Formation of NO(A 2Σ+, C 2Πr, D 2Σ+) by the ion-ion neutralization reactions of NO+ with C6F5Cl-, C6F5Br-, and C6F5- at thermal energy

Article abstract of DOI:10.1063/1.472133

The ion-ion neutralization reactions of NO⁺(Χ ¹Σ⁺:υ″ = 0) with C₆F₅Cl^-, C₆F₅Br^-, and C₆F₅^- have been spectroscopically studied in the flowing helium afterglow. The NO(Α ²Σ⁺ - Χ ²Π_r,C ²Π_r-Χ ²Π_r,D ²Σ⁺-Χ ²Π_r) emission systems are observed in the NO⁺/C₆F₅Cl^- reaction with the branching ratios of 0.96, 0.017, and 0.028, respectively, while only the NO(Α-Χ) emission system is found in the NO⁺/C₆F₅Br^- and NO⁺/C₆F₅^- reactions. The vibrational and rotational distributions of NO(Α, C, D) indicate that only 1%-11% of the excess energy is deposited into vibration and rotation of NO(Α, C, D) for all the reactions. In the NO⁺/C₆F₅X^- (X=Cl,Br) reactions, a major part of the excess energy is expected to be partitioned into the relative translational energy of the neutral products and the vibrational energy of C₆F₅X. A comparison of the observed vibrational and rotational distributions with the statistical prior ones indicates that the reaction dynamics is not governed by a simple statistical theory because of the large impact parameter. The excitation mechanism of NO(Α, C, D) in the ion-ion neutralization reactions of NO⁺ with C₆F₅X^- (X=F,Cl,Br,CF₃) and C₆F₅^- is discussed.

Full text of DOI:10.1063/1.472133

Formation of NO(A 2Σ+,C 2Π r ,D 2Σ+) by the ion–ion neutralization reactions of NO+
with C6F5Cl−, C6F5Br−, and C6F− 5 at thermal energy
Masaharu Tsuji, Hiroaki Ishimi, Yukio Nishimura, and Hiroshi Obase
Citation: The Journal of Chemical Physics 105, 2701 (1996); doi: 10.1063/1.472133
View online: http://dx.doi.org/10.1063/1.472133
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2
2
2
Formation of NO(A ⌺؉,C ⌸r ,D ⌺؉) by the ion–ion neutralization
reactions of NO؉ with C6F5Cl؊, C6F5Br؊, and C6F؊5 at thermal energy
Masaharu Tsuji, Hiroaki Ishimi, and Yukio Nishimura
Institute of Advanced Material Study and Department of Molecular Science and Technology, Graduate
School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816, Japan
Hiroshi Obase
Department of Industrial Chemistry, Faculty of Engineering, Tohwa University, Chikushigaoka, Minami-ku,
Fukuoka 815, Japan
͑Received 18 December 1995; accepted 6 May 1996͒
The ion–ion neutralization reactions of NOϩ(X ⌺ϩ: Љϭ0) with C F ClϪ, C F BrϪ, and C FϪ
1
v
6
5
6
5
6 5
have been spectroscopically studied in the flowing helium afterglow. The NO(A ⌺ϩ –
2
2
2
2
2
2
X ⌸r ,C ⌸r –X ⌸r ,D ⌺ϩ –X ⌸r) emission systems are observed in the NOϩ/C6F5ClϪ
reaction with the branching ratios of 0.96, 0.017, and 0.028, respectively, while only the NO(A–X)
emission system is found in the NOϩ/C6F5BrϪ and NOϩ/C6F5Ϫ reactions. The vibrational and
rotational distributions of NO(A,C,D) indicate that only 1%–11% of the excess energy is deposited
into vibration and rotation of NO(A,C,D) for all the reactions. In the NOϩ/C6F5XϪ ͑XϭCl,Br͒
reactions, a major part of the excess energy is expected to be partitioned into the relative
translational energy of the neutral products and the vibrational energy of C6F5X. A comparison of
the observed vibrational and rotational distributions with the statistical prior ones indicates that the
reaction dynamics is not governed by a simple statistical theory because of the large impact
parameter. The excitation mechanism of NO(A,C,D) in the ion–ion neutralization reactions of
NOϩ with C6F5XϪ ͑XϭF,Cl,Br,CF3͒ and C6FϪ5 is discussed. © 1996 American Institute of
Physics. ͓S0021-9606͑96͒01331-1͔
2
2
Only the NO(A ⌺ϩ –X ⌸ ) emission system from Јϭ0
I. INTRODUCTION
v
r
was observed for the reaction with SFϪ6 , while the
NO(A ⌺ϩ –X ⌸r ,C ⌸r –X ⌸r ,D ⌺ϩ –X ⌸r) emis-
2
2
2
2
2
2
Mutual neutralization reactions between positive and
negative ions are important because they remove efficiently
charges from cold plasmas. Since the first study in 1896 by
Thomson and Rutherford,1 ion–ion neutralization reactions
have been investigated by using flowing-afterglow and
merged-beam methods.2–9 The major purpose of these stud-
ies was the determination of reaction rate constants. There
are only a few optical spectroscopic studies that have deter-
mined the final states of the reaction products.10–12 Smith
et al.13 have spectroscopically studied ion–ion neutralization
process ͑1͒ by using a flowing-afterglow method
sion systems from Јϭ0 were found for the reactions with
v
C6FϪ6 and C6F5CFϪ3 . This shows that the product electronic
state distribution in the ion–ion neutralization reactions de-
pends strongly on the negative ion.
