Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hyperfine Paschen-Back regime of Potassium D$_2$ line observed by
  Doppler-free spectroscopy   Selective reflection of a laser radiation from an interface formed by a
dielectric window and a potassium atomic vapour confined in a nano-cell with
$350~$nm gap thickness is implemented for the first time to study the atomic
transitions of K D$_2$ line in external magnetic fields. In moderate
$B$-fields, there are 44 individual Zeeman transitions which reduce to two
groups (one formed by $\sigma^+$ the other one by $\sigma^-$
circularly-polarised light), each containing eight atomic transitions, as the
magnetic field increases. Each of these groups contains one so-called "guiding"
transition whose particularities are to have a probability (intensity) as well
as a frequency shift slope (in MHz/G) that are constant in the whole range of
$0 - 10~$kG magnetic fields. In the case of $\pi$-polarised laser radiation,
among eight transitions two are forbidden at $B = 0$, yet their probabilities
undergo a giant modification under the influence of a magnetic field. We
demonstrate that for $B$-fields $> 165~$G a complete hyperfine Paschen-Back
regime is observed. Other peculiarities of K D$_2$ line behaviour in magnetic
field are also presented. We show a very good agreement between theoretical
calculations and experiments. The recording of the hyperfine Paschen-Back
regime of K D$_2$ line with high spectral resolution is demonstrated for the
first time.

33
The categories of this paper are: plasm-ph
8
The categories of this paper are: hep-ex
25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
13
The categories of this paper are: quant-ph
10
The categories of this paper are: q-bio
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
26
The categories of this paper are: bayes-an
22
The categories of this paper are: chao-dyn