九州大学大学院総合理工学府報告第 43 巻 第 1 号 22-31 頁 令和 3 年 9 月

Engineering Sciences Reports, Kyushu University
Vol. 43, No. 1, pp. 23-31, SEP, 2021

Isotope Effects in Dissociative Excitation of C2H2 andC2D2 by Collisions with Metastable Ne(3P0,2) Atoms
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 May 24, 2021, accepted June 7, 2021)
Isotope effects in dissociative excitation of C2H2 and C2D2 by collisions with metastable Ne(3P0,2)
atoms have been studied in the Ne flowing afterglow. The CD(A2−X2r, B2−−X2r, and C2+−X2r)and C2(d3g−a3u C1g−A1u e3g−a3u and D1u+−X1g+) emission systems have been identified in
the UV and visible region. The emission rate constants of CD(A,B,C) and C2(d,C,e,D) were determined
to be 1.7, 0.25, 0.014, and 5.4, 0.39, 0.12, 0.0046 × 10-13 cm3 molecule-1 s-1, respectively. The isotopic
ratios of kH(CH*)/kD(CD*) and kH(C2*)/kD(C2*) were determined to be 0.85 and 1.73, respectively.
Although the reduction of emission rate constant of C2* from C2D2 is consistent with a simple
dissociation model from the RRKM theory, the increase in the emission rate constant of CD* does notagree with it. The inverse isotope effect for CD* is explained by the fact that the formation of C2* andCH* or CD* occurs competitively, so that faster dissociation of C–H bonds suppresses the formation ofCH*. Other possible mechanisms to explain the inverse isotope effect for CD* are discussed. The
nascent vibrational and rotational distributions of CD(A:𝑣 =0–2,B:𝑣 =0) and C2(d:𝑣 =0–6, C:𝑣 =0–3,
e:𝑣 =0–6, D:𝑣 =0–2) were determined and compared with those obtained in the Ne(3P0,2)/C2H2 reaction.
Key words: Isotope effect, Dissociative excitation, Acetylene, Metastable Ne* atoms, Emission rate constant,
Rovibrational distribution, Superexcited state, Reaction dynamics

1.

Introduction

In a preceding paper, we have studied theenergy-transfer reaction of Ne(3P0,2) with C2H2
in the Ne flowing afterglow.1) The CH(A2−X2r,
B2−−X2r, and C2+−X2r) and C2(d3g−a3u,
C1g−A1u, e3g−a3u, and D1u+−X1g+) emission
systems have been observed in the UV andvisible region. Emission rate constants ofCH(A,B,C) and C2(d,C,e,D) were determined byusingareference
reaction
method.
Rovibrational distributions of CH(A,B) andC2(d,C,e,D) were determined from computer
simulation of observed spectra. Significant
vibrational excitation of C2* and rotational
excitation of CH* and C2* were observed. Whenthe observed vibrational and rotational
distributions of CH(A,B) and C2(d,C,e,D) were*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

compared with statistical prior ones, large
discrepancies were observed in most cases.
These results suggested that CH* and C2*
fragments are not formed through long-lived
near-resonant C2H2** states, where excessenergies are distributed in all internal degrees
of freedom statistically. It was concluded thatthe most important factor, which dominates thereaction dynamics, is molecular structure ofprecursor state. CH(A,B) and C2(d,C,e,D) areformed via trans-bent near-resonant Rydberg
states converging to the first excited 𝐴 2Ag
stateof
C2H2+.
Non-linear
precursor
superexcited states with enlarged C≡C bondand excited CCH bending vibration led tovibrationally excited C2* and rotationallyexcited CH* and C2* fragments.
In the present study, UV and visible emission
spectra resulting from the Ne(3P2: 16.62 eV,
3P0:16.72 eV)/C2D2 reaction are measured in
the Ne afterglow to obtain more information on
the reaction dynamics from isotope effects. The
emission rate constants of each emitting state
and rovibrational distributions of excited states
are determined and compared with thoseobtained from the Ne(3P0,2)/C2H2 reaction.1)

24

Isotope Effects in Dissociative Excitation of C2H2 and C2D2 by Ne(3P0,2) Atoms

Isotope effects obtained are also compared withthose reported in vacuum ultraviolet (VUV)
photoexciation
withsimilarexcitationenergies.2,3)

feature of the Ne(3P2)/C2D2 spectrum is that theintensity ratio of C2(d−a)/CD(A−X) becomessmallincomparison
with
that
of
C2(d−a)/CH(A−X) in the Ne(3P0,2)/C2H2 reaction.