In order to obtain further information about the elec-
*
tronic state selectivity of NO in the ion–ion neutralization
processes, the formation of NO by the reactions of NOϩ
*
with C6F5ClϪ, C6F5BrϪ, and C6FϪ5 have been studied here.
The electronic state distributions and the rovibrational distri-
butions in each reaction are determined. The observed vibra-
tional and rotational distributions are compared with statisti-
cal prior ones in order to obtain dynamical features of the
ion–ion neutralization reactions. The results obtained are
compared with our previous data for reactions ͑3͒ and ͑4͒.
Ϫ
2
1
2
NOϩ X ⌺ϩ͒ϩNO →NO A ⌺ϩ: ϭ0͒ϩNO . ͑1͒
͑
͑
v
Ј
2
Only the NO(A ⌺ϩ –X ⌸ ) emission from Јϭ0 was ob-
2
2
v
r
served. We have recently investigated the excitation pro-
cesses of NO by ion–ion neutralization reactions of NOϩ
with such negative ions as SFϪ6 , C6FϪ6 , and C6F5CF3Ϫ by
using the flowing-afterglow method14–17
II. EXPERIMENT
The flowing-afterglow apparatus used in this study is
identical with that used for the ion–ion neutralizations of
NOϩ with SFϪ6 and C6FϪ6 .14,15 In brief, the metastable
He(2 3S) atoms and Heϩ and Heϩ2 ions were generated by a
microwave discharge of the high purity He gas in a discharge
flow operated at 0.5–1.5 Torr ͑1 Torrϭ133.3 Pa͒. All experi-
ments were carried out by removing the Heϩ and He2ϩ ions
by using a pair of ion-collector grids placed between the
discharge section and the reaction zone.
Ϫ
6
1
NOϩ X ⌺ϩ͒ϩSF
͑
͑2͒
͑3͒
→NO A ⌺ϩ: ϭ0͒ϩSF ,
2
͑
v
Ј
6
Ϫ
6
NOϩ X ⌺ϩ͒ϩC F
1
͑
6
→NO A ⌺ϩ,C ⌸ ,D ⌺ϩ: ϭ0͒ϩC F ,
2
2
2
͑
v
Ј
r
6 6
Ϫ
3
NOϩ X ⌺ϩ͒ϩC F CF
1
͑
6
5
N2O or NO was admixed with the discharge flow about
10 cm downstream from the center of the discharge, whereas
→NO A ⌺ϩ,C ⌸ ,D ⌺ϩ: ϭ0͒ϩC F CF . ͑4͒
2
2
2
͑
v
Ј
r
6
5
3
J. Chem. Phys. 105 (7), 15 August 1996
0021-9606/96/105(7)/2701/9/$10.00
© 1996 American Institute of Physics
2701
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*
Tsuji et al.: NO formation by ion–ion neutralization reactions
2702
C6F5X ͑XϭCl,Br,I͒ were introduced 10 cm further down-
stream from an inlet of N2O or NO. The partial pressure of
N2O or NO in the reaction zone was 15–50 mTorr, while
those of C6F5X ͑XϭCl,Br,I͒ were 1–5 mTorr, as measured
by a capacitance manometer. Typical operating pressures
were 1.0 Torr for He, 50 mTorr for N2O, 17 mTorr for NO,
and 3 mTorr for C6F5X ͑XϭCl,Br,I͒.
At low N2O and NO gas pressures, the vibrationally ex-
1
cited NOϩ(X ⌺ϩ) states arrived at the reaction zone,17 be-
cause the vibrational relaxation rate of NOϩ(X: Ју1) is
v
slow for the buffer He gas, while it is fast for NO and N2O
1
1
NOϩ X ⌺ϩ: у1͒ϩHe→NOϩ X ⌺ϩ: ϭ0͒ϩHe,
͑
v
͑
v
Љ
Љ
͑5͒
͑6͒
͑7͒
k5Ͻ1ϫ10Ϫ13 cm3 sϪ1 ͑Ref. 18),
1
1
NOϩ X ⌺ϩ: у1͒ϩNO→NOϩ X ⌺ϩ: ϭ0͒ϩNO,
͑
v
͑
v
Љ
Љ
*
FIG. 1. Emission spectra of NO resulting from the He afterglow reaction
͑a͒ with C6F5Cl addition and ͑b͒ without C6F5Cl addition. Spectrum ͑c͒ is
the difference spectrum between ͑a͒ and ͑b͒ ͓͑a͒–͑b͔͒. The optical resolution
is 0.19 nm ͑FWHM͒.
