九州大学大学院総合理工学府報告第 43 巻 第 2 号 11-23 頁 令和 4 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 43, No. 2, pp. 11-23, FEB, 2022

Formation of Excited CH(A2) and CD(A2) Radicals by
Collisions of Metastable Ar(3P2) Atoms with C2H, C2D,
and C2H3 Radicals in an Ar 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) and CD(A2−X2r) emission systems have been observed by dissociative excitationof C2H, C2D, and C2H3 radicals by collisions with metastable Ar(3P2:11.55 eV) atoms in the Ar flowing
afterglow. C2H, C2D, and C2H3 radicals were generated by the reactions of F atoms with C2H2, C2D2,
and C2H4, respectively. The nascent vibrational and rotational distributions of CH(A:𝑣 =0–2) andCD(A: 𝑣 =0–2) were determined from computer simulation of observed emission spectra. The
vibrational distributions of CH(A) and CD(A) rapidly decreased with increasing 𝑣 in all threereactions. The rotational distributions of CH(A:𝑣 =0–2) and CD(A:𝑣 =0–2) states were expressed bysingle Boltzmann temperatures of 300–1900 K and 600–3800 K, respectively. High rotational
excitation of CH(A) and CD(A) in the Ar(3P2)/C2H and Ar(3P2)/C2D reactions suggested that bending
modes of precursor C2H** and C2D** Rydberg states are excited. The observed rovibrational
distributions of CH(A) and CD(A) in the Ar(3P2)/C2H and Ar(3P2)/C2D reactions indicated that most ofexcess energies are deposited into relative translational energies of CH(A) + C and CD(A) + C products.
The observed vibrational and rotational distributions were compared with statistical prior ones forcharacterizing dynamical features of reactions.
Key words: Dissociative excitation, Aliphatic hydrocarbon radicals, Metastable Ar* atoms, Flowing afterglow,
Energy transfer, Rovibrational distribution, Statistic dissociation models, Rydberg state

1.

Introduction

The chemical vapor deposition of such simple
aliphatic hydrocarbons as CH4 and C2H2 in aplasma reactor has been widely used formakingcarbonfilms.1-3)
In
general,hydrocarbons are diluted by a rare gas and H2
to maintain stable plasma. This caused
increased interest in studies on dissociation
processes of aliphatic hydrocarbons diluted byrare gases in the plasma reactor. We have madesystematic studies on dissociative excitation of
simple aliphatic hydrocarbons by collisions
with the metastable Kr(3P2), Ar(3P2), Ne(3P0,2),
and He(23S) atoms in the flowing afterglow*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(FA).4-8) Product state distributions wereobtained by analyzing the emission spectra.
As part of the continuing research program
to investigate the energy-transfer processes
between metastable rare gas atoms and simple
aliphatic hydrocarbons, we have previously
extended this study to the reactions of Xe(3P2),
Kr(3P2), and Ar(3P2) with CH3 and C2H5 radicals,
which are the simplest alkyl radicals.9,10) The
excited CH(A,B) states were formed as
dominant emitting exit channels. The
rovibrational distributions were determined bya spectral simulation. These distributions
were compared with the results fromstatistical model calculations for characterizing
the dynamical features of the reactions.
In the present study, emission spectra
resulting from energy-transfer reactions ofAr(3P2) atoms with C2H, C2D, and C2H3 radicals
are measured in the FA. The rovibrational
distributions of CH(A) and CD(A) aredetermined by a computer simulation ofobserved band profiles. The observed

12

Formation of CH(A) and CD(A) radicals by the reactions of Ar(3P2) with C2H, C2D, and C2H3 radicals in the flowing afterglow

rovibrational distributions are compared withthose from statistical two-body and three-bodydissociation models to characterize thedynamical features of the reactions. On the
basis
of
these
data,
energy-transfer
mechanisms are discussed.

2.

Experimental

Figure 1 shows a schematic diagram of the
FA apparatus used in the present study. Thisflowing system was evacuated continuouslywith a 10000 l/min mechanical booster pump
(ULVAC 006) backed by a 1600 l/min oil rotary
pump. The following gases were used in thisstudy: Ar (Nippon Sanso: purity: 99.9999%),
C2H2 (Eto Sanso: 99.0%), C2H4 (Eto Sanso:
99.0%), C2H6 (Eto Sanso: 99.0%), and CF4(Seitetsu
Kagaku:99.5%).
C2D2
wassynthesized by the reaction of CaC2 with D2O
as reported in our previous paper.7)
There are two microwave (MW) dischargesources, which are denoted as MW(A) andMW(B) in Fig. 1. MW(A) was used for thegeneration of F atoms by MW discharge of
Ar/CF4 mixture. MW(B) was used for thegeneration of metastable Ar(3P2: 11.55 eV, 3P0:
11.72 eV) atoms. Since the [Ar(3P0)]/[Ar(3P2)]
ratio has been reported to be about 0.01,11) thecontribution of the upper Ar(3P0) spin-orbit
component could be negligible in our condition.
The C2H, C2D, and C2H3 radicals weregenerated by the following hydrogen
abstraction reaction by F atoms:12,13)
F + C2H2 → C2H + H
F + C2D2 → C2D + D
F + C2H4 → C2H3 + H

(1)
(2)
(3)

where fluorine atoms were produced from a

MW discharge of dilute (3%) CF4 in Ar. Typicalpressures of Ar and CF4 gases in the discharge
tube were 0.16−0.19 Torr (1 Torr = 133.3 Pa)
and 5 mTorr, and those of C2H2, C2D2, and C2H4
were 16, 1, and 16 mTorr, respectively. The
total pressure in the reaction zone was
0.19−0.20 Torr, when both MW(A) and MW(B)were operated.

