九州大学大学院総合理工学府報告第 43 巻 第 2 号 1-10 頁 令和 4 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 43, No. 2, pp. 1-10, FEB, 2022

Formation of Excited CH(A2, B2−and C2+) Radicals
by Collisions of Metastable Ne(3P0,2) Atoms with Simple
C1~C3 Aliphatic Hydrocarbons in a Flowing Afterglow
Masaharu TSUJI*1,2† Takahiro KOMATSU*3 Keiko UTO*1
Jun-Ichiro HAYASHI*1,2 and Takeshi TSUJI*4
E-mail of corresponding author: tsuji@cm.kyushu-u.ac.jp†(Received November 9, 2021, accepted November 24, 2021)
CH(A2−X2r B2−−X2r, and C2+−X2r) emission systems have been observed by dissociative
excitation of such simple C1~C3 aliphatic hydrocarbons as CH4, C2H6, C2H4, and C3H4 bycollisions with metastable Ne(3P2:16.62 and 3P0:16.72 eV) atoms in the flowing afterglow. The
emission rate constants of CH(A,B,C) from CH4, C2H6, C2H4, and C3H4 were determined to be 0.082,
0.039, 0.38, and 0.59 × 10-13 cm3 molecule-1 s-1, respectively. The CH(A) state was the major CH*
product in all reactions, which occupied 64−92% of CH(A,B,C) products. The nascent vibrational androtational distributions of CH(A:𝑣 =0–2 and B:𝑣 =0) were determined. The rotational distributions ofCH(A:𝑣 =0–2 and B:𝑣 =0) states were expressed by single Boltzmann temperatures of 1700–4600 K
and 2500–5500 K, respectively. The CH(A:𝑣 =0 and B:𝑣 =0) states from C3H4 were more rotationallyexcited than those from the other aliphatic hydrocarbons.
Key words: Dissociative excitation, Aliphatic hydrocarbons, Metastable Ne* atoms, Flowing afterglow,
Energy transfer, Rovibrational distribution, Superexcited state, Rydberg state

1.

Introduction

Reactions of rare gas metastable atoms withaliphatic hydrocarbons in discharge plasma areimportant for understanding dissociation andionization mechanisms of mixtures of rare gas
atoms and aliphatic hydrocarbons used for the
fabrication of various carbon films.1-3)
Compared with extensive studies on reactionsof He(23S:19.82 eV) and Ar(3P2:11.55 eV,
3P0:11.72
eV)
withsimplealiphatichydrocarbons,4-9) little work has been carriedout on reactions of Ne(3P0,2) with them becauseof high cost of Ne gas. We have previously
studied reactions of Ne(3P0,2) with CH4 andC2H2 in a Ne flowing afterglow (FA) by
observing optical emission spectra from such
excited fragments as CH* and C2* in theultraviolet (UV) and visible region.10,11)*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

Rovibrational distributions of CH(A,B) were
determined for the Ne(3P0,2)/CH4 reaction by aspectral simulation. Not only rovibrational
distributions of CH(A,B) and C2(d,C,e,D) but
also emission rate constants of CH(A,B,C) andC2(d,C,e,D)
weredeterminedfortheNe(3P0,2)/C2H2 reactions. It was concluded thatrotationally excited CH* and vibrationally androtationally excited C2* are formed via transbent Rydberg (superexcited) C2H2** states withelongated C≡C bond and bending mode
excitation.
In order to obtain more information on
reaction dynamics of acetylene, isotope effects
between C2H2 and C2D2 were examined.11)
Although the reduction of emission rate
constant of C2* from C2D2 was consistent witha simple dissociation model from the RRKM
theory, the increase in the emission rate
constant of CD* did not agree with it. The
inverse isotope effect for CD* was explained by
the fact that the formation of C2* and CH* or
CD* occurs competitively, so that fasterdissociation of C–H bonds suppresses theformation of CH*.
In the present study, emission spectra
resulting from energy-transfer reactions of2Formation of CH(A,B,C) radicals by the reactions of Ne(3P0,2) with aliphatic hydrocarbons in the flowing afterglow

Ne(3P0,2) atoms with CH4, C2H6, C2H4, and C3H4
(allene) are measured in the Ne FA. The
emission rate constants of CH(A,B,C) androvibrational distributions of CH(A,B) aredetermined. On the basis of these data, energytransfer mechanisms are discussed.

