九州大学大学院総合理工学府報告第 44 巻 第 2 号 9-19 頁 令和 5 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 44, No. 2, pp. 9-19, FEB, 2023

Relative Formation Rate Constants andElectronic-State Distributions of Ne* Produced fromthe Ne+/SF6- and Ne+/C6F6- Ion-Ion Neutralization
Reactions in the He Flowing Afterglow
Masaharu TSUJI*1,2† Masahiro HISANO*3 Keiko UTO*1
Jun-Ichiro HAYASHI*1 Takeshi TSUJI*4
E-mail of corresponding author: tsuji@cm.kyushu-u.ac.jp†(Received October 31, 2022, accepted November 10, 2022)
Relative formation rate constants, 𝑘 𝑢 , and electronic-state distributions, 𝑁 𝑢 , of Ne* forupper 𝑢 states in the Ne+/SF6- and Ne+/C6F6- ion-ion neutralization (I-IN) reactions were determinedby observing Ne* atomic emissions in the 200–990 nm region. Ne* emissions from forty-two ns(n=4 and5), ns' (n=4 and 5), np(n=3 and 4), np' (n=3 and 4), and 4d states in the 18.38–20.71 eV region wereidentified in the Ne+/SF6- reaction, whereas those from forty-five ns(n=4 and 5), ns' (n=4 and 5), np(n=3
and 4), np' (n=3 and 4), nd(n=3–5), and nd' (n=3 and 4) states in the 18.38–21.02 eV region were observedin the Ne+/C6F6- reaction. Major product Ne* states were 4s', 3d, and 3d' states in the Ne+/SF6- reaction,
whereas they were 3p, 4s', 3d, 3d', and 5d states in the Ne+/C6F6- reaction. The electronic-state
distributions of Ne* in the Ne+/SF6- and Ne+/C6F6- reactions suggested that Ne* atoms are not formed
through long-lived Ne+–SF6- and Ne+–C6F6- complexes, where excess energies are distributed to
products statistically. It was concluded that electronic-state distributions of Ne* are governed by thetransition probability from the attractive Ne+–SF6- and Ne+–C6F6- entrance potentials to flat Ne*–SF6
and Ne*–C6F6 exit potentials at crossing points.
Key words: Neon ion, SF6-, C6F6-, Ion-ion neutralization reaction, Flowing afterglow, Ne* atomic

emission, Radiative cascade, Relative formation rate constant, Electronic-state
distribution, Curve crossing

1.

Introduction

Due to a strong mutual Coulombic attractive
force, reaction rate constants of recombination
reactions between positive and negative ionsare extremely large ( ≈ 2 × 10-7 × (300/T )1/2
cm3 s-1)1,2) in comparison with those of ion–molecule reactions (k ≈ 10-11–10-9 cm3 s-1).
Therefore, ion–ion reactions play a significant
role as a loss process of ions in natural plasmas
including interstellar gas clouds and in manmade plasmas such as static and flowing*1

Institute for Materials Chemistry and Engineering,and Research and Education Center of GreenTechnology*2 Department of Applied Science for Electronics andMaterials*3 Department of Applied Science for Electronics andMaterials, Graduate Student*4 Department of Materials Science, Shimane
University

afterglows and laser plasmas. Until now
reaction rate constants or cross sections of ion–
ion reactions have been measured for various
reaction systems, and several theoreticalmodels have been proposed to explain theexperimental data.1-5) However, only very few
optical spectroscopic studies have been carriedout on the product-state distributions in ion–
ion recombination reactions.6,7)
Two-body ion–ion reactions between an
atomic ion A and molecules BC leading to
excited neutral species are classified into thefollowing two cases:
Excimerformation
recombination reaction

via

A+ + BC- → AB* + C

dissociative(1)

Mutual neutralization reaction
A+ + BC- → A* + BC

(2)

Electonic-state distributions of Ne* in the Ne+/SF6- and Ne+/C6F6- ion-ion neutralization reactions

10

In process (1), a new A–B bond is created, while
such a rearrangement does not occur, and only
mutual neutralization takes place by anelectron transfer from a negative ion to a
positive ion in process (2).
We have previously studied the following
two-body
ion—ionrecombination
andneutralization reactions by using a flowingafterglow method.6-13)
He+ + C6F5X- (X = F, Cl, Br, CF3)
→ He* + C6F5F
Ar+ + SF6- → ArF* + SF5
Kr+ + SF6- → KrF* + SF5→ Kr* + SF6
Kr+ + C6F6- → KrF* + C6F5
→ Kr* + C6F6
Xe+ + SF6- → XeF* + SF5

(3)
(4)
(5a)
(5b)
(6a)
(6b)
(7)

In this study, the formation of Ne* in the
Ne+/SF6- and Ne+/C6F6- I-IN reactions areinvestigated by observing Ne* emissions in
the He flowing afterglow. The relative
formation rate constants and electronic-state
distributions are determined. The observed
electronic-state distributions are comparedwith our previous results for the He+/C6F5X(X = F, Cl, Br, CF3) I-IN reactions (3) leading
to excited He* atoms.13) Based on theobserved
electronic-state
distributions,reaction dynamics is discussed.

2.