3.

Results and Discussion

3.1
Emission spectra
excitation processes

anddissociative

The minimum energies required for theformation of CH(A:𝑣 =0−2) and CD(A:𝑣 =0−2)
from C2H, C2D, and C2H3 radicals are as follows.
C2H → CH(A:𝑣 =0) + C − 10.57 eV→ CH(A:𝑣 =1) + C − 10.91 eV→ CH(A:𝑣 =2) + C − 11.22 eV

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

C2D → CD(A:𝑣 =0) + C − 10.73 eV→ CD(A:𝑣 =1) + C − 10.98 eV→ CD(A:𝑣 =2) + C − 11.22 eV

(5a)
(5b)
(5c)

C2H3 → CH(A:𝑣 =0) + CH2 – 9.94 eV→ CH(A:𝑣 =1) + CH2 – 10.28 eV→ CH(A:𝑣 =2) + CH2 – 10.60 eV

(6a)
(6b)
(6c)

Therefore, the formation of CH(A:𝑣 =0−2) andCD(A: 𝑣 =0−2) from C2H, C2D, and C2H3 isenergetically accessible.
Ar(3P2) + C2H→ CH(A:𝑣 =0) + C + Ar + 0.98 eV→ CH(A:𝑣 =1) + C + Ar + 0.64 eV→ CH(A:𝑣 =2) + C + Ar + 0.33 eV

(7a)
(7b)
(7c)

Ar(3P2) + C2D→ CD(A:𝑣 =0) + C + Ar + 0.82 eV→ CD(A:𝑣 =1) + C + Ar + 0.57 eV→ CD(A:𝑣 =2) + C + Ar + 0.33 eV

(8a)
(8b)
(8c)

Ar(3P2) + C2H3
→ CH(A:𝑣 =0) + CH2 + Ar + 1.61 eV (9a)→ CH(A:𝑣 =1) + CH2 + Ar + 1.27 eV (9b)→ CH(A:𝑣 =2) + CH2 + Ar + 0.95 eV (9c)Fig. 1. The flowing afterglow apparatus forstudying energy-transfer reactions frommetastable Ar(3P2) atoms to C2H, C2D, andC2H3 radicals.

The heats of formation described above were
calculated using reported thermochemical andspectroscopic data.14,15) Since the heat offormation of C2D has not been reported, it wasestimated to be 559.08 kJ/mol from reported
dissociation energy of D−CCD (46754 cm-1)16)
and heats of formation of C2D2 and D.14)
The following reactions are endoergic.
Therefore, they cannot contribute to the令和 3 年度

九州大学大学院総合理工学報告

formation of CH(A:𝑣 =0−2) and CD(A:𝑣 =0−2)
in the Ar afterglow reactions of C2H2 and C2D2.
Ar(3P2) + C2H2
→ CH(A:𝑣 =0) + CH + Ar – 1.29 eV (10a)→ CH(A:𝑣 =1) + CH + Ar – 0.96 eV (10b)→ CH(A:𝑣 =2) + CH + Ar – 0.64 eV (10c)Ar(3P2) + C2D2
→ CD(A:𝑣 =0) + CD + Ar – 1.46 eV→ CD(A:𝑣 =1) + CD + Ar – 1.20 eV→ CD(A:𝑣 =2) + CD + Ar – 0.97 eV

第 43 巻第2号

13

reaction by subtracting the contribution of the
Ar+*/C2H2 reaction and stray light from
MW(A), assuming that the CH(A-X) emission
resulting from the Ar+*/C2H2 reaction isunchanged by switching on or off MW(A). The
result obtained is shown in Fig. 3a.

(11a)
(11b)
(11c)

On the other hand, the following reactions forC2H3 are exoergic, so that they areenergetically accessible.
Ar(3P2) + C2H4
→ CH(A:𝑣 =0) + CH3 + Ar + 1.55 eV (12a)→ CH(A:𝑣 =1) + CH3 + Ar + 1.21 eV (12b)→ CH(A:𝑣 =2) + CH3 + Ar + 0.89 eV (12c)Emission spectra were measured under
three different conditions. Figures 2a, 2b, and2c show emission spectra obtained byswitching on both MW(A) and MW(B), only
MW(B), and only MW(A), respectively.
CH(A2−X2r) emission spectrum withrotational fine structures is observed in Fig.
2a, where strong stray ArI lines are partiallyoverlapped. It is known that not only
metastable Ar(3P0,2) atoms but also Ar+, Ar2+,
and Ar+*(metastable ions) ions are generated
by MW discharge of Ar in an Ar FA.17-19)
Suzuki and Kuchitsu17) observed CH(A-X)
emission system from the Ar afterglow
reaction of C2H2, where MW was used for thegeneration of Ar active species. In their study,
the CH(A-X) emission reduced its intensity by
application of an electrostatic potential on theion-collector grid placed between the
discharge section and reaction zone. Based on
this fact and energetics, they concluded that
CH(A) is formed by the Ar+*/C2H2 reaction.
We examined the contribution of the
Ar+*/C2H2 reaction to the CH(A-X) emission
by switching off MW(A). As shown in Fig. 2b,
a weak CH(A-X) emission arising from theAr+*/C2H2 reaction is observed. The peakintensity of CH(A-X) in Fig. 2a and 2b is 1:0.33,
indicating that the contribution of the
Ar+*/C2H2 reaction is 33% in Fig. 2a. Figure 2c
was obtained by switching off MW(B), where
weak emissions arising from stray light ofMW(A) are observed. We obtained the CH(AX) emission resulting from the Ar(3P2)/C2H

Fig. 2. Emission spectra obtained from (a)Ar(3P2)/C2H and Ar+*/C2H2, (b) Ar+*/C2H2, and(c) background emission from MW(A) in Fig.
1. Lines marked ○ are stray ArI lines.