k6ϭ5ϫ10Ϫ10 cm3 sϪ1 ͑Ref. 18),
1
NOϩ X ⌺ϩ: у1͒ϩN O→products,
͑
v
Љ
2
k7Ϸ2ϫ10Ϫ10 cm3 sϪ1 ͑Ref. 18).
k
12a
eϪϩC6F5I→ C6F5ϪϩI,
у95%͒
͑12a͒
͑12b͒
͑
The spectral features of NO(A–X) were independent of the
k
12b
NO or N2O gas pressure above 10 mTorr indicating that all
→ C6F5IϪ,
р5%͒
of the reactant NOϩ ions were relaxed completely to the
͑
NOϩ(X ⌺ϩ: Љϭ0) level. Thus all experiments were car-
1
v
k
12aϩk12bϭ3.1ϫ10Ϫ8 cm3 sϪ1
.
ried out at NO and N2O pressures above 10 mTorr, where the
1
contribution of NOϩ(X ⌺ϩ: Љу1) was negligible.
The emission spectra resulting from ion–ion neutraliza-
tion reactions around the C6F5X gas inlet, were dispersed in
the 190–700 nm region with a Spex 1250M monochromator.
All emission spectra presented here are corrected for the
relative sensitivity of the optical detection system.
v
In our flowing-afterglow experiment, electron/positive
ion plasma was created between the two gas inlets of N2O or
NO and C6F5X ͑XϭCl,Br,I͒ due to the following Penning
ionization:
He 2 3S͒ϩN O→N OϩϩHeϩeϪ,
47%͒
͑
͑8a͒
͑8b͒
͑9͒
͑
2
2
III. RESULTS AND DISCUSSION
→NOϩϩNϩHeϩeϪ,
51%͒
͑
A. Excitation processes of NO in the He afterglow
He 2 3S͒ϩNO→NOϩϩHeϩeϪ.
100%͒
͑
Figures 1͑a͒ and 1͑b͒ show typical emission spectra in
the 190–280 nm region obtained by the addition of N2O into
the He afterglow from the first gas inlet with and without
addition of C6F5Cl from the second gas inlet, respectively,
where only He(2 3S) was involved as an initial active spe-
͑
The branching ratios of Eqs. ͑8a͒, ͑8b͒, and ͑9͒ are obtained
from mass spectroscopic data of West et al.19 The electron/
positive ion plasma was converted completely to positive
ion/negative ion one by the addition of C6F5X ͑Cl,Br,I͒ with
large thermal electron attachment coefficients. The following
nondissociative electron attachment leading to parent anions
occurs for C6F5Cl and C6F5Br, while dissociative electron
attachment leading to C6FϪ5 preferentially takes place for
C6F5I8
2
2
cies. The very weak NO(A ⌺ϩ –X ⌸r) ␥ system from
2
2
ϭ0 and the NO(B ⌸ –X ⌸ ) ␤ system from Јϭ0,1
vЈ
v
r
r
are identified in Fig. 1͑b͒ with reference to reported spectral
data.20–22 Most of these NO(A–X) and NO(B–X) emissions
probably result from the three-body N(4S)/O(3P)/M ͑M
ϭHe, N2O͒ reactions.23,24 By the addition of a small amount
of C6F5Cl into the reaction zone, the NO(A ⌺ϩ –X ⌸r)
system is enhanced strongly, as shown in Fig. 1͑a͒. In order
to subtract the contribution from the weak underlying
NO(A–X,B–X) emissions, Fig. 1͑b͒ is subtracted from Fig.
1͑a͒. The resulting spectrum is shown in Fig. 1͑c͒. It is clear
2
2
k
10
eϪϩC6F5Cl→C6F5ClϪ
100%͒
͑
k10ϭ8.4ϫ10Ϫ8 cm3 sϪ1
,
͑10͒
k
11a
from Fig. 1͑c͒ that the NO(A–X) emission from Јϭ0 and
v
eϪϩC6F5Br→ C6F5BrϪ,
у97%͒
͑11a͒
͑
1 appears by the C6F5Cl addition. All emission spectra pre-
sented below were obtained by using the same subtraction
method.
On the basis of the reaction scheme given in Sec. II and
the energetics, the NO(A,C,D) states can be formed by dis-
k
11b
→ BrϪϩC6F5 ,
р3%͒
͑
k
11aϩk11bϭ8.3ϫ10Ϫ8 cm3 sϪ1
,
͑11b͒
J. Chem. Phys., Vol. 105, No. 7, 15 August 1996
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*
Tsuji et al.: NO formation by ion–ion neutralization reactions
2703
measured in the Ar afterglow, where only the N2Oϩ ion was
generated by the Arϩ/N2O charge-transfer reaction26 and
thermal electrons were produced by a microwave discharge
*
of Ar. No evidence of the NO formation due to the
N2Oϩ/C6F5ClϪ reaction was found. It was therefore con-
cluded that only ion–ion neutralization reactions ͑14a͒–͑14c͒
participate in the excitation of NO(A,C,D). Although the
same emission spectra of NO(A,C,D) were obtained by us-
ing NO and N2O as source gases of NOϩ, the signal to noise
ratio of the observed spectra was much better by using N2O
*
because background NO emissions were much weaker.
