In tokamak divertors, certain transitions of C³⁺ ions from n=7 to n=6 (∼722.6 nm) and from n=6 to n=5 (∼466.0 nm) sometimes appear much brighter than expected for electron-impact excitation from the ground state. This situation occurs because of charge exchange between C⁴⁺ ions and hydrogen atoms in the n=2 level [1]. As a result, behavior of such highly-ionized carbon ions can be observed very easily using visible spectral lines, and the observation can be very useful for divertor plasma study.
Until now there is no quantitative estimation of cross sections for charge exchange in collisions of multiply-charged ions with excited H atoms. In this work, cross sections for charge exchange in collisions of C⁴⁺ ions with H*(n=2) atoms have been calculated using a molecular basis close-coupling method [2] in the collision energy range of 60 eV/amu – 6 keV/amu.
The obtained results are summarized as follows:
(1) The maximum value of cross sections in collisions of C⁴⁺ ions with H(1s) atoms is about 30 × 10^-16 cm² at collision energy E = 350 eV/amu. At the same collision energy, our calculated cross sections for H*(n=2) atoms is about 600 × 10^-16 cm². The ratio of these values are close to the n⁵ rule by Macek and Ovchinnikov [3].
(2) Electrons are predominantly captured into C³⁺(n=6) states.
(3) Within the electron capture processes into C³⁺(n=6) states, the capture into lower angular momentum dominates at low collision energies (E < 500 eV/amu), but at higher collision energies, the contributions of each angular momentum component are close to statistical weight.
(4) Energy dependence of the cross sections is weak.
(5) Mainly, only Σ, Π, and Δ states contribute to the electron capture processes.

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