Fig. 3.
(a) observed and (b) simulated
emission spectra of CH(A2−X2r) bandsystem from the Ar(3P2)/C2H reaction. The
observed spectrum was obtained from spectra(a) – (b) – (c) in Fig. 2.

14Formation of CH(A) and CD(A) radicals by the reactions of Ar(3P2) with C2H, C2D, and C2H3 radicals in the flowing afterglow

Figure 4a shows CD(A-X) emission spectrum
obtained from the Ar(3P0,2)/C2D reaction. This
spectrum was obtained by subtracting thecontribution of the Ar+*/C2D2 reaction andstray light from MW(A), as in the case of the
Ar+*/C2H reaction.
Figures 5a, 5b, and 5c show emission spectra
obtained from Ar(3P2)/C2H4 + Ar(3P2)/C2H3 +Ar+*/C2H4, Ar(3P2)/C2H4 + Ar+*/C2H4, andbackground emission from MW(A), respectively.
These emission spectra were observed byswitching on both MW(A) and MW(B), only
MW(B), and only MW(A), respectively. CH(AX) emission is observed in Fig 5a, where strong
stray ArI lines are partially overlapped.
Weaker CH(A-X) emission is also observed inspectrum (b), indicating that Ar(3P2)/C2H4 +Ar+*/C2H4 reactions give CH(A-X) emission. A
comparison of peak intensity of CH(A-X)
emission between Fig. 1a and Fig. 1b showsthat the contribution of the Ar(3P2)/C2H4 +Ar+*/C2H4 reactions to the CH(A-X) emission is53%. This value was estimated assuming thatthe intensity of the CH(A-X) emission from theAr(3P2)/C2H4 + Ar+*/C2H4 reactions was
independent of the switching on or off MW(A).
The CH(A-X) emission from the Ar(3P2)/C2H3
reaction was obtained by subtracting thecontribution of the Ar(3P2)/C2H2 + Ar+*/C2H2
reactions and stray light from MW(A),assuming that the CH(A-X) emission resulting
from the Ar(3P2)/C2H4 + Ar+*/C2H4 reactions isuFig. 4. (a) observed and (b) simulated
emission spectra of CD(A2−X2r) bandsystem from the Ar(3P2)/C2D reaction.

unchanged by switching on or off MW(A). The
result obtained is shown in Fig. 6a.

Fig. 5. Emission spectra obtained from (a)Ar(3P2)/C2H4 + Ar(3P2)/C2H3 + Ar+*/C2H4, (b)Ar(3P2)/C2H4 + Ar+*/C2H4, and (c) backgroundemission from MW(A) in Fig. 1. Lines marked○ are stray ArI lines.

Fig. 6.
(a) observed and (b) simulated
emission spectra of CH(A2−X2r) bandsystem from the Ar(3P2)/C2H3 reaction. The
observed spectrum was obtained from spectra(a) – (b) – (c) in Fig. 5.

令和 3 年度

第 43 巻九州大学大学院総合理工学報告

第2号

15

rela
Table 1. Observed and prior vibrational distributions (%) of CH(A) andCD(A) produced from the reactions of Ar(3P2) with C2H, C2D, and C2H3.
C2H
𝑃
𝑣′ 𝑁
0
1
2

100
25
1

C2D
𝑃

𝑃

Processes

Process(a)/(b)

(c)

100
53
19

100
28
4

𝑁
100
25
6

Table 2. Observed rotational temperatures (K)of CH(A) and CD(A) produced from the reactionsof Ar(3P2) with C2H, C2D, and C2H3.
𝑣′=0
𝑣′=1
𝑣′=2

C2H
1900
1200
300

C2D
3800
2400
600

C2H3
1900
1200
800

3.3
Rovibrational
distributionsof
CH(A: 𝒗 =0−2) and CD(A: 𝒗 =0−2) in thereactions of Ar(3P2) with C2H, C2D, and C2H3
radicals
Rovibrational distributions of CH(A:𝑣 =0−2)
and CD(A:𝑣 =0−2) in the Ar(3P2)/M (M = C2H,
C2D, and C2H3) reactions were determined by
a computer simulation of CH(A−X) andCD(A−X) bands. The simulation method usedin this study was the same as that reported in
our previous studies on CH4,5) C2H2,6), andC2D2.7) In Figs. 3b, 4b, and 6b are shown thebest fit spectra of the CH(A−X) and CD(A−X)
bands from C2H, C2D, and C2H3, calculated
assuming
singleBoltzmannrotational
distributions for each 𝑣′ level. Vibrational
distributions and rotational temperatures thus
obtained are given in Tables 1 and 2. From theobserved rovibrational distributions of CH(A)and CD(A), we evaluated the averagevibrational and rotational energies of CH(A)and CD(A) states, denoted as <Ev> and <Er>,and average fractions of excess energies
deposited into vibrational and rotationalenergies of CH(A) and CD(A), denoted as <fv>
and <fr>, respectively. In the Ar(3P2)/C2H andAr(3P2)/C2D reactions, the average fractions ofexcess energies deposited into relativetranslational energy of neutral fragments, <ft>,were estimated from the <fv> + <fr> values.
Results obtained are summarized in Table 3.
As discussed in our previous paper,6-8) it isexpected that the vibrational and rotational

C2H3
𝑃

Processes

Process(a)/(b)

(c)

100
57
26

100
33
7

𝑁
100
25
4

𝑃

𝑃

Process

Process

(d)

(f)

100
24
4

100
17
2

Table 3. Average vibrational and rotationalenergies (eV) deposited into CH(A) andaverage fractions of excess energies (%)
deposited into vibration and rotation of
CH(A) and CD(A) in the reactions of Ar(3P2)
with C2H, C2D, and C2H3.