Experimental

The flowing-afterglow apparatus used in thisstudy was essentially identical with thatreported previously.13,14) We have previously
studied Ne afterglow reactions of simple
molecules.15-18) Then, we found that metastable
Ne(3P0,2) atoms are major active species at low
Ne buffer gas pressure below about 0.2 Torr (1Torr = 133.3 Pa), whereas not only Ne(3P0,2)
atoms but also Ne+ and Ne2+ ions exist as active
species at Ne gas pressures above that. The[Ne2+]/[Ne+] ratio increased with increasing
the Ne gas pressure, because Ne2+ molecularions are formed by the Ne+/2Ne three-bodyreaction.20)
Ne+ + Ne + Ne → Ne2+ + Ne

ｋ8 = (4.4±0.4) × 10-32 cm6 s-1(8)Both Ne+ and Ne2+ can participate in thereaction as positive ion in the Ne afterglow at
Ne gas pressures above 0.2 Torr. To exclude thecontribution of Ne2+, we used here He afterglow
reaction of Ne. We initially generated such He

active species as He(23S), He+, and He2+ andelectrons by a microwave discharge of highpurity He gas (>99.9999%) in a He flowing
afterglow at a microwave power of 100 W.14,18)
The contribution of such charged species as He+,
He2+, and electrons to the observed emissions
was examined by using a charged-particle
collector grid placed between the discharge
section and the reaction zone. A small amountof Ne gas (26 mTorr) was added from the firstgas inlet placed 10 cm downstream from thecenter of microwave discharge. We used a low
Ne partial pressure to suppress the formationof Ne2+ by the three-body reaction (9).
Ne+ + Ne + He → Ne2+ + He

(9)

The booster pump used in the present
apparatus was equipped with a continuously
variable gate valve. The flow tube pressure wasvaried from 0.9 Torr to 7.3 Torr by opening or
closing the gate valve.
By the reactions of He(23S), He+, and He2+
with Ne, the following energy-transfer andcharge-transfer reactions take place near the
Ne gas inlet.15,19,20)
He(23S) + Ne → Ne*(4s,4s') + He
k10 = 3.85 × 10-12 cm3 s-1(10)

He+ + Ne → Ne+ + He (0.87)→ HeNe+ + h (0.17)
k11a = (1.20 × 10-15) ±30% cm3 s-1

(11a)
(11b)

He2+ + Ne → Ne+ + 2He
k12 = (6.00 × 10-10) ±30% cm3 s-1(12)

Since the rate constant of reaction (12) is larger
than that of (11) by five orders of magnitude,
Ne+ atomic ions are preferentially formed byreaction (12) in our He flowing-afterglow
reaction of Ne.
We added an electron scavenger SF6 or C6F6
from the second gas inlet placed further 10 cm
downstream from the first gas inlet. The partialpressure of SF6 or C6F6 was 3–5 mTorr. Aroundthe second gas inlet, SF6- or C6F6- anions wereformed by a fast electron attachment to SF6 or
C6F6,e- + SF6 → SF6k13 = 2.2 × 10-7 cm3 s-1(13)(Ref. 21)e- + C6F6 → C6F6k14 = 2.1 × 10-7 cm3 s-1(14)(Ref. 22)

令和 4 年度

九州大学大学院総合理工学報告

Then, excited Ne* atoms can be formed throughthe following I-IN reactions around the second
gas inlet:
Ne+ + SF6- → Ne* + SF6
Ne+ + C6F6- → Ne* + C6F6

(15)
(16)

Under our experimental conditions, excited
Ne* states can be formed by energy-transfer
reaction (10) and I-IN reaction (15) or (16).
Without applying an electrostatic potential tothe charged-particle collector grids, Ne*emissions resulting from both reactions (10)and (15) or (16) are observed, whereas thoseresulting only from reaction (10) are obtainedby applying an electrostatic static potential togrids. Thus, the Ne* emissions resulting only
from I-IN reaction (15) or (16) are obtained bysubtracting the contribution of energy-transfer
reaction (10). There are two spin-orbit
components in the ground state of Ne+, 2P3/2 and
2P1/2, with recombination energies of 21.56 and21.66 eV, respectively. Their relative
concentration was not estimated in this study.
A reaction flame around the second gas inlet
was dispersed in the 200–990 nm region with aSpex 1250M monochromator equipped withcooledphotomultipliers(Hamamatsu
Photonics
R376
and
R316-02).
The
monochromator and the optical detection
system were corrected by using standard D2
and halogen lamps. All spectra presented here
were corrected for the wavelength response of
the detection system.

3.

第 44 巻

第2号

reported atomic spectral tables,23,24) and theexcitation energies of Ne* were obtained from
reported tables.24.25) In the present study, we
identified forty-two ns(n=4 and 5), ns' (n=4 and5), np(n=3 and 4), np' (n=3 and 4), and 4d levelsof Ne* in the 18.38–20.71 eV region. The
recombination energies of Ne+(2P3/2) andNe+(2P1/2) are 21.56 and 21.66 eV, respectively,
and the electron affinity of SF6 is 1.03 eV.26)
Therefore, the total available energies in the
Ne+(2P3/2)/SF6- and Ne+(2P1/2)/SF6- I-IN
reactions, H0o + 3RT, are 20.62 eV and 20.71
eV, respectively.
Ne+(2P3/2) + SF6- → Ne(1S0) + SF6 + 20.54 eV(17a)
Ne+(2P1/2) + SF6- → Ne(1S0) + SF6 + 20.63 eV(17b)
Low-energy Ne* states in the 18.38–20.57 eVregion can be produced through both theNe+(2P3/2)/SF6- and Ne+(2P1/2)/SF6- reactions. On
the other hand, high energy Ne* states in the
20.66–20.71 eV region can only be formed
though the Ne+(2P1/2)/SF6- reaction. These
results show that the upper Ne+(2P1/2) spinorbit component participates in the formationof Ne* in our experimental condition. In Table
1, both nL and nL' states of Ne* with 2p5(2P3/2)nL

