiat. Transfer 53, 1–22 (1995). [CrossRef]

26.

A. G. Maki and J. S. Wells, “New wavenumber calibration tables from heterodyne frequency measurements,” J. Res. Natl. Inst. Stand. Technol. 97, 409–470 (1992).

27.

R. C. M. Learner, A. P. Thorne, I. Wynne-Jones, J. W. Brault, and M. C. Abrams, “Phase correction of emission line Fourier transform spectra,” J. Opt. Soc. Am. A 12, 2165–2171 (1995). [CrossRef]

28.

National Solar Observatory, “Digital library,” http://diglib.nso.edu/nso_user.html.

29.

A. E. Kramida, “The program LOPT for least-squares optimization of energy levels,” Comput. Phys. Commun. 182, 419–434 (2011). [CrossRef]

30.

V. Kaufman, “Wavelengths, energy levels, and pressure shifts in mercury 198,” J. Opt. Soc. Am. 52, 866–870 (1962). [CrossRef]

31.

S. H. Emara, “Wavelength shifts in Hg198 as a function of temperature,” J. Res. Natl. Bur. Stand. 65A, 473–474 (1961).

32.

M. L. Salit, C. J. Sansonetti, D. Veza, and J. C. Travis, “Investigation of single-factor calibration of the wave-number scale in Fourier-transform spectroscopy,” J. Opt. Soc. Am. B 21, 1543–1550 (2004). [CrossRef]

33.

C. J. Sansonetti and D. Veza, “Doppler-free measurement of the 546nm line of mercury,” J. Phys. B 43, 205003 (2010). [CrossRef]

OCIS Codes
(300.6210) Spectroscopy : Spectroscopy, atomic
(300.6300) Spectroscopy : Spectroscopy, Fourier transforms
(300.6540) Spectroscopy : Spectroscopy, ultraviolet

ToC Category:
Spectroscopy

History
Original Manuscript: November 29, 2010
Revised Manuscript: January 18, 2011
Manuscript Accepted: January 18, 2011
Published: March 15, 2011

Virtual Issues
April 14, 2011 Spotlight on Optics

Citation
Gillian Nave and Craig J. Sansonetti, "Wavelengths of the 3d6(5D)4s a6D−3d5(6S)4s4p y6P multiplet of Fe II (UV 8)," J. Opt. Soc. Am. B 28, 737-745 (2011)
http://www.opticsinfobase.org/josab/abstract.cfm?URI=josab-28-4-737


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References

  1. P. J. Mohr, B. N. Taylor, and D. B. Newell, “The 2006 CODATA Recommended Values of the Fundamental Physical Constants” (Web Version 5.2). Available: http://physics.nist.gov/constants [2010, May 11]. National Institute of Standards and Technology, Gaithersburg, MD 20899 (2007).
  2. M. T. Murphy, J. K. Webb, and V. V. Flambaum, “Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra,” Mon. Not. R. Astron. Soc. 345, 609-638 (2003). [CrossRef]
  3. H. Chand, R. Srianand, P. Petitjean, B. Aracil, R. Quast, and D. Reimers, “Variation of the fine-structure constant: very high resolution spectrum of QSO HE 0515-4414,” Astron. Astrophys. 451, 45-56 (2006). [CrossRef]
  4. V. A. Dzuba, V. V. Flambaum, and J. K. Webb, “Space-time variation of physical constants and relativistic corrections in atoms,” Phys. Rev. Lett. 82, 888-891 (1999). [CrossRef]
  5. J. N. Bahcall, W. L. W. Sargent, and M. Schmidt, “An analysis of the absorption spectrum of 3c 191,” Astrophys. J. 149, L11-L15 (1967). [CrossRef]
  6. J. C. Pickering, A. P. Thorne, and J. K. Webb, “Precise laboratory wavelengths of the Mg I and Mg II resonance transitions at 2853, 2803 and 2796 Angstroms,” Mon. Not. R. Astron. Soc. 300, 131-134 (1998). [CrossRef]
  7. M. Aldenius, S. Johansson, and M. T. Murphy, “Accurate laboratory ultraviolet wavelengths for quasar absorption-line constraints on varying fundamental constants,” Mon. Not. R. Astron. Soc. 370, 444-452 (2006). [CrossRef]
  8. M. Aldenius, “Laboratory wavelengths for cosmological constraints on varying fundamental constants,” Phys. Scr. T134, 014008 (2009). [CrossRef]
  9. E. J. Salumbides, S. Hannemann, K. S. E. Eikema, and W. Ubachs, “Isotopically resolved calibration of the 285 nm Mg I resonance line for comparison with quasar absorptions,” Mon. Not. R. Astron. Soc. 373, L41-L44 (2006). [CrossRef]
  10. S. Hannemann, E. J. Salumbides, S. Witte, R. T. Zinkstok, E.-J. van Duijn, K. S. E. Eikema, and W. Ubachs, “Frequency metrology on the Mg3s2S1-->3s4pP1 line for comparison with quasar data,” Phys. Rev. A 74, 012505 (2006). [CrossRef]
  11. V. Batteiger, S. Knünz, M. Herrmann, G. Saathoff, H. A. Schüssler, B. Bernhardt, T. Wilken, R. Holzwarth, T. W. Hänsch, and T. Udem, “Precision spectroscopy of the 3s-3p fine-structure doublet in Mg+,” Phys. Rev. A 80, 022503 (2009). [CrossRef]
  12. M. T. Murphy, J. K. Webb, and V. V. Flambaum, “Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra,” Mon. Not. R. Astron. Soc. 345, 609-638 (2003). [CrossRef]
  13. G. Nave and C. J. Sansonetti, “Reference wavelengths in the spectra of Fe, Ge, and Pt in the region near 1935 Å,” J. Opt. Soc. Am. B 21, 442-453 (2004). [CrossRef]
  14. G. Nave, S. Johansson, and A. P. Thorne, “Precision vacuum-ultraviolet wavelengths of Fe II measured by Fourier-transform and grating spectrometry,” J. Opt. Soc. Am. B 14, 1035-1042(1997). [CrossRef]
  15. S. Johansson, “The spectrum and term system of Fe II,” Phys. Scr. 18, 217-265 (1978). [CrossRef]
  16. J. C. Pickering, M. P. Donnelly, H. Nilsson, A. Hibbert, and S. Johansson, “The FERRUM project: experimental oscillator strengths of the UV 8 multiplet and other UV transitions from the yP6 levels of Fe II,” Astron. Astrophys. 396, 715-722(2002). [CrossRef]
  17. V. Kaufman and K. L. Andrew, “Germanium vacuum ultraviolet Ritz standards,” J. Opt. Soc. Am. 52, 1223-1237 (1962). [CrossRef]
  18. R. C. M. Learner and A. P. Thorne, “Wavelength calibration of Fourier-transform emission spectra with applications to Fe I,” J. Opt. Soc. Am. B 5, 2045-2059 (1988). [CrossRef]
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