Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: High-precision laser spectroscopy of the CO A$^1\Pi$ - X$^1\Sigma^+$
  (2,0), (3,0) and (4,0) bands   High-precision two-photon Doppler-free frequency measurements have been
performed on the CO A$^1\Pi$ - X$^1\Sigma^+$ fourth-positive system (2,0),
(3,0), and (4,0) bands. Absolute frequencies of forty-three transitions, for
rotational quantum numbers up to $J = 5$, have been determined at an accuracy
of $1.6\times10^{-3}$ cm$^{-1}$, using advanced techniques of two-color 2+1'
resonance-enhanced multi-photon ionization, Sagnac interferometry,
frequency-chirp analysis on the laser pulses, and correction for AC-Stark
shifts. The accurate transition frequencies of the CO A$^1\Pi$ - X$^1\Sigma^+$
system are of relevance for comparison with astronomical data in the search for
possible drifts of fundamental constants in the early universe. The present
accuracies in laboratory wavelengths of $\Delta\lambda/\lambda = 2 \times
10^{-8}$ may be considered exact for the purpose of such comparisons.

21
The categories of this paper are: adap-org
32
The categories of this paper are: atom-ph
15
The categories of this paper are: nucl-ex
6
The categories of this paper are: astro-ph
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The categories of this paper are: acc-phys
33
The categories of this paper are: plasm-ph
1
The categories of this paper are: cs
28
The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
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The categories of this paper are: comp-gas
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The categories of this paper are: dg-ga
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The categories of this paper are: eess
2
The categories of this paper are: math
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The categories of this paper are: q-fin
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The categories of this paper are: stat
36
The categories of this paper are: cmp-lg