2.

Experimental

The FA apparatus equipped with an opticalemission detection system was the same as
those reported previously.10,11,13) The following
gases were used in this study: Ne (Nippon
Sanso: purity 99.55%), CH4 (Eto Sanso: 99.0%),
C2H6 (Eto Sanso: 99.0%), C2H4 (Eto Sanso:
99.0%), C2H6 (Eto Sanso: 99.0%), C3H4 (Nippon
Sanso: 99.3%), and SF6 (Seitetsu Kagaku:
99.7%).
In brief, the metastable Ne(3P0,2) atoms weregenerated by a microwave discharge of highpurity Ne gas operated at Ne pressures of
0.1−0.15 Torr (1 Torr = 133.3 Pa). The effect ofionic active species was examined by using an
ion-collector grid placed between the discharge
section and the reaction zone. A small amountof aliphatic hydrocarbons (CH4, C2H6, C2H4,
and C3H4) was injected into the Ne afterglow
from a nozzle placed 10 cm downstream fromthe center of the discharge. The partialpressure of reagent gas in the reaction zone was
1−6 mTorr.
Emission spectra were observed through aquartz window placed around a reagent gas
inlet. A Spex 1.25 m monochromator equipped
withaHamamatsu
Photonics
R376
photomultiplier was used for spectral
measurements in the 300–700 nm region. The
relative sensitivity of the optical detection
system was calibrated using standard D2 andhalogen lamps.

3.

Results and Discussion

3.1
Emission spectra
excitation processes

anddissociative

Figures 1a−1d show typical emission spectra
obtained from the Ne afterglow reaction of CH4,
C2H6, C2H4, and C3H4, respectively. The
CH(A2−X2r, B2−−X2r, and C2+−X2r
emission systems are identified in the 300−450
nm region. In the 305−320 nm region, a broad
OH(A2+−X2) emission system14) resulting
from dissociative excitation of impurity H2O isoverlapped with the CH(C−X) emission with asharp peak at 314 nm. When the contributionof ionic active species to the observed emissions
was examined by using the ion-collector grid,Fig. 1. Emission spectra obtained from Ne
afterglow reactions of (a) CH4, (b) C2H6, (c)C2H4, and (d) C3H4 (allene).
no appreciable changes were observed in theintensities of CH(A−X, B−X, and C−X). These
results indicate that Ne+ and Ne2+ ions do notparticipate in the formation of CH(A,B,C).
Based on above findings, Ne(3P0,2) atoms are
responsible for the production of these emitting
excited states.
Figures 2−5 show energy-level diagrams ofionization states and CH(A,B,C) states
produced from CH4, C2H6, C2H4, and C3H4,
respectively, where ionization potentials (IPs)and dissociation limits (DLs) for the formationof each state are shown. Thermochemical andspectroscopic data reported in Refs. 15−17 areused for the calculations. The lowest IPs of CH4,
C2H6, C2H4, and C3H4 are 10.1−12.0 eV. Since
these values are lower than the energies ofNe(3P0,2:16.62 and 16.72 eV), M+ (M = CH4,
C2H6, C2H4, and C3H4) ions can be formed by
Penning ionization (1).
Ne(3P0,2) + M → M+ + Ne + e(M = CH4, C2H6, C2H4, and C3H4)(1)

In the cases of C2H6, C2H4, and C3H4, not only
the ground ionic state but also excited ionic令和 3 年度

九州大学大学院総合理工学報告第 43 巻第2号

3statesFig. 2.
Energy-level diagrams for theformation of CH(A,B,C) states from CH4. IP
and DL mean ionization potential anddissociation limit, respectively.

Fig. 3.
Energy-level diagrams for theformation of CH(A,B,C) states from C2H6.
states can be formed energetically. The DLs of
CH(A,B) from CH4 and C2H4, and CH(A) from
C2H6 are lower than first IPs of M. Therefore,
formation of CH(A,B) via two-body electron-ion
recombination processes are energeticallypossible.
M+ + e- → CH(A,B) + neutral products (2)(M = CH4, C2H6, C2H4)
To examine the contribution of above processes,
effects of the addition of a typical electron
scavenger, SF6, was examined. SF6 scavenges
thermal electrons very rapidly:
e- + SF6 → SF6k3 = 2.2 × 10-7 cm3 s-1 (Ref. 18).