C2H

C2D

C2H3

<𝐸 >

0.073

<𝐸 >

0.15

<𝑓 >

7.4

<𝑓 >

15.4

<𝑓 > + <𝑓 >

22.8

<𝑓 >

77.2

<𝐸 >

0.072

<𝐸 >

0.29

<𝑓 >

8.8

<𝑓 >

35.5

<𝑓 > + <𝑓 >

44.3

<𝑓 >

55.7

<𝐸 >

0.086

<𝐸 >

0.15

<𝑓 >

5.4

<𝑓 >

9.3

<𝑓 > + <𝑓 >14.7

relaxation of CH(A) by collisions with the bufferrare gas is insignificant within their radiative
lifetimes of about 500 ns for CH(A),20,21) so thatobserved rovibrational distributions in the Ar
FA reflect nascent distributions.
The following tendencies are found for theobserved rovibrational distributions of CH(A)and CD(A).
(1) The vibrational population of CH(A:𝑣 =0–2)
and CD(A: 𝑣 =0–2) rapidly decrease withincreasing 𝑣 . The 𝑁1 / 𝑁0 ratio for thethree reactions were the same (0.25),
whereas the 𝑁2 /𝑁0 ratio decreases from

16

Formation of CH(A) and CD(A) radicals by the reactions of Ar(3P2) with C2H, C2D, and C2H3 radicals in the flowing afterglow

0.06 to 0.01 in the order of C2D, C2H3, and
C2H.
(2) The rotational temperatures of CH(A:𝑣 =0–2) and CD(A: 𝑣 =0–2) decrease withincreasing𝑣.
The
rotationaltemperatures of CD(A: 𝑣 =0–2) from C2D
are higher than those of CH(A:𝑣 =0–2) fromC2H by a factor of 2. The rotationaltemperatures of CH(A:𝑣 =0–2) from C2H3
are similar to those from C2H.
(3) There is no significant difference in the <fv>value between C2H and C2D, whereas the<fr> value of C2D is larger that of C2H by afactor of 2.3. The <fv> and <fr> values ofC2H3 are smaller than those of C2H and
C2D.
(4) The <ft> values of C2H and C2D are large(≥56%), indicating that most of excessenergies are partitioned into relativetranslational energies of neutral CH(A) + C
and CD(A) + C fragments. It is therefore
reasonable to assume that CH(A) andCD(A) are formed through highly repulsive
potentials.
3.4 The formation mechanisms of CH(A) andCD(A) from the reactions of Ar(3P2) with C2H,
C2D, and C2H3 radicals
3.4.1

Statistical models

The formation dynamics of CH(A) and CD(A)from the Ar(3P2)/C2H and Ar(3P2)/C2D reactions
is discussed assuming two-body and three-bodydissociation mechanisms, which have beenused in the formation of CH(A2) from
Ar(3P2)/CH3 and Ar(3P2)/C2H5,9,10) OH(A2+)from Ar(3P0,2)/H2O,22) and NH(A3i) from
Ar(3P0,2)/NH3.23) Possible processes for CH(A)are as follows. Similar processes can be applied
to the formation of CD(A) from the Ar(3P2)/C2D
reaction.
(a) Two-body dissociation process throughresonant excitation transfer
Ar* + C2H → C2H* + Ar
C2H* → CH(A) + C(13)(b) Two-body dissociation process throughion-pair formation
Ar* + C2H → Ar+C2H− → ArCH* + C
ArCH* → CH(A) + Ar
(14)(c) Three-body dissociation process throughcollision complex

Ar* + C2H → ArC2H**
ArC2H** → CH(A) + C + Ar

(15)

In process (a), energy is resonantly transferredfrom Ar* to C2H, and then C2H** dissociates
into CH(A) + C. In processes (b) and (c), CH(A)is produced through collision complexes,
ArCH* and ArC2H*, respectively. ArCH*
originates from an ion-pair potential V(Ar+–C2H−). In order to produce ArCH*, the ion-pairpotential must be sufficiently low to couple
with the entrance V(Ar*–C2H) potential. Since
the electron affinity of C2H is relatively high(2.97 eV),24) V(Ar+–C2H−) potential is locatedjust 1.24 eV above the entrance V(Ar*–C2H)
potential at dissociation limits. Therefore, theion-pair potential is expected to be sufficiently
low to interact strongly with the entrance
potential. Thus, the second process cannot be
excluded from the formation mechanism ofCH(A).
Statistical vibrational and rotational
distributions were calculated for the above
three processes. According to a simple
statistical theory,25-27) the probability offorming a CH A: 𝑣 , 𝑁 rovibrational level anda CH(A:𝑣 ) vibrational level through processes(a) and (b) is expressed by the same equation
𝑃 ′ ′ ∝ 2𝑁′

1 𝐸

𝑃′∝ 𝐸

𝐸

∗

𝐸
/

∗

𝐸

/

∗

(16a)
(16b)

.