Results and Discussion

3.1 Emission spectra of Ne* resulting fromthe Ne+/SF6- and Ne+/C6F6- I-IN reactions
Figure 1(a) shows a typical emission
spectrum resulting from the He(23S)/Ne +Ne+/SF6- reactions in the 500–700 nm region at
a He gas pressure of 7.3 Torr, where Ne* lines
from the 3p, 3p', 4d, and 5s' states and a He line
are identified. Most of the observed Ne* lines
become weak, when charged species were
removed from the He afterglow on applying anelectrostatic potential to the grid, as shown in
Fig. 1(b). Emission spectrum resulting from the
Ne+/SF6- I-IN reaction is shown in Fig. 1(c),which is obtained by subtracting spectrum (b)from spectrum (a).
The upper electronic states of the observed
Ne* lines in the 500–970 nm region and their
excitation energies are listed in Table 1. The
spectral assignment was made by referring to

11

Fig. 1. Emission spectra resulting from (a)He(23S)/Ne + Ne+/SF6-, (b) He(23S)/Ne, and(c) Ne+/SF6- reactions in the 500–700 nm
region.

12

Electonic-state distributions of Ne* in the Ne+/SF6- and Ne+/C6F6- ion-ion neutralization reactions

Thus, all of the observed Ne* states in the
18.38–21.02 eV region can be produced throughboth the Ne+(2P3/2)/C6F6- and Ne+(2P1/2)/C6F6reactions. The formation of both nL and nL'
states of Ne* indicates that Ne* states having
both Ne+(2P3/2) and Ne+(2P1/2) ion-core
configurations are produced in the Ne+/C6F6reaction.
3.2 Relative formation rate constants andelectronic-state distributions of Ne* in the
Ne+/SF6- and Ne+/C6F6- I-IN reactions
The formation rate constant of an upper Ne*
state, 𝑘 𝑢 , and its steady-state electronic
distribution, 𝑁 𝑢 , in the Ne+/SF6- andNe+/C6F6- reactions were evaluated from theemission intensity of a 𝑢, 𝑙 transition of Ne*,𝐼 , using similar relations given in thepreceding paper for the Ne+/2e- CRR reaction.14)
Ne ∗ 𝑢 ⁄𝑑𝑡
𝑘 𝑢 NeFig. 2. Emission spectra resulting from (a)He(23S)/Ne + Ne+/C6F6-, (b) He(23S)/Ne, and (c)Ne+/C6F6- reactions in the 400–600 nm region.
and 2p5(2P1/2)nL' (n=3–5, L=s, p, d) electron
configurations, respectively, are produced.
Based on this fact, Ne* states having both
Ne+(2P3/2) and Ne+(2P1/2) ion-core configurations
are formed in the Ne+/SF6- reaction.
Figures 2(a)-2(c) show emission spectra
resulting from the (a) He(23S)/Ne + Ne+/C6F6-,(b) He(23S)/Ne, and (c) Ne+/C6F6- reactions in
the 400–600 nm region at a He pressure of 0.91
Torr. Ne* lines from the 3p, 4d, and 5d states, a
He* line, and C6F6+(B-X) molecular ion
emission13,27) are observed. In Table 2 are listed
the upper electronic states of the observed Ne*
transitions in the 300–990 nm region and their
excitation energies. In the present study, fortyfive ns(n=4 and 5), ns' (n=4 and 5), np(n=3 and 4),
np' (n=3 and 4), nd(n=3–5), and nd' (n=3 and 4)
states of Ne* are identified with excitationenergies of 18.38–21.02 eV. Since the electron
affinity of C6F6 is 0.53 eV,26) the total available
energiesintheNe+(2P3/2)/C6F6and+2
Ne ( P1/2)/C6F6 I-IN reactions, H0o + 3RT, are21.11 eV and 21.21 eV, respectively.
Ne+(2P3/2) + C6F6- → Ne(1S0) + C6F6 + 21.03 eV(18a)
Ne+(2P1/2) + C6F6- → Ne(1S0) + C6F6 + 21.13 eV
(18b)

X

=

𝑘 ℎ NeX

𝑘 𝑢 NeX𝑏

19

where X = SF6 or C6F6, and 𝑏 is the opticalbranching ratio for the ℎ → 𝑢 radiative
cascade transition:
Equation (19) gives
𝑘 𝑢

𝑘 𝑢

𝑏

= 𝑘 𝑢

𝑘 ℎ

𝑘 𝑢: cascade .

20

The rate constant for the direct excitation of a𝑢 level, 𝑘 𝑢 , is deduced from Eq. (20), andthe corresponding population, 𝑁 𝑢 , iscalculated from the relation:
𝑁 𝑢 ∝ 𝑘0 𝑢 /

𝐴𝑢𝑙

21

𝑙

In Tables 1 and 2 are given 𝑘 𝑢 ,𝑘 𝑢: 𝑐𝑎𝑠𝑐𝑎𝑑𝑒 , 𝑘 𝑢 , and 𝑁 𝑢 values for the
Ne+/SF6- and Ne+/C6F6- reactions obtained fromtotal intensities of observed emission lines andunobserved lines, respectively. Intensities ofunobserved lines were estimated from
intensities of observed lines and reported 𝐴
values for the same upper levels.24) Figures 3
and 4 show the dependence of 𝑘 𝑢 ,令和 4 年度

九州大学大学院総合理工学報告第 44 巻第2号

13values on the
𝑘 𝑢: 𝑐𝑎𝑠𝑐𝑎𝑑𝑒 , and 𝑘 𝑢
0000000000
Table 1. Relative formation rate constants and electronic-state distribution of Ne* in the Ne+/SF6I-IN reaction in the He flowing afterglow.
Energy
/ eV
18.38
18.56
18.58
18.61
18.64
18.69
18.70
18.71
18.73
18.97
19.66
19.76
19.78
20.03
20.03
20.03
20.04
20.04
20.05
20.05
20.14
20.14
20.14
20.15
20.19
20.20
20.21
20.21
20.26
20.29
20.30
20.30
20.37
20.56
20.57
20.66
20.70
20.71
20.71
20.71
20.71
20.71