(3)

Little intensity changes are observed before
and after SF6 addition. Based on this result, weconcludethat
two-body
electron-ion
recombination processes (2) do not participate
in the formation of CH(A,B) from CH4, C2H6,
and C2H4. Thus, two-body energy-transfer
reactions between Ne(3P0,2) and M are
responsible for the formation of CH(A,B).

Fig. 4.
Energy-level diagrams for theformation of CH(A,B,C) states from C2H4.

Fig. 5.
Energy-level diagrams for theformation of CH(A,B,C) states from C3H4
(allene).
3.2 Emission rate constants of CH(A,B,C) in
the Ne(3P0,2)/M (M = CH4, C2H6, C2H4, andC3H4) reactions
Energetically possible excitation processes ofCH(A,B,C) by the Ne(3P2)/M (M = CH4, C2H6,
C2H4, and C3H4 ) atoms are as follows:
Ne(3P2) + CH4→ CH(A) + H2 + H + Ne + 4.55 eV,→ CH(B) + H2 + H + Ne + 4.24 eV,→ CH(C) + H2 + H + Ne + 3.49 eV,→ CH(A) + 3H + Ne + 0.03 eV.

(4a)
(4b)
(4c)
(4d)

Ne(3P2) + C2H6
→ CH(A) + CH4 + H + Ne + 5.23 eV,→ CH(B) + CH4 + H + Ne + 4.92 eV,→ CH(C) + CH4 + H + Ne + 4.17 eV,→ CH(A) + CH3 + H2 + Ne + 5.20 eV,→ CH(B) + CH3 + H2 + Ne + 4.90 eV,→ CH(C) + CH3 + H2 + Ne + 4.14 eV,→ CH(A) + CH + 2H2 + Ne + 1.43 eV,→ CH(B) + CH + 2H2 + Ne + 1.12 eV,→ CH(C) + CH + 2H2 + Ne + 0.36 eV,→ CH(A) + CH3 + 2H + Ne + 0.68 eV,→ CH(B) + CH3 + 2H + Ne + 0.38 eV,

(5a)
(5b)
(5c)
(5d)
(5e)
(5f)
(5g)
(5h)
(5i)
(5j)
(5k)

4

Formation of CH(A,B,C) radicals by the reactions of Ne(3P0,2) with aliphatic hydrocarbons in the flowing afterglow→ CH(A) + CH2 + H2 + H + Ne + 0.45 eV,(5l)→ CH(B) + CH2 + H2 + H + Ne + 0.14 eV.
(5m)
Ne(3P2) + C2H4
→ CH(A) + CH3 + Ne + 6.62 eV,→ CH(B) + CH3 + Ne + 6.31 eV,→ CH(C) + CH3 + Ne + 5.56 eV,→ CH(A) + CH + H2 + Ne + 1.97 eV,→ CH(B) + CH + H2 + Ne + 1.66 eV,→ CH(C) + CH + H2 + Ne + 0.91 eV,→ CH(A) + CH2 + H + Ne + 1.86 eV,→ CH(B) + CH2 + H + Ne + 1.56 eV,→ CH(C) + CH2 + H + Ne + 0.80 eV.

(6a)
(6b)
(6c)
(6d)
(6e)
(6f)
(6g)
(6h)
(6i)

Ne(3P2) + C3H4
→ CH(A) + C2H3 + Ne + 6.45 eV,(7a)→ CH(B) + C2H3 + Ne + 6.15 eV,(7b)→ CH(C) + C2H3 + Ne + 5.39 eV,(7c)→ CH(A) + C2H2 + H + Ne + 4.94 eV, (7d)→ CH(B) + C2H2 + H + Ne + 4.64 eV, (7e)→ CH(C) + C2H2 + H + Ne + 3.88 eV, (7f)
→ CH(A) + C2H + H2 + Ne + 4.61 eV, (7g)→ CH(B) + C2H + H2 + Ne + 4.30 eV, (7h)
→ CH(C) + C2H + H2 + Ne + 3.55 eV, (7i)
→ CH(A) + C2H + 2H + Ne + 2.06 eV, (7j)
→ CH(B) + C2H + 2H + Ne + 1.75 eV, (7k)
→ CH(C) + C2H + 2H + Ne + 1.00 eV, (7l)
→ CH(A) + CH3 + H + Ne + 0.62 eV, (7m)
→ CH(B) + CH3 + H + Ne + 0.31 eV.
(7n)
For the reactions with higher-energy
metastable Ne(3P0) atoms, 0.10 eV higher
excess energies are released in the above
reactions. Based on our previous experiment ontheNe(3P0,2)/Xe+(2P3/2)
excitation-transfer
reaction,19) the [Ne(3P0)]/[Ne(3P2] ratio in theflow system is expected to be ≈0.13.
The emission rate constants for reactions (4)–(7) were determined by using a reference
reaction method. The NO(A-X) emission fromthe Ne(3P0,2)/NO reaction was used as areference reaction.11)