2.

3.2 Emission rate constants of CD* and C2*Experimental

The FA apparatus equipped with UV andvisible optical emission detection system was
the same as that reported in the preceding
paper.1) C2D2 was synthesized by the reactionof CaC2 with D2O using a liquid nitrogen to
suppress a rapid reaction.
CaC2 (g) + 2D2O (l)→ Ca(OD)2 (s) + C2D2 (g)(1)

Purity of CaC2 and D2O reagents were higher
than 99.9%. The partial pressure of C2D2 in thereaction zone was 1−6 mTorr (1 Torr = 133.3
Pa).

3.

Results

3.1
Emission spectra
excitation processes

anddissociative

Figure 1a shows a typical emission spectrum
obtained from the Ne afterglow reaction of C2D2.
The CD(A2−X2r and B2−−X2r) andC2(d3g−a3u and C1g−A1u) emission
systems are identified in the 330−570 nm
region. In addition, CD(C2+−X2r) andC2(e3g−a3u and D1u+−X1g+) emission
systems are identified in the 314 nm and210−300 nm region, respectively (not shown in
Fig. 1a). For comparison, emission spectrum
obtained from the Ne(3P0,2)/C2H2 reaction isshown in Fig. 1b, where corresponding CH(A−X
and B−X) and C2(d−a and C−A) emission
systems are observed. The most outstanding

CD(A,B) and C2(d,C,e,D) are formed from thefollowing energy-transfer reactions between
Ne(3P2) atoms and C2D2:
Ne(3P2) + C2D2
→ CD(A) + CD(X) + Ne + 3.61 eV,→ CD(B) + CD(X) + Ne + 3.29 eV,→ CD(C) + CD(X) + Ne + 2.55 eV,→ C2(d) + 2D + Ne + 3.17 eV,→ C2(d) + D2 + Ne + 7.76 eV,→ C2(C) + 2D + Ne + 1.40 eV,→ C2(C) + D2 + Ne + 6.00 eV,→ C2(e) + 2D + Ne + 0.63 eV,→ C2(e) + D2 + Ne + 5.23 eV,→ C2(D) + 2D + Ne + 0.41 eV,→ C2(D) + D2 + Ne + 5.01 eV.

The Ho values were calculated using reported
theremochemcial and spectroscopic data.4,5) For
the reactions with higher-energy Ne(3P0) atoms,
0.10 eV higher excess energies are released in
the above reactions. The excess energies
released in the Ne(3P2)/C2D2 reaction aresmaller than those in the Ne(3P2)/C2H2
reaction1) by 0.05–0.17 eV.
Among above processes, highly exoergic
reactions (3b), (3d), (3f), and (3h), in which D2
molecules are formed, will be unfavorable,
because large excess energies should be
released as rovibrational energies and kinetic
energies of products.
The emission rate constants for reactions (2)–(3) were determined by using the reference
reaction method reported in our previous
paper.1) The Ne(3P0,2)/NO reaction was used asa reference reaction.
Ne(3P0,2) + NO → NO(A2+) + Ne
k4 = 4.8 × l0-11 cm3 molecule-1 s-1

Fig. 1. Emission spectra obtained from the(a) Ne(3P0,2)/C2D2 and (b) Ne(3P0,2)/C2H2
reactions.

(2a)
(2b)
(2c)
(3a)
(3b)
(3c)
(3d)
(3e)
(3f)
(3g)
(3h)

(4)

Emission rate constants of each species were
estimated by comparing the integrated
emission intensity of each band system withthat of NO(A–X) in prepared mixtures of
C2D2/NO. Results obtained are given in Table 1
along with our results for the Ne(3P0,2)/C2H2
reaction.1) If there is no nonradiative decay andpumping from the observation region, theemission rate constants equal the formation
ones. Hereafter, kH(X) and kD(X) represent

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0000
Table 1. Emission rate constants of theexcitedfragments
producedintheNe(3P0,2)/C2D2 and Ne(3P0,2)/C2H2 reactions.
(× 10-13 cm3 molecule-1 s-1).
Ne(3P0,2)/C2D2This work