On the other hand, that through process (c) is
given by𝑃 ′ ′ ∝ 2𝑁′

1 𝐸

𝑃′∝ 𝐸

𝐸

∗

𝐸

∗

𝐸

∗

.

(17a)
(17b)

The
prior
vibrationalandrotational
distributions of CH(A: 𝑣 =0–2) calculated forprocesses (a)/(b) and (c) are compared with theobserved ones in Table 1 and Figs. 7a–7c. The
prior vibrational distribution of CH(A: 𝑣 =0–2)
and rotational distributions of CH(A: 𝑣 =0–2)
predict higher vibrational and rotational
excitation than the observed ones for both twobody models (a)/(b) and three-body dissociation
model (c).
The deviation from prior vibrational androtational distributions can often be expressedin the form of surprisal,27)
𝐼𝑣 = −ln[P(𝑣)/P o(𝑣)], = 𝜆0 + 𝜆𝑣 𝑓𝑣(18a)

IR = −ln[P(𝑣,N)/Po(𝑣,N)] = 𝜃0  + 𝜃 𝑔𝑁

(18b)

令和 3 年度

九州大学大学院総合理工学報告

where 𝑓 = 𝐸 /𝐸 , 𝑔𝑁 = 𝐸 ⁄ 𝐸
𝑁 and 𝑃 𝑣, 𝑁 = 𝑁 ′ ′ .

𝐸 , P(𝑣) =Figure A1 in Appendix shows vibrational androtational surprisal plots of CH(A: 𝑣 =0–2).
When the two-body and three-body dissociation
models for processes (a)/(b) and (c) are used for
𝑃 ′ ′ and 𝑃𝑜𝑣′ , good linear relationships are foundin the surprisal plots of CH(A: 𝑣 =0–2) in the fv
and 𝑔 range given in Table A1 in Appendix.
These results imply that an exponential gap
behavior is present between the experimental
and statistical distributions in most cases.
From slopes of each surprisal plot, positive
vibrationalandrotationalsurprisalparameters are obtained in all cases for thecases of C2H (Table A1 in Appendix) because
the observed vibrational and rotational
distributions are lower than prior ones. The
000

第 43 巻第2号

17

rotational surprisal parameter of CH(A: 𝑣 =0–2) increases with increasing 𝑣 , in bothprocesses (a)/(b) and (c), suggesting that thedeviation from prior distribution increases withincreasing 𝑣 .
The prior vibrational and rotational
distributions of CD(A: 𝑣 =0–2) calculated forprocesses (a)/(b) and (c) are compared with theobserved ones in Table 1 and Figs. 8a–8c. The
prior vibrational distribution of CD(A: 𝑣 =0–2)
for processes (a)/(b) is much higher than theobserved one, whereas that for process (c) isslightly higher than the observed one (Table 1).
The
prior
rotational
distributionsofCD(A: 𝑣 =0–2) for process (a)/(b) are muchhigher than observed ones. On the other hand,a reasonable agreement is found between priorrotational distributions of CD(A: 𝑣 =0,1) forprocess (c) for low 𝑁 levels below 𝑁 ≈15,000000(a) CH(A;v’=0)

80
60
40

Obs.
Processes (a)/(b)Process (c)20Rotational distributions of CD(A:v'=0)

Rotational distributions of CH(A:v'=0)(a) CH(A;v’=0)

100

0
0
N'

100

Obs.
Processes (a)/(b)Process (c)

80
60
40
20
0

5
10
15
20
Rotational level of CH(A: v' = 0)

0

5
N'

10
15
20
25
Rotational level of CD(A: v' = 0)30(b) CH(A;v’=1)

80
60
Obs.
Processes (a)/(b)40

Process (c)20Rotational distributions of CD(A:v'=1)

Rotational distributions of CH(A:v'=1)(b) CH(A;v’=1)

100

100

Obs.
Processes (a)/(b)80

Process (c)

60
40
20
0

0
0

0

5
10
15
20
N' Rotational level of CH(A: v' = 1)

5
N'

10

15

20

25

Rotational level of CD(A: v' = 1)(c) CH(A;v’=2)

80
60
Obs.
Processes (a)/(b)Process (c)4020

Rotational distributions of CD(A:v'=2)Rotational distributions of CH(A:v'=2)(c) CH(A;v’=2)

100

100

Obs.
Processes (a)/(b)80

Process (c)

60
40
20
0

0
0
N'

5
10
Rotational level of CH(A: v' = 2)

15

Fig. 7. Observed and two-body and threebody prior rotational distributions ofCH(A: 𝑣 =0–2) in the Ar(3P2)/C2H reaction.

0
N'

5
10
15
Rotational level of CD(A: v' = 2)

20

Fig. 8. Observed and two-body and threebody prior rotational distributions ofCD(A: 𝑣 =0–2) in the Ar(3P2)/C2D reaction.