Ne(3P0,2) + NO → NO(A2+) + Ne
k8 = 4.8 × l0-11 cm3 molecule-1 s-1(8)

Emission rate constants were estimated bycomparing the integrated emission intensity ofeach CH* band system with that of NO(A–X) in
prepared mixtures of M/NO. When there is nononradiative decay and pumping from theobservation region, the emission rate constants
correspond to the formation ones.
Emission rate constants thus obtained are
given
in
Table
1.
For
comparison,corresponding data for C2H2 obtained in ourprevious work11) is given. The total emission
rate constant of CH(A,B,C) from C2H2, 1.72 ×
10-13 cm3 molecule-1 s-1, is largest. The
kCH*(M)/kCH*(C2H2) ratio decreases in the orderof C3H4, C2H4, CH4, and C2H6 from 0.34 to 0.22,
0.048, and 0.023, respectively. In all cases, theCH(A) state is major CH* products, which
occupies 64−92% of CH(A,B,C). Minor CH(B)and CH(C) states occupy 7.9−36 and0.061−1.2% of CH(A,B,C), respectively.
The quenching rate constant of Ne(3P0,2) by
CH4 has been measured to be 11 × l0-11 cm3
molecule-1 s-1.20) A comparison between thisvalues and the emission rate constant of CH*
obtained in this work suggests that thebranching ratio of dissociative excitation
leading to CH(A,B,C) is only 7.5 × 10-3 % in
the Ne(3P0,2)/CH4 reaction. Based on this fact,
dissociative excitation (4a)−(4d) is minor
product channel, so that Penning ionizationand dissociation into non-emitting neutral
products are expected to be major productchannels in the Ne(3P0,2)/CH4 reaction.
Unfortunately, quenching rate constants ofNe(3P0,2) by C2H6, C2H4, and C3H4 have not beenmeasured. Therefore, information on total
quenching rate constants and Penningionization rate constants is required todetermine branching ratios of CH(A,B,C)
formation in the Ne(3P0,2)/M (M = C2H6, C2H4,
and C3H4) reactions.

Table 1. Emission rate constants (× 10-13 cm3 molecule-1 s-1) of CH(A,B,C)
produced from the reactions of Ne(3P0,2) with aliphatic hydrocarbons.
observa
CH4

C2H6

C2H4

C2H2

C3H4

This work

This work

This work

Ref. 11This work

CH(A)

0.070

0.025

0.35

1.5

0.53

CH(B)

0.012

0.014

0.030

0.22

0.052

CH(C)

3.3 E(-4)

4.5 E(-4)

2.3 (E-4)

4.6 (E-3)

6.8 (E-3)

sum

0.082

0.039

0.38

1.72

0.59

令和 3 年度

九州大学大学院総合理工学報告

3.3 Rovibrational distributions of CH(A,B) in
the Ne(3P0,2)/M (M = CH4, C2H6, C2H4, andC3H4) reactions
Rovibrational distributions of CH(A,B) in theNe(3P0,2)/M (M = C2H6, C2H4, and C3H4)

Fig. 6.
(a) observed and (b) simulated
emission spectra of CH(A2−X2r) bandsystem in the Ne(3P0,2)/C2H6 reaction.

Fig. 7.
(a) observed and (b) simulated
emission
spectra
of
(0,0)
band
of
CH(B2−−X2r)bandsysteminthe
Ne(3P0,2)/C2H6 reaction.

第 43 巻第2号

5

reactions were determined by a computer
simulation of CH(A−X, B−X) bands because
rotational lines were not fully resolved. The
simulation method used in this study was thesame as that reported in our previous studies
on CH410) and C2H2.11)
In Figs. 6−11 are shown the observed andFig. 8. (a) observed and (b) simulated emission
spectra of CH(A2−X2r) band system in theNe(3P0,2)/C2H4 reaction.