Ne(3P0,2)/C2H2

kH
Ref. 1

kH/kD

CD(A)

1.7

CH(A)

1.5

0.88

CD(B)

0.25

CH(B)

0.22

0.88

CD(C)

0.014

CH(C)

0.0046

0.33

sum

2.0

sum

1.7

0.85

C2(d)

5.4

C2(d)

9.3

1.72

C2(C)

0.39

C2(C)

0.69

1.77

C2(e)

0.12

C2(e)

0.22

1.83

C2(D)

0.0046C2(D)0.0069

1.50

sum

5.9

sum

10.2

1.73

total

7.9

total

11.9

1.51

kD

emission rate constants of excited species X
produced from C2H2 and C2D2, respectively.
The kD (C2*) and kH (C2*) values are larger than
the kD (CD*) and kH (CH*) values by factors of
3.0 and 6.0, respectively. Here, C2*, CD*, andCH* stand for C2(d,C,e,D), CD(A,B,C), andCH(A,B,C), respectively. Results obtained
imply that C2* are more favorable excitedproducts than CD* or CH* in the Ne(3P0,2)/C2D2
and
Ne(3P0,2)/C2H2
reactions.
Among
CD(A,B,C) or CH(A,B,C) and C2(d,C,e,D)
products, the CD(A) or CH(A) and C2(d) states
are major product states. Actually, thekD(CD(A)) and kH(CH(A)) values occupy 85%
and 88% of kD(CD*) and kH(CH*), whereas thekD(C2(d)) and kH(C2(d)) values occupy 92% and
91% of kD(C2*) and kH(C2*), respectively.
When deuterium isotope effects are
examined, the following ratios are obtained forthe total emission rate constants of CH* or CD*
and C2*.

kH (CH*) / kD (CD*) = 0.85
kH (C2*) / kD (C2*) = 1.73

(5a)
(5b)

It should be noted that the formation ofCD(A,B,C) by CC scission is enhanced by the
deuterium substitution, whereas the formationof C2(d,C,e,D) by two C−D bond scissions aresuppressed by the deuterium substitution.
When the dependence of isotope ratio on the
product state is examined, the isotope ratio of
the major CH(A,B) states is the same (0.88),

第 43 巻第1号

25

whereas that of the minor CH(C) state is verysmall (0.33). The isotope ratios for the major
C2(d,C,e) states are nearly the same (1.72−1.83),
whereas that for the minor highest C2(D) state(1.50) is smaller than the above major states.
These results show that the dependence of
isotope effects on the CH* and C2* electronic
states is small for the major CH(A,B) andC2(d,C,e) states. Based on the above facts, we
believe that similar reaction dynamics takes
part in the formation of CH* or CD* and C2*from dissociation of C2H2** and C2D2**superexcitedstates.
Further
detailed
discussion about isotope effects will be given in
the Discussion section.
3.3 Rovibrational distributions of CD(A,B)
and C2(d,C,e,D) in the Ne(3P0,2)/C2D2 reaction
Rovibrational distributions of CD(A,B) andC2(d,C,e,D) in the Ne(3P0,2)/C2D2 reaction were
determined by a computer simulation ofemission spectra. The simulation procedure
was the same as that used in the Ne(3P0,2)/C2H2
reaction.1)
In Figs. 2−7 are shown the observed and best
fit spectra of the CD(A−X, B−X) bands andC2(d−a, C−A e−a and D−X) bands obtained
assuming
singleBoltzmannrotational
distributions for each 𝑣′ level. Vibrational
distributions and rotational temperatures thus
obtained for CH(A,B) and C2(d,C,e,D) are given
in Tables 2 and 3, respectively. From theobserved rovibrational distributions, weestimated the average vibrational androtational energies of each CD* and C2* state,denoted as <Ev> and <Er>, and the average
yields of total available energy into these
degrees of freedom, denoted as <fv> and <fr>,respectively. Symbol <ft> represents theaverage yield of total available energy into therelative translational energy. The <Ev>, <Er>,<fv>, <fr>, and <ft> values obtained for CD* andC2* are summarized in Tables 4 and 5,
respectively. For comparison, corresponding
data of CH* and C2* obtained in theNe(3P0,2)/C2H2 reaction1) are given.
On the basis of observed rovibrational
distributions of CD* and C2*, the following
tendencies are found.
(A) Formation of CD*(1) The vibrational population and rotationaltemperatures of CD(A:𝑣 =0–2) are nearly
the same as those of CH(A:𝑣 =0–2) from
C2H21) except for a lower rotationaltemperature of 800 K for CD(A:𝑣 =2). The