18

Formation of CH(A) and CD(A) radicals by the reactions of Ar(3P2) with C2H, C2D, and C2H3 radicals in the flowing afterglow

whereas the prior rotational distribution ofCD(A: 𝑣 =2) for process (c) is higher than theobserved one.
Figure A2 in Appendix shows vibrational androtational surprisal plots of CD(A: 𝑣 =0–2).
When the two-body and three-body dissociation
models for processes (a)/(b) and (c) are used for
𝑃 ′ ′ and 𝑃𝑜𝑣′ , good linear relationships are foundin the surprisal plots of CD(A: 𝑣 =0–2) in the fv
and 𝑔 range given in Table A1 in Appendix.
The vibrational surprisal parameter ofCD(A: 𝑣 =0–2) for processes (a)/(b) gave apositive value (2.5) because the observed
vibrational distribution is lower than that
expected from prior one. On the other hand, thevibrational surprisal parameter for process (c)(0.18) is small, suggesting that there is no
significant difference between the observed andprior distributions. The rotational surprisal
parameters for CD(A: 𝑣 =0–2) for processes(a)/(b) are positive because the observed
rotational distributions are lower than those
expected from prior ones. The rotationalsurprisal parameters for CD(A: 𝑣 =0,1) forprocess (c) are small negative values (−0.50 and−0.38), suggesting that the observed rotational
distributions are slightly higher than those
expected from prior ones. The rotationalsurprisal parameter of CD(A: 𝑣 =2) is positive(3.3), indicating that the observed distribution
is lower than the prior one. The rotationalsurprisal parameter of CD(A: 𝑣 =0–2) increases
with increasing 𝑣 , in both processes (a)/(b)and (c), suggesting that the deviation fromprior distribution increases with increasing 𝑣 .
It should be noted that deviations of observed
vibrational and rotational distributions of C2H
and C2D from prior ones for three-body process(c) are smaller than those for two-bodyprocesses (a) and (b). Therefore, three-bodyprocesses are more favorable if energy-transfer
processes are governed by statistical models.
Vibrationalandrotationalsurprisalparameters obtained for C2D are smaller thanthose for C2H, indicating that deviations fromprior distributions are small for the case of C2D.
The following three processes may be
responsible for the production of CH(A) fromthe Ar(3P2)/C2H3 reaction.
(d) Two-body dissociation process throughresonant excitation transfer
Ar* + C2H3 → C2H3** + Ar
C2H3** → CH(A) + CH2(19)(e) Two-body dissociation process throughion-pair formation
Ar* + C2H3 → Ar+C2H3−→ ArCH* + CH2
ArCH* → CH(A) + Ar
(20)
(f)

Three-body dissociation process throughcollision complex
Ar* + C2H3 → ArC2H3*
ArC2H3* → CH(A) + CH2 + Ar

(21)

The electron affinity of C2H3 is small (0.667
eV),14) V(Ar+–C2H3−) potential is located 3.54 eV
above the entrance V(Ar*–C2H3) potential atdissociation limits. Therefore, the ion-pairpotential is too high to interact strongly withthe entrance potential at a long distance
between Ar* and C2H3. Thus, it is reasonable toassume that the ion-pair process (e) is notimportant compared with the case of C2H. Thus,
process (e) was excluded from possible
processes in this study.
The probability of forming a CH A: 𝑣 , 𝑁
rovibrational level and a CH(A:𝑣 ) vibrational
level through process (d) is expressed by theequation𝑃 ′ ′ ∝ 2𝑁′

1 𝐸

𝑃′∝ 𝐸

𝐸

∗

𝐸

∗

𝐸

(22a)

∗

(22b)

.

On the other hand, that through process (f) is
given by𝑃 ′ ′ ∝ 2𝑁′

1 𝐸

𝑃′∝ 𝐸

𝐸

𝐸
.

∗

.

∗

𝐸

∗

.

(23a)
(23b)

The
prior
vibrational
distributionofCH(A: 𝑣 =0–2) and rotational distributions ofCH(A: 𝑣 =0–2) calculated for processes (d) and(f) are compared with the observed ones inTable 1 and Figs. 9a–9c. Surprisal plots andsurprisal parameters obtained for CH(A) fromC2H3 are given in Figure A3 and Table A1 in
Appendix. Although a good agreement isobserved between the prior vibrational
distribution and observed one for process (d),
the
prior
vibrational
distributionofCH(A: 𝑣 =0–2) for process (f) is lower than theobserved one. The prior rotational distributionof CH(A: 𝑣 =0–2) for processes (d) and (f)predict higher vibrational and rotational
excitation than the observed ones for both twobody model (d) and three-body dissociation

Rotational distributions of CH(A:v'=0)

令和 3 年度

九州大学大学院総合理工学報告

100(a) CH(A;v’=0)

Obs.
Process (d)Process (f)

80
60
40
20
0
0

5
10
15
20
25
N' Rotational level of CH(A: v' = 0)

Rotational distributions of CH(A:v'=1)

100

30

(b) CH(A;v’=1)

Obs.
Process (d)Process (f)

80
60
40
20
0

Rotational distributions of CH(A:v'=1)

0

5
10
15
20
N' Rotational level of CH(A: v' = 1)

100

25

Obs.
Process (d)Process (f)80(c) CH(A;v’=2)

60
40
20
0
0
N'

5
10
15
Rotational level of CH(A: v' = 1)

20

Fig. 9. Observed and two-body and threebody prior rotational distributions ofCH(A: 𝑣 =0–2) in the Ar(3P2)/C2H3 reaction.
model (f). The rotational surprisal parameter ofCH(A: 𝑣 =0–2) increases with increasing 𝑣 , inboth processes (d) and (f), suggesting that thedeviation from prior distributions increases
with increasing 𝑣 , as in the cases of C2H andC2D. The deviation of observed vibrational
distribution of CH(A) from prior one for twobody process (d) is smaller than that for threebody process (f). On the other hand, deviationsof observed rotational distributions ofCH(A: 𝑣 =0–2) from prior ones for two-bodyprocess (d) are larger than those for three-bodyprocess (f).
3.4.2

Dissociative excitation process

In general, energy transfer from metastablerare gas atoms to molecules proceeds throughnear-resonant
processes,
where
spin-