Fig. 9. (a) observed and (b) simulated emission
spectra of (0,0) band of CH(B2−−X2r) bandsystem in the Ne(3P0,2)/C2H4 reaction.

6

Formation of CH(A,B,C) radicals by the reactions of Ne(3P0,2) with aliphatic hydrocarbons in the flowing afterglow

best fit spectra of the CH(A−X, B−X) bands
from C2H6, C2H4 and C3H4 calculated assuming
single Boltzmann rotational distributions foreach 𝑣′ level. Vibrational and rotationaltemperatures thus obtained are given in Table
2 along with our previous data for CH410) andC2H2.11) From the observed rovibrational

distributions of CH(A,B), we evaluated theaverage vibrational and rotational energies ofeach CH* state, denoted as <Ev> and <Er>,
respectively. The <Ev> and <Er> values
obtained for CH(A,B) are summarized in Tables
3 along with our previous data for CH4 andC2H2.10,11) As discussed in our previous paper,11)
it is expected that the vibrational androtational relaxation of CH(A,B) by collisions
with the buffer Ne gas is insignificant within
their radiative lifetimes of about 500 ns forCH(A)21) and 345–357 ns for CH(B),22) so thatobserved rovibrational distributions reflect
nascent distributions.
The following tendencies are found for theobserved rovibrational distributions of CH*.

Fig. 10. (a) observed and (b) simulated
emission spectra of CH(A2−X2r) bandsystem in the Ne(3P0,2)/C3H4 reaction.

Fig. 11. (a) observed and (b) simulated
emission spectra of (0,0) band of
CH(B2−−X2r) band system in theNe(3P0,2)/C3H4 reaction.

(1) The vibrational population of CH(A:𝑣 =0–2)
decreases with increasing 𝑣 . The
rotational temperature of CH(A) decreases
from 3300–4600 K to 1700–2500 K withincreasing 𝑣 from 0 to 2. The rotationaltemperature of CH(B: 𝑣 =0) is 2500–5500 K.
(2) The CH(A) states from CH4 and C2H2 are
more vibrationally excited than those from
C2H6, C2H4 and C3H4. The rotationaltemperature of CH(A:𝑣 =0) for CH4, C2H2,
and C3H4 (4200–4600 K) are higher thanthose for C2H6 and C2H4 (3300–3400 K).
The CH(A: 𝑣 =0 and B: 𝑣 =0) states from
C3H4 are more rotationally excited than
those
from
the
other
aliphatichydrocarbons.
(3) The <𝐸 > values of CH(A) for CH4 and C2H2
(0.13–0.15) are larger than those for C2H6,
C2H4 and C3H4 (0.09–0.10). The < 𝐸 >values of CH(A) for CH4, C2H2, and C3H4
(0.31–0.34) are larger than those for C2H6
and C2H4 (0.26–0.27). The <𝐸 > value ofCH(B) for C3H4 (0.47) is larger than those
for CH4, C2H2, and C2H4 (0.22–0.26).
(4) Tokeshi et al.23) measured vibrational androtational distributions of CH(A) from C2H6,
C2H4, and C2H2 by electron impact at 17–
100 eV range. The N1/N0 andN2/N0 values
at 17 eV, which are close to the energies ofNe(3P0,2), were 0.30 and 0.058 for C2H6, 0.29
and 0.044 for C2H4, and 0.44 and 0.078 for
C2H2, respectively. These values are in
reasonable agreement with those obtained
from the Ne(3P0,2) reactions (Table 2).
TherotationaltemperaturesofCH(A:𝑣 =0) and CH(A:𝑣 =1) under electron
impact at 17 eV were 4400 and 3200 K for
C2H6, 5100 and 3400 K for C2H4, and 3200
and 2000 K for C2H2, respectively.
Rotational temperatures obtained from the令和 3 年度

九州大学大学院総合理工学報告第 43 巻

第2号

7

Table 2. Rovibrational distributions of CH(A,B) produced from the Ne(3P0,2)/aliphatichydrocarbons reactions.