26

Isotope Effects in Dissociative Excitation of C2H2 and C2D2 by Ne(3P0,2) Atoms

rotational temperatures of CD(B:𝑣 =0) is
higher than that of CH(B:𝑣 =0) by 25%.1)(2) The < 𝐸 > and <fv> values of CH(A) arelarger than those of CD(A) by 25–30%. The<𝐸 > and <fr> values of CH(A) are nearlyhe000

the same as those of CD(A). The <𝐸 > and<fr> values of CH(B) are smaller than thoseof CD(B) by 19–24%.
(3) The <fv> + <fr> value for CD(A) is 11.5%,
whereas that for CD(B) is 8.2%. These
results suggest that almost all (89% for
CD(A) and 92% for CD(B)) of the total
available energies are channeled into
00000
000

Fig. 2. (a) observed and (b) simulated
emission spectra of CD(A2−X2r) bandsystem obtained from the Ne(3P0,2)/C2D2
reaction.

Fig. 4. (a) observed and (b) simulated
emission spectra of C2(d3g−a3u) bandsystem obtained from the Ne(3P0,2)/C2D2
reaction.

Fig. 3. (a) observed and (b) simulated
emission spectra of (0,0) band ofCD(B2−−X2r) band system obtained fromthe Ne(3P0,2)/C2D2 reaction.

Fig. 5. (a) observed and (b) simulated
emission spectra of C2(C1g−A1u) bandsystem obtained from the Ne(3P0,2)/C2D2

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rovibrational energies of CD(X) and relativetranslational energy of CD(A or B) + CD(X)products, as in the case of CH* from C2H2.

Fig. 6. (a) observed and (b) simulated
emission spectra of C2(e3g−a3u) bandsystem obtained from the Ne(3P0,2)/C2D2
reaction.

第 43 巻第1号

27(B) Formation of C2*(1) The vibrational populations of C2(d,C,e,D)from C2D2 are the same as those from
C2H2.1) The rotational temperatures ofC2(d: 𝑣 =0–6) from C2D2 are higher thanthose from C2H2 by 62–82%, whereas thoseof C2(C: 𝑣 =0–2) are higher than those fromC2H2 by 10–25%.1) The rotationaltemperatures of C2(e: 𝑣 =0–6) andC2(D: 𝑣 =0–2) are the same as those from
C2H2.1)(2) The <𝐸 > value of the lowest C2(d) statefrom C2D2 is highest (0.53 eV), whereas
those of the higher C2(C,e,D) states are
small (0.12–0.24 eV). The <𝐸 > value of thelowest C2(d) state (0.33 eV) is also higher
than the other C2(C,e,D) states (0.063–0.15
eV).
(3) The <fv> values for C2(d,CeD) + 2D from
C2D2 are larger than the <fr> values byfactors of 1.4–2.9. These results indicatethat deposition of excess energy into
vibration is more favorable than that into
rotation for the formation of C2* + 2D from
C2D2, as in the case of C2* + 2H from C2H2.1)(4) The <fv> + <fr> values for C2(d,CeD) + 2Dfrom C2D2 are 27, 28, 32, and 60%,
respectively. The C2H2/C2D2 ratios of <fv> +<fr> values for C2(d,CeD) are 0.77–0.90.
These results suggest that more excessenergies are deposited into vibrational androtational energies of C2* by deuterium
substitution.
3.4 Statistical models
As typical examples for the formation of CD*
and C2*, the formation dynamics of major
CD(A) and C2(d) products from theNe(3P0,2)/C2D2 system is discussed assuming
two-body
andthree-bodydissociation
000000000
Table. 2. Rovibrational distributions of CD(A,B)
produced from the Ne(3P0,2)/C2D2 reaction. 𝑁
and 𝑃
represent observed and calculated
prior vibrational distributions, respectively. 𝑃
were calculated assuming two-body and threebody dissociation processes.
𝑁

Emitting species

Fig. 7. (a) observed and (b) simulated
emission spectra of C2(D1u+−X1g+) bandsystem obtained from the Ne(3P0,2)/C2D2
reaction.