第 43 巻第2号

19

conservation rule holds between reactants andproducts.11,28-31) Therefore, it is likely that C2H,
C2D, and C2H3 radicals are initially excited intonear-resonant doublet and/or quartet states atthe energy of Ar(3P2:11.55 eV) because theground states of theses radicals and CH(A) areboth doublets and the ground state of carbon
atom is 3P. The ionization potentials (IPs) of
C2H, C2D, are C2H3 were measured as 11.61,
11.58, and 8.25 eV, respectively.14,32) Here, IP of
C2D was estimated from reported appearance
potential of C2D+ under electron-impact
ionization and dissociation energy of
D(D−CCD).14,32) The IPs of C2H and C2D areslightly higher than the energy of Ar(3P2:11.55eV), whereas that of C2H3 is lower than theenergy of Ar(3P2). It is therefore reasonable toassume that precursor states for C2H and C2D
are not superexcited states above their first IPs,
whereas that for C2H3 is superexcited state
located above first IP of C2H3. In theAr(3P2)/C2H reactions, most probable precursor
states are doublet and/ or quartet Rydberg
states converging to the ground ionic states of
C2H. According to ab initio calculations byKoures and Harding,33) many doublet andquartet Rydberg states exist below first IP for
C2H. Some of them will be precursor states.
Higher energy deposition into rotation thaninto vibration even though the ground state of
C2H is linear34) suggests that bending modes ofprecursor C2H** Rydberg states are excited
after energy transfer.
Analogous Rydberg states will exist for C2D
below its IP. It is therefore expected that theformation of CD(A) from the Ar(3P2)/C2D
reaction proceeds though similar Rydberg
states converging to the ground state of C2D+.
A higher rotational excitation of CD(A) than
CH(A) is explained by larger torque in CD(A)after dissociation of C2D** due to larger
moment of inertia. The high position of excessenergies into translational energies of CH(A) +C and CD(A) + C indicates that CH(A) andCD(A) are formed through curve crossing
between precursor Rydberg states and highly
repulsive CH(A) + C and CD(A) + C potentials.
The formation of CH(A) from C2H3 probably
proceeds through superexcited states above thefirst IP of C2H3. Unknown Rydberg states ofC2H3** converging to excited ionic states of
C2H3+ are possible precursor states leading toCH(A) + CH2. Although excess energies in theAr(3P2)/C2H3 reaction are larger than those in
the Ar(3P2)/C2H reaction, no significant
enhancement of deposition of excess energy

20

Formation of CH(A) and CD(A) radicals by the reactions of Ar(3P2) with C2H, C2D, and C2H3 radicals in the flowing afterglow

into vibration and rotation of CH(A) was
observed. Since the degree of freedom of CH2
fragment is much larger than C from theAr(3P2)/C2H reaction, CH2 can accept more
excess energy after reaction. This will be one
reason for the low energy deposition of excessenergy into vibration and rotation of CH(A) in
the Ar(3P2)/C2H3 reaction.
We found that the observed vibrational androtational distributions are lower thanstatistical
two-body
andthree-bodydissociation models in most cases. One reason
may be the fact that lifetimes of intermediate
states are too short to randomize excessenergies into all degrees of freedom. Anotherreason may be the fact that electronic
structures and molecular structures ofprecursor Rydberg states and repulsive
potential curves leading to CH(A) and CD(A)
play
more
important
role
in
their
decomposition processes.

Acknowledgments
This work was supported by the Mitsubishi
foundation (1996) and JSPS KAKENHI Grant
number 09440201 (1997-2000).
References
1)
2)

3)
4)
5)
6)
7)
8)
9)

4.

Summary and Conclusion

Dissociative excitation of C2H, C2D, and C2H3
radicals by collisions with metastable Ar(3P2)
atoms has been studied by observing CH(A-X)
and CD(A-X) emissions in the Ar FA. The
vibrationalandrotational
nascent
distributions of CH(A:𝑣 =0–2) and CD(A:𝑣 =0–2) were determined from spectral simulation.
The vibrational distributions of CH(A) andCD(A) rapidly decreased with increasing 𝑣 inall reactions. The rotational distributions ofCH(A:𝑣 =0–2) and CD(A:𝑣 =0–2) states wereexpressed by single Boltzmann temperatures of
300–1900 K and 600–3800 K, respectively. In
most cases, the observed vibrational androtational distributions are lower thanstatistical prior ones obtained by assuming
two-body and three-body dissociation models.
In the Ar(3P2)/C2H and Ar(3P2)/C2D reactions,more excess energies are deposited into
rotational modes than into vibrational ones ofCH(A) and CD(A). Taking account of the linear
C2H and C2D molecules in the ground states,
we concluded that rotational excitation of
CH(A) and CD(A) arises from excitation ofbending modes of precursor C2H** and C2D**
Rydberg states. It was found that most ofexcess energy (≥56%) is deposited into relativetranslational energies of CH(A) + C and CD(A)+ C products Therefore, we concluded that
CH(A) and CD(A) are formed through highly
repulsive potentials.

10)
11)
12)

13)
14)

15)

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Reference Database 17, Version 7.0 (Web Version),
Release 1.6.8., Data Version 2015.09, and references
therein.
J. Berkowitz, C. A. Mayhew, and B. Ruščić, J. Chem.
Phys., 88, 7396 (1988).
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Number69(2018):
http://webbook.nist.gov/chemistry.
K. P. Huber and G. Herzberg, “Molecular Spectra andMolecular Structure, IV. Constants of Diatomic
Molecules ”, Van Nostrand Reinhold, New York (1979).