When C6F5BrϪ and C6F5Ϫ were used as negative ions, the
*
same emission spectra of NO were obtained by using N2O
*
and NO. No evidence of the NO formation by the
N2Oϩ/C6F5BrϪ and N2Oϩ/C6FϪ5 reactions was found when
N2Oϩ was selectively formed from the Arϩ/N2O reaction.
*
These findings led us to conclude that NO was excited by
the NOϩ/C6F5BrϪ and NOϩ/C6FϪ5 reactions.
Figure 2 shows emission spectra resulting from the ion–
ion neutralization reactions of NOϩ with C6F5ClϪ, C6F5BrϪ,
and C6FϪ5 . For comparison, our previous data for C6FϪ6 and
C6F5CFϪ3 are also given. The Јϭ0 Љ progression of the
v
v
NO(A–X) emission system is observed strongly in all spec-
2
2
2
2
tra. Although the NO(C ⌸r –X ⌸r ,D ⌺ϩ –X ⌸r)
systems can be clearly identified in the NOϩ/C6F6Ϫ
and NOϩ/C6F5CFϪ3 reactions, these systems and the NO(A–
X) system from ЈϾ0 are either weak or absent for the other
v
*
FIG. 2. Emission spectra of NO
resulting from the NOϩ/
C6F5XϪ͑XϭF,Cl,Br,CF3͒ and NOϩ/C6FϪ5 reactions. The optical resolution is
0.19 nm ͑FWHM͒.
sociative ion–ion neutralization reactions ͑13a͒–͑13c͒ and/or
nondissociative ion–ion neutralization reactions ͑14a͒–͑14c͒
N OϩϩC F ClϪ→NO A͒ϩNϩC F Clϩ2.00 eV,
͑
2
6
5
6 5
͑13a͒
͑13b͒
→NO C͒ϩNϩC F Clϩ0.99 eV,
͑
6
5
→NO D͒ϩNϩC F Clϩ0.87 eV,
͑
6
5
͑13c͒
͑14a͒
͑14b͒
͑14c͒
NOϩϩC F ClϪ→NO A͒ϩC F Clϩ3.30 eV,
͑
6
5
6 5
→NO C͒ϩC F Clϩ2.29 eV,
͑
6
5
→NO D͒ϩC F Clϩ2.17 eV.
͑
6
5
Since the dissociation energy of D͑C6F5–Cl͒ is 3.99 eV,25
dissociative neutralization reactions leading to C6F5ϩCl are
closed in the above processes. In order to examine the con-
tribution from NOϩ/C6F5ClϪ reactions ͑14a͒–͑14c͒, the
source gas of positive ions was changed from N2O to NO.
Since only the NOϩ ion is formed in the He(2 3S)/NO Pen-
ning ionization,19 the contribution from reactions ͑13a͒–
͑13c͒ can be removed completely. The emission spectrum of
*
NO obtained by using NO was the same as that found by
using N2O. This implies reactions ͑14a͒–͑14c͒ dominate the
*
FIG. 3. Expanded spectra of NO in the 190–230 nm region resulting from
the NOϩ/C6F5XϪ͑XϭF,Cl,Br,CF3͒ and NOϩ/C6F5Ϫ reactions. The optical
resolution is 0.36 nm ͑FWHM͒.
*
formation of NO . In order to examine the contribution from
N2Oϩ/C6F5ClϪ reactions ͑13a͒–͑13c͒, NO emission was
*
J. Chem. Phys., Vol. 105, No. 7, 15 August 1996
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*
Tsuji et al.: NO formation by ion–ion neutralization reactions
2704
*
TABLE I. Relative formation rates of NO in the ion–ion neutralization
reactions of NOϩ with C6F5XϪ͑XϭF,Cl,Br,CF3͒ and C6FϪ5 .
Relative formation rate
2
ϩ
2
ϩ
Anion
NO(A
⌺
)
NO(C 2⌸r)
NO(D
⌺ )
C6FϪ6
C6F5ClϪ
C6F5BrϪ
C6FϪ5
Ref. 15
This work
This work
This work
Ref. 16
1.00
1.00
1.00
1.00
1.00
0.13Ϯ0.04
0.018Ϯ0.002
0.00
0.24Ϯ0.04
0.029Ϯ0.003
0.00
C6F5CFϪ3
0.041Ϯ0.03
0.060Ϯ0.010
NO(B–X,E–A) transitions. The NO(C–X,D–X) transi-
tions were detected in the NOϩ/C6F5XϪ ͑XϭF,Cl,CF3͒ reac-
tions.