CD(A)

CD(B)

𝑃

𝑃

Two-body Three-body

𝑇 /K

𝑣′=0

100

100

100

4200

𝑣′=1

45

77

70

3000

𝑣′=2

8

60

49

800

𝑣′=0

100

3125

28

Isotope Effects in Dissociative Excitation of C2H2 and C2D2 by Ne(3P0,2) Atoms

Table 3. The rovibrational distributions of C2(d,C,e,D) produced from the Ne(3P0,2)/C2D2 reaction.
C2(d)

C2(C)

C2(e)

C2(D)

𝑣′

𝑁

𝑃

𝑇 /K

𝑁

𝑃

𝑇 /K

𝑁

𝑃

𝑇 /K

𝑁

𝑃

𝑇 /K

0
1
2
3
4
5
6

100
95
88
55
63
58
50

100
81
65
51
40
30
23

5200
4500
4100
3500
2900
2500
2100

100
60
50
40

100
60
33
15

2250
1650
1250
1000

100
60
27
12
7
3
1

100
51
22
14
7
1
0

1000
1200
700
400
300
300
300

83
100
43

100
9
0

800
800
450

Table 4. Average vibrational and rotationalenergies (eV) deposited into CD (A,B) or
CH(A,B) and average fractions (%) ofvibrationalandrotationalenergies
deposited into CD(A,B) or CH(A,B) in theNe(3P2)/C2D2 and Ne(3P2)/C2H2 reactions.

CD(A)
or
CH(A)

CD(B)
or
CH(B)

<𝐸 >
<𝐸 >
<𝑓 >
<𝑓 >
<𝑓 > +<𝑓 >
<𝐸 >
<𝐸 >
<𝑓 >
<𝑓 >
<𝑓 > +<𝑓 >

C2D2

C2H2

This
work

Ref. 1

0.10
0.32
2.77
8.75
11.52
0.0
0.27
0.0
8.18
8.18

0.13
0.32
3.46
8.60
12.06
0.0
0.22
0.0
6.21
6.21

Table 5. Average vibrational and rotationalenergies deposited into C2(d,C,e,D) andaverage fractions of vibrational, rotational,
and translational energies deposited intoC2(d,C,e,D) in the Ne(3P2)/C2D2 andNe(3P2)/C2H2 reactions.

C2H2
/C2D2

1.30
1.00
1.25
0.98
1.05

C2(d)

0.81
0.76
0.76

C2(C)

mechanisms. The same models have been
applied to the formation of CH(A,B) andC2(d,C,e,D) from the Ne(3P2)/C2H2 reaction.1)(a) Two-body dissociation process throughresonant excitation transfer
Ne* + C2D2 → C2D2** + Ne
C2D2** → CD(A) + CD(X)(6)(b) Three-body dissociation process throughresonant excitation transfer
Ne* + C2D2 → NeC2D2**NeC2D2** → CD(A) + CD(X) +Ne(7)(c) Three-body dissociation process throughresonant excitation transfer
Ne* + C2D2 → C2D2** + Ne
C2D2** → C2(d,C,e,D) + 2D

C2(e)

(8)

C2(D)

C2H2
/C2D2

C2D2

C2H2

This
work

Ref. 1

<𝐸 >
<𝐸 >
<𝑓 >
<𝑓 >
<𝑓 > +<𝑓 >
<𝑓 >
<𝐸 >
<𝐸 >
<𝑓 >
<𝑓 >
<𝑓 > +<𝑓 >
<𝑓 >
<𝐸 >
<𝐸 >
<𝑓 >
<𝑓 >
<𝑓 > +<𝑓 >
<𝑓 >
<𝐸 >

0.53
0.33
16.63
10.34
26.97
73.03
0.24
0.15
17.44
10.50
27.94
72.06
0.12
0.082
18.62
12.88
31.50
68.50
0.18

0.53
0.19
16.01
5.75
21.76
78.24
0.24
0.13
16.63
8.60
25.23
74.77
0.12
0.082
15.59
10.78
26.37
73.63
0.18

