Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Reference wavelengths of Si II, C II, Fe I, and Ni II   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 10$^8$,
corresponding to a radial velocity of $\approx$~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 45 lines with wavelength
uncertainties over an order of magnitude smaller than previous measurements.
Improved wavelengths for 8 lines of Ni II were measured and Ritz wavelengths
from optimized energy levels 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.

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The categories of this paper are: quant-ph
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The categories of this paper are: nucl-th
7
The categories of this paper are: hep-th
33
The categories of this paper are: plasm-ph
31
The categories of this paper are: ao-sci
28
The categories of this paper are: alg-geom
9
The categories of this paper are: nlin
37
The categories of this paper are: supr-con
8
The categories of this paper are: hep-ex
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The categories of this paper are: econ
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The categories of this paper are: stat
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The categories of this paper are: q-fin
36
The categories of this paper are: cmp-lg
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The categories of this paper are: cond-mat
32
The categories of this paper are: atom-ph
5
The categories of this paper are: gr-qc