Emitting
species
𝑣′=0

CH(A)

𝑣′=1
𝑣′=2

CH(B)

𝑣′=0

CH4
Ref. 10C2H6This work

C2H4
This work

C2H2
Ref. 11C3H4This work

𝑁𝑣′

𝑇𝑟 / K𝑁𝑣′

𝑇𝑟 / K𝑁𝑣′

𝑇𝑟 / K𝑁𝑣′

𝑇𝑟 / K𝑁𝑣′

𝑇𝑟 / K

100
58
9
100

4200
2800
2400
3000

100
27
6
100

3400
2200
1750
3000

100
32
4
100

3300
2200
1700
2750

100
45
7
100

4200
3000
2500
2500

100
28
4
100

4600
2200
1700
5500

Table 3. Average vibrational and rotational energies (eV) deposited into CH (A, B) in theNe(3P0,2)/aliphatic hydrocarbons reactions.
CH4
Ref. 10C2H6This work

C2H4
This work

C2H2
Ref. 11C3H4This work

Emitting
species

<𝐸 >

<𝐸 >

<𝐸 >

<𝐸 >

<𝐸 >

<𝐸 >

<𝐸 >

<𝐸 >

<𝐸 >

<𝐸 >

CH(A)
CH(B)

0.15
0.0

0.31
0.26

0.10
0.0

0.27
0.26

0.10
0.0

0.26
0.24

0.13
0.0

0.32
0.22

0.09
0.0

0.34
0.47

Ne(3P0,2) reactions are lower than those byelectron impact excitation by 1000 K forC2H6 and by 1200–1800 K for C2H4,
whereas they are higher than those byelectron impact excitation by 1000 K for
C2H2 (Table 2).
3.4 The formation mechanisms of CH* from
simple aliphatic hydrocarbons
The formation mechanisms of CH* fromaliphatic hydrocarbons are discussed frommolecular structure and electronic structure ofsuperexcited states of each hydrocarbon. As
shown in energy-level diagrams (Figs. 2−5),energies of Ne(3P2 ＝ 16.60 eV, 3P0 = 16.72 eV)are higher than those of the first IPs of CH4,
C2H6, C2H4, and C3H4 by 4.1−6.7 eV. Therefore,
not only the ground ionic states but also someexcited ionic states can be formed by Penningionization between Ne(3P0,2) and M.
Ne(3P0,2) + M → M+ and M+* + Ne + e- (9)(M = CH4, C2H6, C2H4, and C3H4)
Penning ionization
In addition, the formation of superexcited M**
states below 16.72 eV is also possible, fromwhich autoionization and dissociation occur
competitively.

Ne(3P0,2) + M → M** + Ne
(10a)
(M = CH4, C2H6, C2H4, and C3H4)
M** → M+ and M+* + e- (autoionization)(10b)M** → m1 + m2 (dissociation)(10c)where m1 and m2 are neutral fragments.
There are two possible energy-transfer
mechanisms in process (10a). One is nearresonant energy transfer without significant
momentum transfer. The other is non-resonant
energy transfer with a significant momentum
transfer
Ne(3P0,2) + M → M** + Ne + T(11a)
M** → CH(A,B,C) + neutral fragments(11b)where T represents a translational energy
released after energy transfer. If entrance
Ne(3P0,2) + M potentials are strongly attractive,
such non-resonant energy transfer can occur
through accidental avoided crossing between
attractive entrance surfaces and low energy
M** + Ne exit potentials. Actually, such nonresonant energy transfer has been observed in
the reactions for open shell atoms andmolecule: e.g., Ar(3P0,2) + O and Cl and Ar(3P2)+ C2H5 and Kr(3P2) + C2H5.24,25) For the cases ofM (M = CH4, C2H6, C2H4, and C3H4), groundstates are closed shell singlet states. Therefore,
rather flat entrance Ne(3P0,2) + M potentials are8Formation of CH(A,B,C) radicals by the reactions of Ne(3P0,2) with aliphatic hydrocarbons in the flowing afterglow

expected. It is therefore reasonable to assume
that energy-transfer from Ne(3P0,2) to M takeplace via near-resonant excitation processes
without significant momentum transfer.
Based on our optical spectroscopic studies on
the dissociative excitation of simple polyatomic
molecules by rare gas metastable He(23S) andRg(3P0,2) atoms,26-30) spin-conservation rule
generally holds during the reaction. Therefore,
triplet superexcited M** states below 16.72 eV
are expected to be formed after near-resonant
energy transfer, because the ground states of M
are singlet. Although superexcited states ofCH4, C2H6, and C2H4 have been extensively
studied by using vacuum ultraviolet photons
and fast electron-impact excitation,23,31-40) they
are spin-allowed singlet states. It is highly
likely that similar triplet superexcited states
exist just below corresponding singlet states,
although little information has been obtainedon the triplet superexcited states of aliphatichydrocarbons in the 13−17 eV range.
3.4.1

CH4

The ground state methane in Td symmetry
has the electronic configuration,36)
1A1: (1a1)2(2a1)2(1t2)6.

