Charge Exchange Cross Sections for Noble Gas Ions and N2 between 0.2 and 5.0 keV
Article
Charge Exchange Cross Sections for Noble Gas Ions and
N2 between 0.2 and 5.0 keV
Steven Bromley * , Corey Ahl, Chad Sosolik and Joan Marler
Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA; (C.A.);
(C.S.); (J.M.)
* Correspondence:
Received: 30 June 2019; Accepted: 8 October 2019; Published: 14 October 2019
Abstract: Charge transfer of an electron from a neutral atom to an ion is a fundamental interaction that
plays a dominant role in the energy balance of atmospheric and astrophysical plasmas. The present
investigation measured the charge exchange cross sections of noble gas ions (He+ , Ne+ , Ar+ , Kr+ ) with
N2 in the intermediate energy range 0.2–5.0 keV. The systems were chosen because there remains a lack
of consensus amongst previous measurements and regions where there were no previous measurements.
A description of the mechanical design for an electrically floated gas cell is described herein.
Keywords: cross section; charge exchange; noble gas ions; N2 ; molecules; scattering
1. Introduction
Charge exchange between ions and neutrals is an important process in a number of fields including
fusion science [1], astrophysics [2–5], and atmospheric physics [6–8]. Recently, charge exchange between
stellar wind ions (H+ , He+ , He2+ ) and molecules has been investigated because of its relevance in
astronomy e.g., with CO and CO2 present in cometary atmospheres [9,10] and N2 present in planetary
atmospheres [11]. The charge exchange process is written
A+ + B → A + B+ + ∆E,
(1)
where ion A+ carries the majority of the collision energy compared to neutral B. The change of internal
energy as a result of the electron transfer is known as the energy defect ∆E.
When ∆E = 0, the process is resonant as in the case of symmetric collisions (i.e., A = B) where the
cross section is well explained by semi-classical calculations over most energy ranges [12,13]. Asymmetric
reactions (A 6= B) may exhibit similar behavior for reactants with small energy defects, e.g., H+ + O →
H + O+ + ∆E(∼ 0) [7]. For other reactions, the energy defect is large (∆E > 0) and consequently the
cross section is typically smaller [14]. However, cross sections for reactions with a large ∆E may become
significant in certain high collision energy regions. When reactant B is a molecule, the number of possible
reaction channels is large compared to the atomic case, and the energy dependence of the charge exchange
cross section deviates from the single-peaked structure seen in asymmetric reactions such as those compiled
by [15]. This is illustrated in particular by collisions between N2 and atmospheric ions H+ and O+ in
the summary of [7]. In the case of the He+ -N2 reaction, discrepancies still remain between a variety of
experimental efforts [11,16–21], including as much as a factor of three in magnitude and qualitatively
different behaviors as a function of collision energy.
Atoms 2019, 7, 96; doi:10.3390/atoms7040096
www.mdpi.com/journal/atoms
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Charge exchange cross sections can be measured either with a gas cell apparatus or crossed-beam
apparatus. While a crossed-beam set-up more easily allows for extraction of the target products, a gas
cell allows for precise measurement of the target pressure and the effective path length. For a more
in-depth discussion of these two techniques, see the review by [7]. Absolute cross sections are dependent
on the ability to measure the ion beam currents and neutral target pressures accurately. The experiments
performed herein took place in a gas cell apparatus [22] with the goal of producing absolute cross section
measurements. The gas cell was designed with a small effective path length in order to measure charge
exchange cross sections with large magnitudes (10−15 cm2 ) and mitigate the problem of large angle
scattering.
In Section 2, we present new measurements for the charge exchange cross sections for the
near-resonant reactions between noble gas ions and N2 (He+ ∆E = 9 eV, Ne+ ∆E = 6 eV, Ar+ ∆E = 0.2 eV,
Kr+ ∆E = −1.6 eV) for ion energies between 0.2 and 5.0 keV. In Section 3, we present details on the
updated gas cell apparatus designed to provide better absolute pressure measurements in the gas cell
region and improve data taking efficiency; we also discuss future directions.
2. Charge Exchange with N2
2.1. Experimental Details
The gas cell (shown in Figure 1) is mounted on a 3D translational plus rotational ultra high vacuum
(UHV) manipulator, with the intention of having the most flexibility to optimize the alignment with
various ion beam sources. This so-called manipulator-mounted gas cell (MGC) includes two skimmers
(1 mm and 2 mm apertures), a front end cap (3 mm aperture), a gas cell body (40 mm length), a back
end cap (4 mm aperture), a retarding field analyzer (5 mm aperture), and a suppression electrode (6 mm
aperture, −120 V). See [22] for more details.
Figure 1. (a) Pressure-dependent current loss data obtained for the Ar+ + Ar charge exchange process at
incident ion energies of 1–5 keV. (b) Schematic of the gas cell used for charge exchange measurements:
1. Faceplate, 2. Skimmer, 3. Gas cell, 4. Retarding field analyzer, 5. Suppression electrode, 6. Faraday cup [22].
Reprinted from Ref. [Symmetric charge exchange for intermediate velocity noble gas projectiles. J. Phys. B
At. Mol. Opt. Phys. 2019, 52, 215203], c IOP Publishing. Reproduced with permission. All rights reserved”
with an appropriate reference to the other paper (https://doi.org/10.1088/1361-6455/ab42d1).
Beams of ions A+ (A = He, Ne, Ar, Kr) are produced in an Omicron ISE 10 Sputter Ion Source. The gas
cell is aligned to be colinear with the ion beam by finding the position which maximizes the current
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collected in the Faraday cup. The copper Faraday cup has an aspect ratio of 0.3 to minimize errors due to
secondary electrons. The acceptance angle for detection is 2.0◦ for collisions occurring near the front of the
cell and 13.0◦ at the back of the cell
The measurement procedure was performed as follows: The beam current I0 (e) at each axial kinetic
energy e is maximized with an empty gas cell by scanning the focal and extraction electrodes on the source
for each energy studied. Then, the gas cell is filled with the target gas. The target pressure is measured
with a Bayard–Alpert gauge.
After the pressure stabilizes, I (e), the beam current with gas in the cell is measured for each of the
energy values by manually adjusting the energy, focus, and extraction voltages to the values previously
determined. This is repeated for four different cell pressures spaced evenly between 1 and 8 ×10−4 mbar.
I (e)− I (e)
The fractional current loss, 0 I (e) , is plotted as a function of pressure for each e and fitted to a linear
0
function, as shown in Figure 1a. The charge exchange cross section is
k T
σcx (e) = b
PL
I0 (e) − I (e)
I0 (e)
,
(2)
where k b is the Boltzma (...truncated)