*
B. Internal state distributions of NO
The relative formation rates of NO(A,C,D) in each re-
action were determined by comparing the total emission in-
tensities of the NO(A–X,C–X,D–X) systems. The results
obtained are given in Table I along with our previous results
for C6FϪ6 and C6F5CF3Ϫ. These values were obtained assum-
ing that the C→A and D→A radiative-cascade processes are
negligible for the formation of NO(A).14,15 The relative for-
mation rates of NO(A,C,D) were unchanged in the He pres-
sure range 0.5–1.5 Torr, leading us to conclude that the elec-
tronic quenching of NO(A,C,D) by collisions with the
FIG. 4. Potential energy diagram of NO and NOϩ. Adopted from Refs. 22
and 27.
three reactions. In order to examine whether the
NO(C–X,D–X) systems and the NO(A–X) system from
Ͼ0 are present or absent, higher-sensitivity measurements
vЈ
buffer He gas is insignificant during short radiative lifetimes
of the spectra in the 190–230 nm region were carried out.
The spectra obtained are shown in Fig. 3 along with our
previous data for C6FϪ6 and C6F5CF3Ϫ, where the NO(C–
X,D–X) systems appear strongly. Although weak NO(C–
2
of ␶ ϭ͑192–202͒Ϯ14 ns for NO(A ⌺ϩ: Јϭ0,1),20 р2.7
v
Ϯ1.5 ns for NO(C ⌸ : Јϭ0),28 and 16.1Ϯ0.7 ns for
2
v
r
NO(D ⌺ϩ: Јϭ0).28 The relative formation rates of
2
v
NO(C,D) decrease with increasing mass of halogen atom
substituted to the C6F5 group and they become zero for the
case of the heaviest C6F5BrϪ ion. The relative formation
rates of NO(C,D) increase again by the substitution of a
heavy CF3 molecule.
X,D–X) emissions from Јϭ0 and NO(A–X) emission
v
from Јϭ1 are identified for C F ClϪ, only very weak
v
6
5
NO(A–X) emission from Јϭ1 is observed for C F BrϪ.
v
6
5
No emission is found in the 190–220 nm region for C6FϪ5 .
Summarizing the above facts, neutralization processes ͑15a͒
and ͑16͒ are found in the NOϩ/C6F5BrϪ and NOϩ/C6F5Ϫ re-
actions, though processes ͑15b͒ and ͑15c͒ cannot be ob-
served
Although only the Јϭ0 level is found for NO(C,D) in
v
the NOϩ/C6F5ClϪ reaction, both the Јϭ0 and Јϭ1 levels
v
v
are observed for NO(A) in the NOϩ/C6F5ClϪ and
NOϩ/C6F5BrϪ reactions. The relative vibrational populations
NOϩϩC F BrϪ→NO A͒ϩC F Brϩ3.31 eV,
͑15a͒
͑15b͒
͑15c͒
͑16͒
͑
6
5
6 5
of NO(A: Јϭ0,1) were determined from the intensity ratio
v
→NO C͒ϩC F Brϩ2.30 eV,
͑
6
5
→NO D͒ϩC F Brϩ2.18 eV,
͑
6
5
NOϩϩC FϪ→NO A͒ϩC F ϩ0.38 eV,
͑
6
6 5
5
Since D͑C6F5–Br͒ is 3.5 eV,25 dissociative neutralization re-
actions leading to C6F5ϩBr are excluded from the possible
processes.
Figure 4 shows a potential-energy diagram of NO and
NOϩ, and the energies of NOϩϩC6F5XϪ ͑XϭF,Cl,Br,CF3͒
and NOϩϩC6FϪ5 ion pairs at infinite intermolecular dis-
tances. Among a number of emitting excited states in
2
2
2
2
2
NO,22,27 the A ⌺ϩ, B ⌸r , C ⌸r , D ⌺ϩ, and E ⌺ϩ
states can be detected by observing the A→X, B→X, C→X,
D→X, and E→A emission systems in the ultraviolet and
visible region. Although the NO(A–X) transition was
observed in all reactions, no reactions exhibited the
FIG. 5. The observed and calculated spectra of the ͑0,1͒ band of
2
NO(A
⌺
ϩ –X 2⌸r) obtained from the NOϩ/C6F5ClϪ reaction at thermal
energy. The optical resolution is 0.085 nm ͑FWHM͒.
J. Chem. Phys., Vol. 105, No. 7, 15 August 1996
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*
Tsuji et al.: NO formation by ion–ion neutralization reactions
2705
TABLE II. Observed and calculated prior vibrational populations of NO(A,C,D) produced from the ion–ion
neutralization reactions of NOϩ with C6F5XϪ͑XϭF,Cl,Br,CF3͒ and C6FϪ5 .a
NO(A)
NO(C)
ϭ0
NO(D)
ϭ0
Anion
C6FϪ6
ϭ0
ϭ1
ϭ1
ϭ1
vЈ
vЈ
vЈ
vЈ
vЈ
vЈ
Obs.