1.00
0.58
0.96
0.56
0.81
1.07
1.00
0.87
0.95
0.82
0.90
1.04
1.00
1.00
0.84
0.84
0.84
1.07
1.00

<𝐸 >

0.063

0.063

1.00

<𝑓 >

44.76

34.38

0.77

<𝑓 >

15.44

11.86

0.77

<𝑓 > +<𝑓 >

60.20

46.24

0.77

<𝑓 >

39.80

53.76

1.35

Statisticalvibrationalandrotational
distributions are calculated for the above three
processes using the same equations reported in
the preceding paper.1) As typical examples, theprior vibrational distribution of CD(A: 𝑣 =0–2)

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

and rotational distributions of CD(A: 𝑣 =0) forprocesses (a) and (b) are calculated andcompared with the observed ones in Table 2
and
And
100
80
60
40
Obs.
Process (a)Process (b)

20
0

0

10
20
30
40
N' Rotational level of CD(A: v' = 0)50

Fig. 8. Observed and two-body and threebody prior rotational distributions ofCD(A: 𝑣 =0) in the Ne(3P0,2)/C2D2 reaction.

Rotational distributions of C 2(d: v' = 0-6) / %(a) Obs.
100
80

20
0

0

10

20

30

40

50

60

70

80

N' Rotational level of C2(d)Rotational distributions of C 2(d: v' = 0-6) / %(b) Calc.
10080Discussion

We found significant deuterium isotope
effects on the formation rate constants of CH*(CD*) and C2* from acetylene. Ibuki et al.2,3)
measured isotope effects for the formation of
CH* or CD* and C2* under VUV photoexciation
using NeI lamp and a synchrotron radiation. At
photoexciation energy range of 11.62−11.72 eV,
the following ratios have been determined forthe emission cross sections, which areproportional to the emission rate constants.

kH(CH*)/kD(CD*) = 0.65−0.67
kH(C2*)/kD(C2*) = 1.52−1.53

60
v' = 0v' = 1v' = 2v' = 3v' = 4v' = 5v' = 6

40
20
0
-20

0

10

20

30

29

4.1 Isotope effects in the Ne(3P0,2)/C2H2 andNe(3P0,2)/C2D2 reactions

v' = 0v' = 1v' = 2v' = 3v' = 4v' = 5v' = 640

第1号

and Fig. 8. The prior vibrational distribution ofCD(A: 𝑣 =0–2) and rotational distributions ofCD(A: 𝑣 =0) predict higher vibrational androtational excitation than the observed ones forboth two-body and three-body dissociation
models, as observed in the case of CH(A) from
C2H2.1)The prior vibrational distributions ofC2(d,C,e,D) and rotational distributions ofC2(d: 𝑣 =0–6) calculated for three-body process(c) are compared with the observed ones inTable 3 and Figs. 9a and 9b. It should be notedthat the observed vibrational distributions ofC2(d,C,D) are higher than prior ones, whereasa reasonable agreement between the observed
and prior distributions is found for C2(e). These
results are opposite to those of CD(A), for which
the observed vibrational distribution is lower
than the prior one. On the other hand, theobserved rotational distributions of C2(d: 𝑣 =0–6) are lower than prior ones, as in the case ofCD(A: 𝑣 =0). As shown above, similar
discrepancies are observed between theobservedvibrationalandrotational
distributions and statistical prior ones for theformation of CD* or CH* and C2* from C2D2
and C2H2.

4.

60

第 43 巻

40

50

60

70

80

N' Rotational level of C 2(d)Fig. 9. (a) Observed and (b) calculated
three-body prior rotational distributions ofC2(d: 𝑣 =0–6) in the Ne(3P0,2)/C2D2 reaction.

