Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: New method to study ion-molecule reactions at low temperatures and
  application to the H$_2^+$ + H$_2$ $\rightarrow$ H$_3^+$ + H reaction   Studies of ion-molecule reactions at low temperatures are difficult because
stray electric fields in the reaction volume affect the kinetic energy of
charged reaction partners. We describe a new experimental approach to study
ion-molecule reactions at low temperatures and present, as example, a
measurement of the ${\rm H}_2^+ + {\rm H}_2\rightarrow {\rm H}_3^+ + {\rm H}$
reaction with the ${\rm H}_2^+$ ion prepared in a single rovibrational state at
collision energies in the range $E_{\rm col}/k_{\rm B} = 5$-60 K. To reach such
low collision energies, we use a merged-beam approach and observe the reaction
within the orbit of a Rydberg electron, which shields the ions from stray
fields. The first beam is a supersonic beam of pure ground-state H$_2$
molecules and the second is a supersonic beam of H$_2$ molecules excited to
Rydberg-Stark states of principal quantum number $n$ selected in the range
20-40. Initially, the two beams propagate along axes separated by an angle of
10$^\circ$. To merge the two beams, the Rydberg molecules in the latter beam
are deflected using a surface-electrode Rydberg-Stark deflector. The collision
energies of the merged beams are determined by measuring the velocity
distributions of the two beams and they are adjusted by changing the
temperature of the pulsed valve used to generate the ground-state ${\rm H}_2$
beam and by adapting the electric-potential functions to the electrodes of the
deflector. The collision energy is varied down to below $E_{\rm col}/k_{\rm B}=
10$ K, i.e., below $E_{\rm col}\approx 1$ meV, with an energy resolution of 100
$\mu$eV. We demonstrate that the Rydberg electron acts as a spectator and does
not affect the cross sections, which are found to closely follow a
classical-Langevin-capture model in the collision-energy range investigated.
Because all neutral atoms and molecules can be excited to Rydberg states, this
method of studying

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The categories of this paper are: q-fin
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The categories of this paper are: nucl-ex
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The categories of this paper are: q-alg
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The categories of this paper are: supr-con
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The categories of this paper are: econ
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The categories of this paper are: hep-th
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The categories of this paper are: nlin
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The categories of this paper are: nucl-th
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The categories of this paper are: dg-ga
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The categories of this paper are: gr-qc
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The categories of this paper are: cs
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The categories of this paper are: comp-gas
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The categories of this paper are: plasm-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-ph
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The categories of this paper are: quant-ph