(12)

Kato et al.37) studied superexcited states of CH4around 14.5 eV built on the (1t2)-1 ion core andthose around 22 eV built on the (2a1)-1 ion coreusing synchrotron orbital radiation (SOR).
Although these superexcited states are
optically allowed singlet states, similar triplet
superexcited states are expected to exist below
the corresponding singlet states. They
concluded that CH(A) and CH(B) are produced
from single-hole one-electron states built on the(1t2)-1 ion core ion in the 13–16 eV region undervacuum ultraviolet (VUV) photoexciation. It isreasonable to assume that CH(A) and CH(B)are also produced from similar single-hole oneelectron states built on the (1t2)-1 ion core in theNe(3P0,2)/CH4
reaction.
However,thesesuperexcited states in the 13–16.7 eV are
unknown triplet states, which are not observed
under VUV photoexciation. Since the 1t2
pseudo character,17)
molecular orbital has 𝜋
C–H bond-length and H–C–H angles willchange when an electron is ejected from thisorbital. These geometric changes led tovibrational and rotational excited CH(A,B)
fragments from dissociation of CH4**.
3.4.2

C2H6

The electronic configuration of the ethane
molecule in the ground state is
1A1g : (1a1g)2(1a2u)2(2a1g)2(2a2u)2× (1eu)4(3a1g)2(1eg)4(13)

for the staggered D3d symmetry.23,32) The
vertical ionization potentials of these orbitals
were determined by photoelectron spectroscopy
as 11.99 (12.70) eV for the (1eg)-1 state, 13.5 eV
for the (3a1g)-1 state, and 15.15 (15.9) eV for the(1eu)-1 state; Jahn-Teller components are given
in parentheses.17) On the basis of detailed VUVphotoexcitation studies on C2H6,36) neutral
superexcited states of C2H6** exist in the 11–
16 eV region which cover all Rydberg states
converging to the above ionic states. Unknown
triplet Rydberg states converging to the (1eu)-1
state will be most important precursor statesleading to CH(A,B) based on energy-resonant
rule. Molecular structures of Rydberg states
will be similar to those of related ionic states.
pseudoSince the 1eu molecular orbital has 𝜋character, C−H bond length and H−C−H angle
will change after an electron is ejected from thisorbital. Geometrical changes after excitation
into (1eu)-1 ion core Rydberg states led tovibrational and rotational excited CH(A,B)
fragments from C2H6**.
3.4.3

C2H4

The electronic configuration of the ethylene
molecule in the ground state is1g+: (1ag)2(1b1u)2(2ag)2(2b1u)2(1b2u)2× (3ag)2(lb3g)2(1b3u)2(14)

for the D2h symmetry.40) The vertical ionization
potentials of these orbitals below 17 eV were
determined by photoelectron spectroscopy as
10.51 eV for the (1b3u)-1 state, 12.85 eV for the(1b3g)-1 state, 14.66 eV for the (3ag)-1 state, and15.87 eV for the (1b2u)-1 state.17)
The formation of CH(A,B) from VUVphotodissociation of C2H4 in the 11.7–21.4 eV
range has been studied by O’Reilly et al.38) Two
dissociation processes have been observed forthe formation of CH(A,B).
Thermodynamic
thresholdC2H4 → CH(A) + CH3→ CH(B) + CH3→ CH(A) + CH + H2
→ CH(B) + CH + H2

10.0 eV
10.3 eV
14.6 eV14.9 eV

(15a)
(15b)
(16a)
(16b)