(9a)
(9b)

It should be noted that similar inverse
deuterium isotope effects are observed between
CH* or CD* and C2* in both Ne(3P0,2) excitationand photoexcitation at similar excitationenergies. However, the isotope effect on CH*
obtained in the Ne(3P0,2) excitation is smaller
than that in VUV photoexcitation, whereasthat on C2* is larger than that in VUVphotoexciation.
According to a simple RRKM theory,3,6)

30Isotope Effects in Dissociative Excitation of C2H2 and C2D2 by Ne(3P0,2) Atoms

kinetic hydrogen isotope effects for theformation of CH* or CD* and C2 from C2H2 andC2D2 are expressed as follows.

k H(CH*)/ k D(CD*) = (µC-CD/µC-CH)1/2 =1.02(10a)k H(C2*)/ k D(C2*) = (µC-D/µC-H)1/2 × (µC-D/µC-H)1/2= µC-D/µC-H =1.86(10b)Here, is the reduced mass. The observed
isotope ratio for C2* in the Ne(3P0,2) reaction,
1.73, is in reasonable agreement with thecalculated value from Eq. 10b (1.86). It seems
therefore reasonable to assume that theobserved isotope effect on C2* in the Ne(3P0,2)
reaction originates from normal kinetic isotope
effect between C–H and C–D bond scissions,which is independent on the spin-multiplicityof the precursor superexcited states.
The inverse isotope effect between CH andCD cannot be explained by the RRKM theory,
which gives a larger value above 1. To explain
the inverse isotope effects on C2* and CH* or
CD*, Ibuki et al.3) proposed two different
mechanisms below 80 nm (>15.50 eV) andabove 80 nm (<15.50 eV) region. The primarily
important effect at the wavelength below 80 nm
is the hydrogen isotope effect in the C–H and
C–D bond dissociation which causes k H(C2*) >k D(C2*). Because of the competition with theseC–H and C–D bond dissociation channels, theprocesses leading to CH* and CD* radicals
show an inverse isotope effect. They proposedfour-membered cyclic isomer (Process A in
Scheme 1), as one of the intermediate
structures. The CC bond dissociation (A1) andthe isomerization (A2) compete in the fourmembered cyclic isomer. The probability of H
atom migration in reaction (A2) should behigher than that of D atom due to the tunneling
effect across the energy barrier. After theformation of C=CH2** vinylidene, dissociation
to C2* (B2) and the dark reactions (B3) and (B4)
occur competitively, as shown in process B in
Scheme 1. Dark dissociation processes (B3) and(B4) are more important for C2H2** because of
faster formation rate of C=CH2**. The fact thatthe more C=CH2** formation the weaker
emission of CH* causes the inverse hydrogenisotope effect between the kH(CH*) andkD(CD*) values.
At the wavelengths longer than 80 nm, thedark processes (B3) and (B4), which are
initiated by the isomerization via a trans-bent
superexcited state to vinylidene, become
important (Process B in Scheme 1). A tunneling
000000

Process (A)Ne* + C2H2

C2H2** (linear)

Process (B)Ne* + C2H2

H

C

C**
H

H

H

CH* + CH

C

C

C

(A1)

CH2** (A2)

H
C

CH

C

CH2** (B1)trans-bent

C

CH2**

C2* + H2

(B2)

C + CH2(B3)

2C + H2(B4)

Scheme 1. Possible decomposition scheme ofC2H2** in the Ne(3P0,2)/C2H2 reaction.
effect on the H and D migration in theintermediates causes the inverse hydrogenisotope effect on the CH* and CD* emission
intensities because the more C=CH2**
formation the weaker CH* emission.
In the Ne(3P0,2) reaction, precursor C2H2**
superexcited states of CH* and C2* are triplet
Rydberg states, which are different fromsinglet Rydberg states formed under VUVphotoexciation. However, it is reasonable toassume that similar mechanisms as proposed
to explain isotope effects under VUVphotoexciation are involved in the Ne(3P0,2)
reaction. In the case of photoexciation, C2H2
molecules are excited into a single resonant
state with the same energy as that of incident
photons. According to our systematic FA optical
spectroscopic studies on energy-transfer
reactions of rare gas metastable Ar(3P0,2),
Kr(3P0,2), and Xe(3P0,2) atoms with such small
molecules as N2 and CO, energy-transfer from
Rg(3P0,2) to molecules occurs near resonantly inmost cases, where a few selective vibrational
levels of triplet states are formed.7-12) However,there was an exceptional case, where a wide
vibrational distribution involving off-resonancestates has been observed (e.g. Kr(3P0,2) + N2 →N2(B3g:𝑣 =3−12) + Kr).13) Therefore, we cannot
exclude the possibility of the formation ofseveral triplet C2H2** or C2D2** states withwide energy distributions in the Ne(3P0,2)
reaction. If products in wide energy
distributions are formed, more than one
mechanism will take part in the observed
inverse isotope effect on CH* and CD*.
In our previous paper on the Ne(3P0,2)/C2H2
reaction,1) we concluded that CH(A,B) andC2(d,C,e,D) are formed via trans-bent nearresonant np ← 3g Rydberg states~ 2
converging to the first excited A
Ag state of+C2H2 . Non-linear precursor superexcitedstates with enlarged C≡C bond and excited
CCH bending vibration led to vibrational
excitation of C2* and rotational excitation of令和 3 年度

