Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Precise frequency measurements of the D-lines and fine-structure
  interval in K   We use a diode laser locked to a Rb transition as the frequency reference
along with a scanning Michelson interferometer to make precise measurements on
the $D_1$ and $D_2$ lines of potassium. The Rb reference frequency is known
with sub-MHz accuracy. We obtain the following values for the energy levels: 12
985.169 60(13)(15) cm$^{-1}$ for the $4P_{1/2}$ state ($D_1$ line); 13 042.875
95(14)(15) cm$^{-1}$ for the $4P_{3/2}$ state ($D_2$ line); and 57.706
35(19)(15) cm$^{-1}$ for the $4P_{3/2}-4P_{1/2}$ fine-structure interval. The
leading source of systematic error cancels in the determination of the
interval. The errors represent about an order of magnitude improvement over
tabulated values.

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The categories of this paper are: atom-ph
7
The categories of this paper are: hep-th
15
The categories of this paper are: nucl-ex
14
The categories of this paper are: math-ph
6
The categories of this paper are: astro-ph
18
The categories of this paper are: econ
8
The categories of this paper are: hep-ex
20
The categories of this paper are: acc-phys
19
The categories of this paper are: eess
29
The categories of this paper are: funct-an
28
The categories of this paper are: alg-geom
4
The categories of this paper are: cond-mat
37
The categories of this paper are: supr-con
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
9
The categories of this paper are: nlin