16) S. H. S. Wilson, C. L. Reed, D. H. Mordaunt, M. N. R.
Ashfold, amd M. Kawasaki, Bull. Chem. Soc. Jpn., 69,
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令和 3 年度

九州大学大学院総合理工学報告

27) R. D. Levine, Bull. Chem. Soc. Jpn., 61, 29 (1988).
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第 43 巻

第2号

2131) M. Tsuji, K. Kobarai, H. Kouno, H. Obase, and Y.
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Appendix
3.0
2.5
2.0

(a) CH(A;v’=0-2)

Processes (a)/(b)1.5

Process (c)-ln[Pv/Pov]

1.5

1.0

1.0

-ln[Pv/Pov]

0.5
0.0
-0.5
-1.0
0.00

(a) CD(A;v’=0-2)

Processes (a)/(b)Process (c)

0.5

0.0
0.20

0.40

0.60

fv
-0.5
0.00

3.0

-ln[PN / PNo ]2.0

Processes (a)/(b)Process (c)-ln[PN / PNo ]

-1.0

0.20

0.40

0.60

0.80

-2.0
Processes (a)/(b)Process (c)

-3.0

-5.0
0.00

4.0

0.20

0.40

1.00

o

0.0

-ln[PN / PNo ]

o

0.0
-1.0

0.40

0.60

0.80

-4.0
Processes (a)/(b)Process (c)

-6.0
-8.0

1.00

-10.0
0.00

15.0

(c) CD(A;v’=1)

-2.0

o

0.20

gN

0.20

0.40

0.60

0.80

1.00

gN

(d) CH(A;v’=2)
4.0

10.0

3.0

o

-ln[PN / PN]

0.80

2.0

1.0

-ln[PN / PNo ]5.0

Processes (a)/(b)0.0Process (c)
-5.0
0.00

0.60

gN

(c) CH(A;v’=1)

Processes (a)/(b)Process (c)2.0-ln[PN / PN](b) CD(A;v’=0)

-1.0

-4.0

1.00

gN

-2.0
0.00

0.60

0.0

0.0

3.0

0.40

1.0

1.0

-2.0
0.00

0.20

fv

(b) CH(A;v’=0)

0.20

0.40

0.60

0.80

1.00

gN

Fig. A1. Vibrational and rotational surprisalplots of CH(A: 𝑣 =0–2) in the Ar(3P2)/C2H
reaction.

Processes (a)/(b)Process (c)(d) CD(A;v’=2)

2.0
1.0
0.0
-1.0
-2.0
0.00

0.20

0.40

0.60

0.80

1.00

gN

Fig. A2. Vibrational and rotational surprisalplots of CD(A: 𝑣 =0–2) in the Ar(3P2)/C2D
reaction.

22

Formation of CH(A) and CD(A) radicals by the reactions of Ar(3P2) with C2H, C2D, and C2H3 radicals in the flowing afterglow

0.2(a) CH(A;v’=0-2)

0.0-ln[Pv/Pov]

-0.2
-0.4
Process (d)-0.6Process (f)
-0.8
-1.0
0.00

0.10

0.20

0.30

0.40

fv
5.0

(b) CH(A;v’=0)-ln[PN / PNo ]

0.0
-5.0

-10.0

Processes (a)/(b)Process (c)

-15.0
-20.0
0.00

0.20

0.40

0.60

0.80

1.00

gN
4.0

(c) CH(A;v’=1)-ln[PN / PNo ]

2.0
0.0
-2.0
Process (d)Process (f)

-4.0

o

-6.0
-8.0

-10.0
0.00

0.20

0.40

0.60

0.80

1.00

gN
4.0

-ln[PN / PNo ]3.0(d) CH(A;v’=2)

Process (d)Process (f)

2.0
1.0
0.0
-1.0
-2.0
0.00

0.20

0.40

0.60

0.80

gN

Fig. A3. Vibrational and rotational surprisalplots of CH(A: 𝑣 =0–2) in the Ar(3P2)/C2H3
reaction.

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

23

Table A1. Vibrational and rotational surprisal parameters for CH(A) and CD(A) produced
from the reactions of Ar(3P2) with C2H, C2D, and C2H3.
Vibrational surprisal
parameter
Three-bodyprocess
(c)

4.4

1.9
5.2

2.8

CH(A: 𝑣 =1)

5.4

3.2

𝑔

≤ 0.6511.7

𝑔≤ 0.90≤ 0.66

CD(A: 𝑣 =0-2)

8.5
2.5

fv ≤ 0.60

0.18

CD(A: 𝑣 =0)

1.8

−0.50

𝑔

CD(A: 𝑣 =1)

2.0

−0.38

𝑔

≤ 0.7

CD(A: 𝑣 =2)

5.6

3.3

𝑔

≤ 0.67

Two-body
process(d)Three-bodyprocess(f)

Two-body
process(d)Three-bodyprocess
(f)

0.21

−1.7

CH(A: 𝑣 =0)

3.3

1.4

fv ≤ 0.41
𝑔 ≤ 0.47

CH(A: 𝑣 =1)

5.8

3.9

𝑔

≤ 0.46

CH(A: 𝑣 =2)

7.3

5.3

𝑔

≤ 0.47CH(A: 𝑣 =0-2)
C2H3

Three-bodyprocess(c)

CH(A: 𝑣 =0)
CH(A: 𝑣 =2)

C2D

Two-body
processes(a)/(b)fv or 𝑔range

fv ≤ 0.67
𝑔 ≤ 0.7

CH(A: 𝑣 =0-2)
C2H

Two-body
processes(a)/(b)Rotational surprisal
parameter