Calc.
1.00
1.00
0.00
2.6͑Ϫ2͒
1.00
1.00
0.00
2.6͑Ϫ3͒
1.00
1.00
0.00
2.0͑Ϫ3͒
Ref. 15
This work
This work
This work
Ref. 16
b
Obs.
Calc.
1.00
1.00
2.1͑Ϫ3͒
4.1͑Ϫ2͒
1.00
1.00
0.00
7.8͑Ϫ3͒
1.00
1.00
0.00
7.1͑Ϫ3͒
C6F5ClϪ
C6F5BrϪ
C6FϪ5
Obs.
Calc.
1.00
1.00
1.1͑Ϫ2͒
4.2͑Ϫ2͒
0.00
1.00
0.00
8.1͑Ϫ3͒
0.00
1.00
0.00
7.3͑Ϫ3͒
Obs.
Calc.
1.00
1.00
0.00
1.2͑Ϫ21͒
Obs.
Calc.
1.00
1.00
0.00
8.3͑Ϫ3͒
1.00
1.00
0.00
3.8͑Ϫ4͒
1.00
1.00
0.00
2.4͑Ϫ4͒
C6F5CFϪ3
aValues in parentheses are power of 10 multiplying the entry.
bThis value was erroneously reported as 0.26 in Ref. 15.
between the ͑0,0͒ and ͑1,0͒ bands of NO(A–X) and their
known Einstein coefficients.20 The N1/N0 ratios are very
small, as shown in Table II, for both reactions. The rotational
distributions of NO(A,C,D) resulting from the reactions
with C6F5ClϪ, C6F5BrϪ, and C6FϪ5 were determined by a
computer simulation of the NO(A–X,C–X,D–X) emis-
sions. The simulation method was the same as that reported
previously.14,15 As a representative result, the observed
NO(A–X) spectrum in the NOϩ/C6F5ClϪ reaction is com-
pared with the best fit one ͑Fig. 5͒. The observed NO(A–
X,C–X,D–X) spectra can be reproduced by single Boltz-
mann rotational temperatures given in Table III. For
distributions of NO(A) were identical between the two ex-
periments. On the basis of these findings, the vibrational and
rotational relaxation of NO(A,C,D) by collisions with He
atoms is insignificant during short radiative lifetimes of 2.7–
202 ns20,28 under the operating conditions.
By using established rovibrational distributions of
NO(A,C,D), we determined the average vibrational and ro-
tational energies of NO(A,C,D) and their average fractions
of the total excess energy, which are denoted by E , f
͘ ͗
,
͘
͗
v
v
E , and f , respectively. The same relations as those re-
͗
͘
͗ ͘
r
r
ported previously14–16 were used for the evaluation of these
values. The E
,
f
, E , and
f
values obtained are
͗ ͘
͗
͘ ͗ ͘ ͗
͘
v
v
r
r
comparison, the rotational temperatures of NO formed in
given in Table IV together with our previous data for C6F6Ϫ
and C6F5CFϪ3 . Either no or very small amount of the excess
energy is deposited into the vibrational mode of NO(A),
while no energy is partitioned into the vibration of NO(C,D)
*
the reactions with C6F6Ϫ and C6F5CFϪ3 are given in Table III.
It should be noted that the rotational temperatures are rela-
tively low for all the reactions. The Tr values of NO(A),
NO(C), and NO(D) in the Јϭ0 levels are independent of
because of the lack of emission from Ју1. Since the f
v
͗ ͘
v
v
the negative ion.
and f values are very small for all cases, the f ϩ f
͗ ͘ ͗ ͘
͗
͘
r
v
r
The vibrational and rotational distributions of
NO(A–X,C–X,D–X) were independent of the He buffer
gas pressure over the range of 0.5–1.5 Torr. In order to fur-
ther examine the effects of the collisional relaxation, the vi-
brational and rotational distributions of NO(A) produced
from the He(2 3S)/NO reaction in the He flowing afterglow
at 0.5–1.5 Torr were compared with those measured using a
beam apparatus29 at 3 mTorr. The vibrational and rotational
values are small ͑р11%͒. On the basis of these facts, most of
the excess energy must be released as the internal energy of
C6F5X ͑XϭF,Cl,Br,CF3͒ and C6F5 and/or the relative trans-
lational energy of the products.
Since the formation of NO(A,C,D) takes place via
strongly attractive ion-pair potentials, long lived
͑NOϩ–C6F5XϪ͒ and ͑NOϩ–C6FϪ5 ͒ complexes can be postu-
lated. According to a simple statistical theory,30–32 the rota-
ϩ
TABLE III. Rotational temperatures of NO produced from the ion–ion neutralization reactions of NO with C6F5XϪ͑XϭF,Cl,Br,CF3͒ and C6F5Ϫ .