State

J

𝑘 𝑢

𝑘 𝑢: cascade

𝑘 𝑢

𝑁 𝑢

3p [1/2]
3p [5/2]
3p [5/2]
3p [3/2]
3p [3/2]
3p' [3/2]
3p' [3/2]
3p [1/2]
3p' [1/2]
3p' [1/2]
4s [3/2]°
4s' [1/2]°
4s' [1/2]°
3d [1/2]°
3d [7/2]°
3d [7/2]°
3d [3/2]°
3d [3/2]°
3d [5/2]°
3d [5/2]°
3d' [5/2]°
3d' [5/2]°
3d' [3/2]°
4p [1/2]
4p [5/2]
4p [5/2]
4p [3/2]
4p [3/2]
4p [1/2]
4p' [3/2]
4p' [1/2]
4p' [3/2]
4p' [1/2]
5s [3/2]°
5s [3/2]°
5s' [1/2]°
4d [1/2]°
4d [7/2]°
4d [7/2]°
4d [3/2]°
4d [5/2]°
4d [5/2]°

1
3
2
1
2
1
2
0
1
0
2
0
1
1
4
3
2
1
2
3
2
3
2
1
3
2
1
2
0
1
1
2
0
2
1
1
1
4
3
2
2
3

1.11E-01
1.00E-01
7.15E-02
2.34E-01
8.79E-02
5.13E-01
2.06E-01
1.32E-02
2.46E-01
1.86E-02
1.04E-01
2.93E-02
1.00E+00
2.45E-02
3.95E-02
2.45E-02
1.06E-02
9.80E-02
6.78E-02
1.95E-02
7.09E-01
1.00E-02
2.89E-01
1.05E-03
1.07E-03
3.94E-03
2.40E-03
1.99E-03
7.31E-04
1.41E-03
9.19E-04
2.46E-03
3.09E-04
1.54E-03
8.09E-02
3.45E-02
1.72E-03
1.67E-03
1.47E-03
1.40E-03
4.73E-04
7.38E-04

8.87E-02
1.00E-01
7.15E-02
2.34E-01
8.79E-02
5.13E-01
2.06E-01
1.32E-02
2.46E-01
1.86E-02
3.73E-02

2.26E-02
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
6.66E-02
2.93E-02
1.00E+00
2.45E-02
3.95E-02
2.45E-02
1.06E-02
9.80E-02
6.78E-02
1.95E-02
7.09E-01
1.00E-02
2.89E-01
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
1.54E-03
8.09E-02
3.45E-02
1.72E-03
1.67E-03
1.47E-03
1.40E-03
4.73E-04
7.38E-04

3.94E-02-4.33

1.81E-01
7.98E-02
5.77E-01
2.18E-02
5.43E-02
3.46E-02
1.44E-02
5.53E-02
9.98E-02
2.87E-02
1.00E+00
1.43E-02
4.08E-01

-3.32
-2.53
-1.65
-4.93
-5.11
-5.31
-5.85
-3.99
-3.91
-5.50
-1.61
-6.19
-2.51

8.01E-03
1.07E-01
5.77E-02
1.78E-03
6.35E-03
5.84E-03
5.29E-03
1.91E-03
2.98E-03

-6.44
-3.34
-3.95
-7.43
-7.26
-7.09
-6.85
-7.87
-7.76

a) Ref. 24.

∑ 𝐴

1.05E-03
1.07E-03
3.94E-03
2.40E-03
1.99E-03
7.31E-04
1.41E-03
9.19E-04
2.46E-03
3.09E-04

is the Einstein coefficient for spontaneous emission.

ln 𝑁 𝑢 /𝑔∑ 𝐴(s-1)a)3.93E+07
5.15E+07
4.90E+07
5.12E+07
4.98E+07
5.08E+07
5.27E+07
6.06E+07
5.49E+07
6.24E+07
2.52E+07
2.52E+07
1.19E+08
7.71E+07
4.99E+07
4.85E+07
5.05E+07
1.22E+08
4.66E+07
4.66E+07
4.86E+07
4.79E+07
4.85E+07
4.84E+06
7.35E+06
7.01E+06
7.77E+06
8.81E+06
1.14E+07
7.67E+06
8.40E+06
7.55E+06
1.58E+07
1.32E+07
5.20E+07
4.10E+07
6.60E+07
1.81E+07
1.73E+07
1.82E+07
1.69E+07
1.70E+07

Electonic-state distributions of Ne* in the Ne+/SF6- and Ne+/C6F6- ion-ion neutralization reactions

14

Table 2. Relative formation rate constants and electronic-state distribution of Ne* in the Ne+/C6F6I-IN reaction in the He flowing afterglow.
Energy
/ eV
18.38
18.56
18.58
18.61
18.64
18.69
18.70
18.71
18.73
18.97
19.66
19.69
19.76
19.78
20.03
20.03
20.03
20.04
20.04
20.05
20.05
20.14
20.14
20.14
20.15
20.19
20.20
20.21
20.21
20.26
20.29
20.30
20.30
20.37
20.56
20.57
20.66
20.70
20.71
20.71
20.71
20.71
20.81
21.02
21.02