令和 3 年度

九州大学大学院総合理工学報告

The observed threshold energies for (15a) and(15b) were 10.4±0.5 eV and <12 eV, whereas
those for (16a) and (16b) were 14.6±0.8 eV and
14.6±0.8 eV, respectively. Processes (16a) and(16b) are more near resonant than processes(15a) and (15b) in the Ne(3P0,2)/C2H4 reactions.
Therefore, they will be major formation
processes of CH(A,B) in the Ne(3P0,2)/C2H4
reaction, if the Ne(3P0,2)/C2H4 reaction proceeds
through similar processes as observed VUVphotodissociation. Unknown triplet Rydberg
states in the 14.6–16.7 eV converging to the(3ag)-1 and (1b2u)-1 ionic states would beintermediates for the formation of CH(A,B).
Since the (3ag)-1 and (1b2u)-1 molecular orbital
pseudo characters, C=C
and 𝜋have σand C−H bond lengths and H−C−H angels
change, when an electron is ejected from 3ag or
1b2u orbital. Vibrational and rotational
excitation of CH(A,B) fragments are expectedfrom C2H4** Rydberg states because ofgeometrical changes in the C2H4** states after
excitation from the ground C2H4 state.
3.4.4

C3H4

The electronic configuration of the allene
molecule in the ground state is
1

A1 : (la1)2(lb2)2(2a1)2(3a1)2(2b2)2× (4a1)2(3b2)2(le)4(2e)4(17)

for D2d symmetry.35) The vertical ionization
potentials of these orbitals were determined byphotoelectron spectroscopy as 10.06 and 10.60
eV for the (2e)-1 state, 14.70 eV for the (1e)-1state, 15.47 eV for the (3b2)-1 state, and 17.47
eV for the (4a1)-1 state.17)Thevalence-shell
photoabsorption,
photodissociation and photoionization crosssections of allene have been measured from theionization threshold to 35 eV using SOR.35)
Although no information on the CH(A,B) from
C3H4 has been obtained, small cross sections
corresponding to dissociation into neutral
products are observed in the 14–17 eV region.
Dissociation cross sections are much smaller
than ionization cross sections, indicating thatneutral dissociation processes are minor exit
channels in this energy region under VUVphotoexciation.
Dissociation
includingdissociative excitation may also be minor exit
channel in the Ne(3P0,2)/C3H4 reaction because
the emission rate constant of CH(A,B,C) from
C3H4 obtained here was very small.
Based on energy-resonant and spinconservation rules, CH(A,B) will be formed via

第 43 巻第2号

9superexcited states in the 14.0–16.7 eV region.
Possible candidates are triplet Rydberg states
converging to the (1e)-1, (3b2)-1, and (4a1)-1 states, σ , and C2s characters,
having 𝜋
respectively. It should be noted that theCH(A: 𝑣 =0 and B: 𝑣 =0) states are morerotationallyexcited
than
the
other
hydrocarbons. High rotational excitation may
arise from rotationally excitation of precursor
superexcited C3H4** states and/or vibrational
excitation of H−C−H bending modes of C3H4**.

4.

Summary and Conclusion

Dissociative excitation of CH4, C2H6, C2H4,
and C3H4 by collisions with metastable
Ne(3P0,2) atoms has been studied by observing
CH(A–X,B–X,C–X) emissions in the Ne FA. The
effect of SF6 addition into the Ne afterglow
indicatedthat
secondaryelectron-ion
recombination reactions do not participate in
the formation of CH(A,B.C) and they areformed through the primary Ne(3P0,2)/M (M =CH4, C2H6, C2H4, and C3H4) reactions. The
emission rate constants of CH(A,B,C) were
determined by using a reference reaction
method. A comparison between the observed
emission rate constant and reported total
quenching rate constant in the Ne(3P0,2)/CH4
reaction indicated that the branching ratio of
dissociative excitation leading to CH(A,B,C) is
only 7.5 × 10-3 %. Major exit channels in theNe(3P0,2)/CH4 reaction was expected to be
Penning ionization and dissociation into nonemitting neutral products. The nascentvibrational and rotational distributions ofCH(A: 𝑣 =0–2,B: 𝑣 =0) were determined.
Vibrational and rotational excitation ofCH(A,B) originates from geometrical changes
in CC, CH, and HCH bonds after excitation into
M** superexcited states. A high rotational
excitation of CH(A,B) from C3H4 suggested that
precursor superexcited C3H4** is rotationallyexcited and/or vibrational excitation of H−C−H
bending modes of C3H4**.
Acknowledgments
Authors thank Prof. Takeshi Odagiri (Sophia
Univ.) for sending a copy of Ref. 37. This work
was supported by the Mitsubishi foundation(1996) and JSPS KAKENHI Grant number
09440201 (1997-2000).
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