*
Tr ͑K͒
Anion
NO(A: ϭ0)
vЈ
NO(A: ϭ1)
vЈ
NO(C: ϭ0)
vЈ
NO(D: ϭ0)
vЈ
C6FϪ6
C6F5ClϪ
C6F5BrϪ
C6FϪ5
Ref. 15
This work
This work
This work
Ref. 16
500Ϯ50
500Ϯ50
500Ϯ50
500Ϯ50
500Ϯ50
300Ϯ50
300Ϯ50
400Ϯ50
400Ϯ50
500Ϯ50
400Ϯ50
C6F5CFϪ3
300Ϯ50
400Ϯ50
J. Chem. Phys., Vol. 105, No. 7, 15 August 1996
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*
Tsuji et al.: NO formation by ion–ion neutralization reactions
2706
TABLE IV. Average vibrational and rotational energies deposited into
NO(A,C,D) and their fractions of the total available energies in the ion–ion
neutralization reactions of NOϩ with C6F5XϪ͑XϭF,Cl,Br,CF3͒ and C6FϪ5 .
*
Anion
C6FϪ6
NO
Ev ͑meV͒ Er ͑meV͒
f
f
͗ ͘
r
͗
͘
v
Ref. 15
NO(A)
NO(C)
NO(D)
0
43Ϯ4
26Ϯ4
34Ϯ4
43Ϯ4
26Ϯ4
34Ϯ4
43Ϯ4
43Ϯ4
43Ϯ4
26Ϯ4
34Ϯ4
0
0
0
0.015
0.014
0.019
0
0
C6F5ClϪ
This work NO(A)
NO(C)
0.6
0
0
0.0002 0.013
0
0
0.011
0.016
0.013
0.11
0.015
0.014
0.019
NO(D)
C6F5BrϪ This work NO(A)
3.2
0
0
0.001
C6FϪ5
This work NO(A)
0
0
0
0
C6F5CFϪ3
Ref. 16
NO(A)
NO(C)
NO(D)
0
0
tional and vibrational populations of a given ( Ј,JЈ) level
v
are given by the relations
P0 ϰ E ϪE A,C,D:
͒
v ͔
Ј
,
͑17a͒
͑17b͒
sϩ3
͓
͑
tot
v
Ј
P0 ϰ 2J ϩ1͒ E ϪE A,C,D: ,J ͒
͔
Ј Ј
.
sϩ2
͑
͓
͑
v
Ј
tot
J
Ј
Here, Etot is the total excess energy released in each process,
E(A,C,D: Ј,JЈ) is a rovibrational energy of the ( Ј,JЈ)
v
v
level, and s is the number of normal modes ͑3nϪ6 for an n
atom nonlinear molecule:sϭ30 for C6F5X and sϭ27 for
C6F5͒. By using Eq. ͑17a͒, the prior vibrational distributions
for the lowest Јϭ0 and 1 levels are estimated for each
v
FIG. 6. ͑a͒ Observed and calculated prior rotational distributions and ͑b͒
exothermic process, as shown in Table II. The statistical
theory predicts very low vibrational excitation of
NO(A,C,D), being consistent with the experimental obser-
vation. However, it was found that NO(A) from the
NOϩ/C6F5BrϪ reaction is more vibrationally excited than
that from the NOϩ/C6F6Ϫ and NOϩ/C6F5ClϪ reactions, even
though similar vibrational excitation is expected among the
three reactions based upon the statistical theory.
surprisal plot for NO(A: ϭ0) produced from the NOϩ/C F ClϪ reaction.
vЈ
6
5
*
fore concluded that the formation of NO by the ion–ion
neutralization reactions of NOϩ with C6F5ClϪ, C6F5BrϪ, and
C6FϪ5 does not proceed through long-lived ion-pair com-
plexes, where the excess energy is randomized statistically to
neutral products. The ion–ion neutralization reactions have
an enormous cross section, due to the strong coulombic at-
traction between the ion pair. Because of the large impact
parameter, the conservation of angular momentum inhibits
close contact, i.e., a strong collision. Therefore the excess
energy will not be randomized statistically in the reaction
products.
As a representative result, the prior rotational distribu-
tion of NO(A: Јϭ0) in the NOϩ/C F ClϪ reaction is com-
v
6
5
pared with the observed one in Fig. 6͑a͒. The calculated prior
distribution is more excited than the observed one. Similar
results have also been obtained for NO(A: Јϭ1),
v
NO(C: Јϭ0), and NO(D: Јϭ0) in the NOϩ/C F ClϪ re-
v
v
6
5
action and NO(A: Јϭ0,1) in the NOϩ/C F BrϪ reaction.
v
6
5
On the other hand, the prior distribution of NO(A) from the
NOϩ/C6FϪ5 reaction is less excited than the observed one.