State

J

𝑘 𝑢

𝑘 𝑢: cascade

3p [1/2]
3p [5/2]
3p [5/2]
3p [3/2]
3p [3/2]
3p' [3/2]
3p' [3/2]
3p [1/2]
3p' [1/2]
3p' [1/2]
4s [3/2]°
4s [3/2]°
4s' [1/2]°
4s' [1/2]°
3d [1/2]°
3d [7/2]°
3d [7/2]°
3d [3/2]°
3d [3/2]°
3d [5/2]°
3d [5/2]°
3d' [5/2]°
3d' [3/2]°
3d' [3/2]°
4p [1/2]
4p [5/2]
4p [5/2]
4p [3/2]
4p [3/2]
4p [1/2]
4p' [3/2]
4p' [1/2]
4p' [3/2]
4p' [1/2]
5s [3/2]°
5s [3/2]°
5s' [1/2]°
4d [1/2]°
4d [7/2]°
4d [7/2]°
4d [3/2]°
4d [5/2]°
4d' [3/2]°
5d [7/2]°
5d [5/2]°

1
3
2
1
2
1
2
0
1
0
2
1
0
1
1
4
3
2
1
2
3
2
2
1
1
3
2
1
2
0
1
1
2
0
2
1
1
1
4
3
2
2
1
3
2

1.82E-01
1.63E-01
1.14E-01
1.11E-01
8.62E-02
1.87E-01
1.43E-01
2.18E-02
1.48E-01
2.49E-02
1.31E-01
5.54E-02
7.18E-02
1.00E+00
2.83E-02
3.38E-02
2.07E-02
1.17E-02
1.49E-01
1.30E-02
2.33E-02
1.64E-01
1.14E-01
2.34E-02
1.88E-03
4.21E-03
1.33E-02
1.76E-02
2.13E-03
7.06E-04
5.51E-03
4.69E-03
1.25E-02
1.67E-03
6.83E-03
1.97E-02
1.74E-01
5.30E-03
5.45E-03
4.21E-03
3.21E-03
3.89E-02
2.12E-03
3.54E-03
3.39E-02

7.58E-02
1.12E-01
7.13E-02
1.11E-01
8.35E-02
1.87E-01
1.43E-01
2.18E-02
1.48E-01
2.49E-02
5.00E-02
2.28E-04
2.88E-04
8.53E-04
3.91E-06a) Ref. 24.

∑ 𝐴

4.65E-07
7.54E-07

1.88E-03
4.21E-03
1.33E-02
1.76E-02
2.13E-03
7.06E-04
5.51E-03
4.69E-03
1.25E-02
1.67E-03

𝑘 𝑢

𝑁 𝑢

1.06E-01 3.22E-01
5.14E-02 1.19E-01
4.26E-02 1.03E-01
0.00E+00
2.76E-03 6.59E-03
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
8.07E-02 3.81E-01
5.52E-02 6.78E-02
7.15E-02 3.38E-01
9.99E-01 1.00E+00
2.83E-02 4.36E-02
3.38E-02 8.06E-02
2.07E-02 5.07E-02
1.17E-02 2.75E-02
1.49E-01 1.45E-01
1.30E-02 3.31E-02
2.33E-02 5.94E-02
1.64E-01 4.01E-01
1.14E-01 2.80E-01
2.34E-02 3.29E-02
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
6.83E-03 6.16E-02
1.97E-02 4.51E-02
1.74E-01 5.05E-01
5.30E-03 9.55E-03
5.45E-03 3.58E-02
4.21E-03 2.90E-02
3.21E-03 2.10E-02
3.89E-02 2.73E-01
2.12E-03 5.79E-03
3.54E-03 6.49E-02
3.39E-02 6.34E-01

is the Einstein coefficient for spontaneous emission.

ln 𝑁 𝑢 /𝑔
-2.23
-4.08
-3.88
-6.63

-2.58
-3.79
-1.09
-1.10
-4.23
-4.72
-4.93
-5.20
-3.03
-5.02
-4.77
-2.52
-2.88
-4.51

-4.40
-4.20
-1.78
-5.75
-5.53
-5.49
-5.47
-2.91
-6.25
-4.68
-2.07

Σ𝐴
(s-1) a)3.93E+07
5.15E+07
4.90E+07
5.12E+07
4.98E+07
5.08E+07
5.27E+07
6.06E+07
5.49E+07
6.24E+07
2.52E+07
9.68E+07
2.52E+07
1.19E+08
7.71E+07
4.99E+07
4.85E+07
5.05E+07
1.22E+08
4.66E+07
4.66E+07
4.86E+07
4.85E+07
8.47E+07
4.84E+06
7.35E+06
7.01E+06
7.77E+06
8.81E+06
1.14E+07
7.67E+06
8.40E+06
7.55E+06
1.58E+07
1.32E+07
5.20E+07
4.10E+07
6.60E+07
1.81E+07
1.73E+07
1.82E+07
1.69E+07
4.35E+07
6.48E+06
6.36E+06

令和 4 年度

1.0

4s’ 3d,

0.8

3d’

4s’

0.6

0.4

4s
18.5

19.0

19.5

5s
20.0

20.5

0.2

0.0
18.0

21.0

3p,3p’
18.5

19.0

Energy / eV(b)k (u:cascade)k (u:cascade)

3p,3p’

0.2

21.0

21.5

18.5

19.0

19.5

0.6

0.4

0.2

4s
20.0

20.5

0.0
18.0

21.0

3p,3p’
4s
18.5

19.0

0.8

(c)

k0 (u)

5s

4s
19.0

19.5

20.5

21.0

21.5

20.0

(c)

0.8

0.4

18.5

20.0

4s’

1.0

0.6

3p

19.5

Energy / eV

4s’ 3d,3d’

1.0

k0 (u)

20.5(b)Energy / eV

0.0
18.0

20.0

0.8

0.4

0.2

19.5

1.0

0.8

0.0
18.0

3d,
5s’
3d’
4s
4d
Energy / eV

1.0

0.6

(a)

0.8

3p,3p’

0.2

15

1.0

(a)

0.4

0.0
18.0

第2号

k (u)

k (u)

0.6

第 44 巻九州大学大学院総合理工学報告

20.5

21.0

Energy / eV

0.6

0.4

0.2

3p

0.0
18.0

18.5

4s
19.0

19.5

3d,
3d’ 5s’
4d
20.0

20.5

21.0

21.5

Energy / eV

Fig. 3. Dependence of relative formation rate
constants on the excitation energy of Ne* in
the Ne+/SF6- reaction: (a) including radiative
cascade, (b) radiative cascade, and (c)excluding radiative cascade.

