Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Cavity Ring-down UV spectroscopy of the C$^2\Sigma^+$-X$^2\Pi$
  electronic transition of CH   Rotationally resolved spectra of the C$^2\Sigma^+$-X$^2\Pi$ electronic system
of the CH radical were measured using cavity ring-down spectroscopy in
supersonically expanding, planar hydrocarbon plasma. The experimental
conditions allowed the study of highly excited rotational levels starting from
vibrationally excited states. Here we present some 200+ new or more accurately
recorded transitions in the 0-0, 1-1 and 2-2 vibronic bands in the ultraviolet
between 30900-32400 cm$^{-1}$ (324-309 nm). The resulting data, compared to
earlier measurements, allows for the determination of more precise molecular
constants for each vibrational state and therefore more precise equilibrium
values. From this an equilibrium bond length of 1.115798(17) \r{A} for the
C$^2{\Sigma}^+$ state is determined. A comprehensive list with observed
transitions for each band has been compiled from all available experimental
studies and constraints are placed on the predissociation lifetimes.

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The categories of this paper are: hep-ph
32
The categories of this paper are: atom-ph
6
The categories of this paper are: astro-ph
23
The categories of this paper are: solv-int
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th
19
The categories of this paper are: eess
18
The categories of this paper are: econ
30
The categories of this paper are: q-alg
2
The categories of this paper are: math
5
The categories of this paper are: gr-qc
10
The categories of this paper are: q-bio
33
The categories of this paper are: plasm-ph
17
The categories of this paper are: q-fin
4
The categories of this paper are: cond-mat