Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the absorption of radiation by the negatively charged hydrogen ion.
  I. General theory and construction of the wave functions   The absorption of infrared and visible radiation from stellar emission
spectra by the negatively charged hydrogen ions H$^{-}$ is considered. We
derive the explicit formulas for the bound-free absorption coefficient
$a_{\nu}$ for the negatively charged hydrogen ion ${}^{\infty}$H$^{-}$ and its
isotopes ${}^{1}$H$^{-}$ (protium) and ${}^{2}$H$^{-}$ (deuterium or D$^{-}$).
The bound-free absorption coefficient $a_{\nu}$ is used to evaluate the actual
absorption of infrared and visible radiation by the H$^{-}$ ion in photospheres
of many cold stars with surface temperatures $T_s \le$ 8,250 $K$.

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The categories of this paper are: cond-mat
27
The categories of this paper are: comp-gas
36
The categories of this paper are: cmp-lg
32
The categories of this paper are: atom-ph
23
The categories of this paper are: solv-int
2
The categories of this paper are: math
28
The categories of this paper are: alg-geom
9
The categories of this paper are: nlin
35
The categories of this paper are: mtrl-th
33
The categories of this paper are: plasm-ph
19
The categories of this paper are: eess
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
31
The categories of this paper are: ao-sci
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The categories of this paper are: gr-qc
21
The categories of this paper are: adap-org