Fig. 4. Dependence of relative formation rate
constants on the excitation energy of Ne* in
the Ne+/C6F6- reaction: (a) including radiative
cascade, (b) radiative cascade, and (c)excluding radiative cascade.

values on the
𝑘 𝑢: 𝑐𝑎𝑠𝑐𝑎𝑑𝑒 , and 𝑘 𝑢
excitation energy of Ne* in the Ne+/SF6- andNe+/C6F6- reactions, respectively. A comparison
between 𝑘 𝑢 and 𝑘 𝑢 values in Tables 1
and 2, and Figs. 3 and 4 suggests that some 3p
and all 3p', 4p, and 4p' levels are formed only
through radiative cascade from upper levels in
the Ne+/SF6- and Ne+/C6F6- reactions. The
cascade-free 𝑘 𝑢values indicate thatfavorable states in the Ne+/SF6- reaction are 4s',
3d, and 3d' states, whereas those in the
Ne+/C6F6- reaction are 3p, 4s, 4s', 3d, 3d', and 5s'
states. Among them, the 4s' state at 19.78 eV isthe most favorable in both reactions.
Figures 5, 6 and 7, 8 show the dependence of𝑁 𝑢and ln 𝑁 𝑢 /𝑔
on the excitationenergy of Ne* in the Ne+/SF6- and Ne+/C6F6reactions, respectively. Favorable populations
are obtained for the 4s, 4s', 3d, 3d', and 5s in the
Ne+/SF6- reaction, whereas they are found forthe 3p, 4s, 4s', 3d, 3d', 5s', and 5d states in the
Ne+/C6F6- reaction. It should be noted that themost populated states are different between thetwo reactions. They are the 3d' state at 20.14 eVin the Ne+/SF6- reaction and the 4s' state at
19.78 eV in the Ne+/C6F6- reaction.

Electonic-state distributions of Ne* in the Ne+/SF6- and Ne+/C6F6- ion-ion neutralization reactions

16

3d,3d’

1.0

1.0

0.8

5d

4s’

0.6

N (u)

N (u)

0.8

0.6

5s’
4s

0.4

0.4

0.2

18.5

19.0

19.5

20.0

20.5

4d

0.2

0.0
18.0

21.0

18.5

19.0

-1.0

0.0

-2.0

-1.0

-3.0

-2.0

-4.0
-5.0

21.0

21.5

-4.0
-5.0

-7.0

-6.0

19.5

20.5

-3.0

-6.0

19.0

20.0

Fig. 7. Electronic-state distribution vs exciationenergy of Ne* in the Ne+/C6F6- reaction.

ln(N(u)/gu)ln(N(u)/gu)

Fig. 5. Electronic-state distribution vs exciationenergy of Ne* in the Ne+/SF6- reaction.

18.5

19.5

Energy / eV

Energy / eV

-8.0
18.0

3d,3d’

3p

5s,5s’,
4d

4s

3p
0.0
18.0

4s’

20.0

20.5

21.0

Energy / eV

-7.0
18.0

18.5

19.0

19.5

20.0

20.5

21.0

21.5

Energy / eV

Fig. 6. ln 𝑁 𝑢 /𝑔 vs excitation energy of Ne*
in the Ne+/SF6- reaction.

Fig. 8. ln 𝑁 𝑢 /𝑔 vs exciation energy of Ne*
in the Ne+/C6F6- reaction.

In Tables 3 and 4 are summarized the total
relative formation rate constants ∑ 𝑘 𝑢
and electronic-state distributions ∑ 𝑁 𝑢 of
Ne* in the Ne+/SF6- and Ne+/C6F6- reactions atthermal energy. Based on the present data
shown in these tables, major populated Ne*
states in the Ne+/SF6- reaction are the 4s', 3d,
and 3d' states, which occupy 40.6, 11.2, and39.8% of ∑ 𝑘 𝑢 and 23.4, 11.0, and 50.7% of∑ 𝑁 𝑢 , respectively. On the other hand, major
populated Ne* states in the Ne+/C6F6- reaction
are the 3p, 4s', 3d, 3d', 5s', and 5d states, which
occupy 8.9, 46.8, 12.2, 13.2, 7.6, and 1.6% of∑ 𝑘 𝑢 and 10.6, 25.9, 8.5, 13.8, 9.8, and13.5% of ∑ 𝑁 𝑢 , respectively. These results
suggest that Ne* states formed in the Ne+/C6F6reaction are populated in more wider energy
range than those produced in the Ne+/SF6reaction. One reason for this is the larger total
available energy of the Ne+/C6F6- reaction thanthat of the Ne+/SF6- reaction, so that higher
energy Ne* states can be formed.
The ∑ 𝑘 𝑢 and ∑ 𝑁 𝑢 values of the
product states having the Ne+(2P3/2) ion core
and the Ne+(2P/1/2) ion core are 18:82% and
24:76%, for the Ne+/SF6- reaction, whereas they
are 32:68% and 50:50% for the Ne+/C6F6reaction, respectively. Since both 2P3/2 and 2P1/2
spin-orbit components of Ne+ exist in thepresent experiment and their relative
concentration is not determined. Therefore, it
is difficult to discuss the relative contributionof the Ne+(2P3/2) and Ne+(2P1/2) components andconservation of ion-core configuration during
the I-IN reactions. One of the spin-orbit
components must be selected to discuss further
the electron-transfer mechanism in thesereaction systems.
AssumingaMaxwell–Boltzmann
distribution, 𝑁 is given by令和 4 年度