九州大学大学院総合理工学報告

CH* and C2* radicals. It has been reported that~ 2
the trans-bent C2H2+(AAg) ion generated ath>16.351 eV (75.8 nm) is unimolecularly~ 2
rearranged into the A
A1 state of vinylidenic
C=CH2+ ion.14) The rearrangement tovinylidene, reaction (B1), is highly probable in
the neutral superexcited acetylene, especiallyin its Rydberg states converging to trans-bent~ 2
C2H2+(AAg) ion. Because of faster tunneling
effect on the H and D migration in theintermediates, the more C=CH2** formation
causes the slower formation rate of CH*. Basedon this fact, direct dissociation of trans-bent
C2H2** and C2D2** into CH* and CD*
completes with rearrangement to vinylidene
form, which suppresses the CH* formation.
In the Ne(3P0,2) reaction, not only
isomerization from trans-bent to threemembered cyclic transition state (B1) as
described above, but also isomerization to nonlinear four-membered cyclic isomer leading to
the C=CH2** formation (A2) may contribute to
the inverse isotope effect between CH* andCD*. The formation of such non-linear
intermediates with a loosen C≡C bond may also
be one reason for the occurrence of highvibrational excitation of C2* and highrotational excitation of CH* and C2*. Further
detailed experimental and theoretical studies
on molecular structures and dissociation
processes of high energy triplet Rydberg states
are required to determine the relative
importance of the above explanations for the
inverse isotope effect for CH* (CD*) in theNe(3P0,2) reaction.

5.

Summary and Conclusion

Isotope effects for the formation of CH* or
CD* and C2* from the energy-transfer reactionsof Ne(3P0,2) with C2H2 and C2D2 have beenstudied in the Ne flowing afterglow. The
emission rate constants of CD(A,B,C) andC2(d,C,e,D) in the Ne(3P0,2)/C2D2 reaction were
determined.
The
isotopicratiosofkH(CH*)/kD(CD*) and kH(C2*)/kD(C2*) were
determined to be 0.85 and 1.73, respectively.
The positive isotope effect for C2* wasconsistent with that predicted from the RRKM
theory because dissociation rate of C–H bond isfaster than that of C–D bond. On the other
hand, negative isotope effect for CH* (or CD*)
does not agree with the prediction from the
RRKM theory. It was explained by the fact thatthe formation of CH* or CD* competes withthat of C2* from dissociation of C–H bonds, so

第 43 巻第1号

31that faster dissociation of C–H bonds than C–D
bonds suppresses the formation of CH*. Other
possible mechanisms to explain the inverse
isotope effect on CH* and CD* were proposed.
The nascent vibrational and rotational
distributions of CD(A: 𝑣 =0–2,B: 𝑣 =0) andC2(d: 𝑣 =0–6, C: 𝑣 =0–3, e: 𝑣 =0–6, D: 𝑣 =0–2)
were determined. The vibrational distributionsof CD(A,B) and C2(d,C,e,D) from theNe(3P0,2)/C2D2 reaction were similar to thosefrom the Ne(3P0,2)/C2H2 reaction. The rotationaltemperatures of CD(A,B) were also similar to
those of CH(A,B). The rotational temperaturesof the lower C2(d,C) states from theNe(3P0,2)/C2D2 reaction were higher than thosefrom the Ne(3P0,2)/C2H2 reaction. On the other
hand, the rotational temperatures of the higher
C2(e,D) states from the Ne(3P0,2)/C2D2 reaction
were the same as those from the Ne(3P0,2)/C2H2
reaction.
Acknowledgments
This work was supported by the Mitsubishi
foundation (1996) and JSPS KAKENHI Grant
number 09440201 (1997-2000).
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