九州大学大学院総合理工学報告

Table 3. Total relative formation rate
constants and electronic-state distribution of
Ne* in the Ne+/SF6- I-IN reaction at thermalenergy.
State

Number of

∑ 𝑘 𝑢

∑ 𝑁 𝑢

sublevel

(%)

(%)

1
1
2
7
3
2
1
6

0.9
2.6
40.6
11.2
39.8
3.3
1.4
0.3

1.4
6.5
23.4
11.0
50.7
4.1
2.1
0.9

3p
4s
4s'
3d
3d'
5s
5s'
4d

Table 4. Total relative formation rate
constants and electronic-state distribution of
Ne* in the Ne+/C6F6- I-IN reaction atthermal energy.
State

Number of

∑ 𝑘 𝑢

∑ 𝑁 𝑢

sublevel

(%)

(%)

4
2
2
7
3
2
1
5
1
2

8.9
5.9
46.8
12.2
13.2
1.2
7.6
2.5
0.1
1.6

10.6
8.7
25.9
8.5
13.8
2.1
9.8
7.1
0.1
13.5

3p
4s
4s'
3d
3d'
5s
5s'
4d
4d'
5d

𝑁

𝑔 exp

𝐸 ⁄𝑘𝑇 ,

第 44 巻

第2号

18.4–21.0 eV region and high-energy 19.7–21.0
eV region, respectively. We have previously
obtained Boltzmann electronic temperatures of
0.070, 0.16, 0.090, and 0.19 eV for He* from the
He+/C6F5X- (X F, Cl, Br, and CF3) I-IN
reactions, respectively.13) Thus, the Boltzmann
electronic temperatures obtained for Ne* fromthe Ne+/SF6- and Ne+/C6F6- reactions are higher
than those of the He+/C6F5X- reactions.
It should be noted that many points are far
from the line of best fit in Figs. 6 and 8. Thus,
very small R2 (coefficient of determination)
values of 0.26 and 0.018 are obtained for the
Ne+/SF6- reaction in the 18.4–20.7 eV region
and for the Ne+/C6F6- reaction 19.7–20.7 eVregion, respectively. It is therefore reasonable
to assume that reaction dynamics of Ne+/SF6and Ne+/C6F6- I-IN reactions is not governed bystatistical model, where such long-lived
intermediates as Ne+–SF6- and Ne+–C6F6- areformed and excess energies are distributed to
all degrees of freedom statistically. Curvecrossings between strongly attractive entrance
Ne+–SF6- and Ne+–C6F6- curves and rather flat
Ne*–SF6 and Ne*–C6F6 exit curves and theprobability of an electron transfer from ananion to a cation at crossing points would playa significant role for the electronic-state
distributions of the present two I-IN reactions.
The crossing points Rc (Å) were calculated
from the relation

Rc = e 2/(IP–EA) = 14.38/(IP–EA),(22)

where 𝐸 is the excitation energy of Ne*. If aplot of ln 𝑁 /𝑔
vs 𝐸 is linear, then the
distribution can be characterized by aBoltzmann electronic temperature 𝑇 . The
dependence of ln(𝑁 /𝑔 ) on 𝐸 was fitted bystraight lines as shown in Figs. 5 and 6.
Although deviations from linearity are large inthe present systems, Boltzmann electronic
temperatures of 0.53 and 0.27 eV were obtained
from slopes of the straight lines for Ne* from the
Ne+/SF6- reaction in the whole 18.4–20.7 eVregion and high-energy 19.7–20.7 eV region,
respectively. On the other hand, Boltzmanntemperatures of 3.63 and 0.83 eV were obtainedfor Ne* from the Ne+/C6F6- reaction in the whole

17

(23)

where IP is the ionization potential of Ne* andEA is the electron affinity of X. The Rc values
calculated for the formation of each Ne* statein the Ne+/SF6- and Ne+/C6F6- reactions are
given in Tables 5 and 6. Most of the productstates are produced via curve crossings at
interparticle distances of 15–37 Å in the
Ne+/SF6- reaction, whereas they are curve
crossings at 5–38 Å in the Ne+/C6F6- reaction.
According to Landau–Zener theory, theprobability of electron transfer falls off rapidly
at large R c because of the rapidly diminishing
nonadiabatic coupling matrix elements, andtherefore the reaction efficiencies of such highenergy states as 5s, 5s', 4d, and 4d' would besmall.
Based on the ab initio calculation of SF6-, thesingly occupied molecular orbital (SOMO) of
SF6- is totally symmetric a1g with S(3s)–F(2p*) character.28) On the other hand, ESR
data in the condensed phase demonstrated that
the SOMO of C6F6- has * character.29) SinceElectonic-state distributions of Ne* in the Ne+/SF6- and Ne+/C6F6- ion-ion neutralization reactions

18

Table 5. Curve crossing points between
entrance Ne+(2P3/2,1/2)–SF6- and exit Ne*–SF6
potentials.

Table 6. Curve crossing points between
entrance
Ne+(2P3/2,1/2)–C6F6andexit
Ne*–C6F6 potentials.

Energy/ eV

StateEnergy
/ eV

State

18.38
19.66
19.76
19.78
20.03
20.03
20.03
20.04
20.04
20.05
20.05
20.14
20.14
20.14
20.56
20.57

3p [1/2]
4s [3/2]°
4s' [1/2]°
4s' [1/2]°
3d [1/2]°
3d [7/2]°
3d [7/2]°
3d [3/2]°
3d [3/2]°
3d [5/2]°
3d [5/2]°
3d' [5/2]°
3d' [5/2]°
3d' [3/2]°
5s [3/2]°
5s [3/2]°

18.38
18.56
18.58
18.64
19.66
19.69
19.76
19.78
20.03
20.03
20.03
20.04
20.04
20.05
20.05
20.14
20.14
20.14
20.56
20.57
20.66
20.70
20.71
20.71
20.71
20.71
20.81
21.02
21.02

3p [1/2]
3p [5/2]
3p [5/2]
3p [3/2]
4s [3/2]°
4s [3/2]°
4s' [1/2]°
4s' [1/2]°
3d [1/2]°
3d [7/2]°
3d [7/2]°
3d [3/2]°
3d [3/2]°
3d [5/2]°
3d [5/2]°
3d' [5/2]°
3d' [3/2]°
3d' [3/2]°
5s [3/2]°
5s [3/2]°
5s' [1/2]°
4d [1/2]°
4d [7/2]°
4d [7/2]°
4d [3/2]°
4d [5/2]°
4d' [3/2]°
5d [7/2]°
5d [5/2]°

Rc (Å)
Ne+(2P3/2)/SF66.69
16.61
18.69
19.17
28.56
29.03
29.04
29.15
29.37
29.84
29.86
36.50
36.52
36.63

Rc (Å)
Ne+(2P1/2)/SF66.40
14.89
16.54
16.91
23.82
24.15
24.16
24.24
24.39
24.72
24.72
29.11
29.12
29.20
205.55241.81

I-IN reactions occur via an electron transferfrom an anion to a cation, the overlapping
between the SOMO orbital with S–F or C–F *
character and a vacant orbital to which an
electron is transferred at the crossing point will
be significant for the final state distribution of
Ne*.

4.

Summary and Conclusion

The Ne+/SF6- and Ne+/C6F6- ion-ion mutual
neutralization reactions were studied byobserving Ne* lines in the He afterglow. Ne*emissions from forty-two ns(n=4 and 5), ns' (n=4
and 5), np(n=3 and 4), np' (n=3 and 4), and 4d
states of Ne* in the 18.38–20.71 eV region wereidentified in the Ne+/SF6- reaction, whereas
those from forty-five ns(n=4 and 5), ns' (n=4 and5), np(n=3 and 4), np' (n=3 and 4), nd(n=3–5), andnd' (n=3 and 4) states of Ne* in the 18.38–21.02
eV region are observed in the Ne+/C6F6reaction. Major product Ne* states in the
Ne+/SF6- reaction are 4s', 3d, and 3d' states,
whereas those in the Ne+/C6F6- reaction are 3p,4s', 3d, 3d', and 5d states. The electronic-state
distributions of Ne* in the Ne+/SF6- andNe+/C6F6- reactions were not expressed by asingle Boltzmann distribution. Therefore, it
was concluded that the reaction dynamics ismainly controlled by transition probability

Rc (Å)
Ne+(2P3/2)/C6F65.41
5.79
5.84
5.98
10.45
10.64
11.24
11.41
14.19
14.30
14.31
14.33
14.39
14.50
14.50
15.91
15.93
15.97
29.96
30.63
38.12
42.58
42.97
42.99
43.15
43.74
61.32
588.66670.25Rc (Å)
Ne+(2P1/2)/C6F65.21
5.56
5.61
5.74
9.74
9.90
10.42
10.57
12.91
13.01
13.01
13.03
13.08
13.17
13.17
14.32
14.34
14.37
24.79
25.25
30.13
32.85
33.08
33.09
33.19
33.54
42.99
115.57118.40

between the strongly attractive Ne+–X- ion-pairpotentials and rather flat exit Ne*–X potentials
at relatively large crossing points.
We studied here Ne* formation from the
Ne+/SF6- and Ne+/C6F6- I-IN reactions (15) and(16). In our previous studies on the Rg+/SF6(Rg=Ar, Kr, Xe) and Rg+/C6F6- (Rg=Kr)
reactions (4)–(7), RgF* (Rg=Ar, Kr, Xe) excimer
emissions were observed as major products inmost cases.6-9,11) It has been known that NeF*
excimer emission occurs in the vacuum
ultraviolet (VUV) region (105–115 nm with apeak at 108 nm).30) In the cases of Ne+/SF6- andNe+/C6F6- reactions, the formation of NeF*
excimer with a transition energy of 11.5 eV isenergetically possible.

令和 4 年度

九州大学大学院総合理工学報告

Ne+ + SF6- → NeF* + SF5
Ne+ + C6F6- → NeF* + C6F5

(24)
(25)

A further optical spectroscopic study in the
VUV region is necessary to clarify whether
excimer formation processes (24) and (25)compete with the formation of Ne* throughI-IN processes (15) and (16) and theirbranching ratios. .
Acknowledgments
The authors acknowledge financial support
from the Mitsubishi foundation (1996) andKyushu University Interdisciplinary Programsin Education and Projects in Research
Development (2001–2002). They also give
thanks to Prof. Kenji Furuya for his carefulreading of our manuscript.
References
D. R. Bates, Adv. Atomic. Mol. Phys., 20, 1 (1985).
M. R. Flanney, Adv. Atomic, Mol. Opt. Phys., 32, 117(1994).
3) M. Zauner-Wieczorek, J. Curtius, and A. Kürten,
Atmos.
Chem.
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