The deviation from the prior distribution can often be repre-
sented in the form of linear surprisal,30–32
C. Reaction mechanism
The ion–ion neutralization reactions studied here pro-
ceed through V͑NOϩ,C6F5XϪ͒ and V͑NOϩ,C6FϪ5 ͒ ion-pair
I g ͒ϭϪln P J ͒/P0 J ͒ ϭ␭0ϩ␪ g
,
͓͑ ͑
J
͑18͒
͑
͑
͔
Ј
Ј
J
v
Ј
Ј
Ј
TABLE V. Rotational surprisal parameters for NO(A,C,D) produced from
the ion–ion neutralization reactions of NOϩ with C6F5ClϪ, C6F5BrϪ, and
C6FϪ5 .
where gJ ϭ EJ /(Etot Ϫ Ev ). As an example, the surprisal
Ј
Ј
Ј
plot for NO(A: Јϭ0) in the NOϩ/C F ClϪ reaction is
v
6
5
shown in Fig. 6͑b͒. A satisfactory linear relationship is
found. Similar linear relationships were found for the other
cases. From the slopes of the surprisal plots, the linear rota-
␪
v
Ј
Anion
NO(A: ϭ0) NO(A: ϭ1) NO(C: ϭ0) NO(D: ϭ0)
vЈ
vЈ
vЈ
vЈ
tional surprisal parameters ␪ are obtained for each neutral-
v
C6F5ClϪ
C6F5BrϪ
C6FϪ5
44.0
44.2
Ϫ31.1
37.1
55.1
55.5
29.9
Ј
ization process ͑Table V͒. As described above, there are sig-
nificant discrepancies between the observed vibrational and
rotational distributions and the statistical ones. It was there-
J. Chem. Phys., Vol. 105, No. 7, 15 August 1996
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*
Tsuji et al.: NO formation by ion–ion neutralization reactions
2707
*
where interactions between neutral NO and C6F5X or C6F5
molecules are small. Thus, a large amount of the kinetic
energy will remain in the neutral products.
Under the Born–Oppenheimer approximation, the
internuclear separations of NOϩ(X), C6F5XϪ, and C6FϪ5
are also unchanged just after the neutralization, since the
1
equilibrium nuclear separation of NOϩ͑X ⌺ϩ:1.063 22 Å͒
2
2
is close to those of NO͑A ⌺ϩ:1.0634 Å͒, NO͑C ⌸r :1.062
Å͒, and NO͑D ⌺ϩ:1.0618 Å͒,22 the formation of
2
NO(A,C,D) with the largest Franck–Condon factors for the
1
NOϩ(X ⌺ϩ)→NO(A,C,D) neutralization will be most fa-
vorable. This prediction is consistent with the preferential
formation of NO(A,C,D) in the Јϭ0 levels. Although no
v
vibrational excitation of NO(A) was found in the
NOϩ/C6FϪ6 , NOϩ/C6F5Ϫ, and NOϩ/C6F5CFϪ3 reactions, the
very low vibrational excitation of NO(A) was observed in
the NOϩ/C6F5ClϪ and NOϩ/C6F5BrϪ reactions. For the latter
reactions, the NO(A) formation occurs via curve crossings at
shorter range than that for the former reactions, so that the
NOϩ(X) potential is slightly perturbed by an access of the
negative ion.
FIG. 7. The entrance NOϩϩC6F5XϪ͑XϭF,Cl,Br,CF3͒ and NOϩϩC6FϪ5 ion-
*
*
pair potentials and the exit NO ϩC6F5X͑XϭF,Cl,Br,CF3͒ and NO ϩC6F5
potentials.
Unfortunately, to the best of our knowledge, detailed
equilibrium geometries of C6F5CFϪ3 and C6F5Ϫ are unknown.
On the other hand, some structural information on the geom-
etry of C6FϪ6 has been obtained from electron spin resonance
͑ESR͒ coupling constants measured in the condensed
phase.35–37 The resulting geometry depends on the theoreti-
cal treatment. Both carbon skeleton distorted to a cyclohex-
ane like chair36 and undistorted carbon skeleton but out of
plane C–F bonds37 have been proposed for the equilibrium
geometry of C6FϪ6 . These geometries are significantly differ-
ent from that of the planer C6F6 molecule. For C6F5ClϪ and
C6F5BrϪ, some changes in the C–Cl and C–Br bond lengths
are expected between the neutral molecules and anions on
the basis of the ESR coupling constants,38 because an excess
*
potentials, as shown in Fig. 7. NO arises from diversion of
trajectories from the entrance V͑NOϩ,C6F5XϪ͒ or
ϩ
Ϫ
*
V͑NO ,C6F5 ͒ potential to the exit V͑NO ,C6F5X͒ or
V͑NO ,C6F5͒ potentials due to a strong coupling between
the two potentials. Thus the electronic state distribution of
NO reflects the different crossing point of the strongly at-
*
*
tractive ion-pair entrance potentials and rather flat covalent
exit potentials at the crossing points and the coupling be-
tween each pair of states. The NOϩ–C6F5XϪ ͑XϭCl,Br͒ and
NOϩ–C6FϪ5 separations at crossing points Rc are calculated
using the same relation as that reported previously.14,15 In the
calculations, the following electron affinities were used:
C6F5Cl ͑0.48 eV͒,33 C6F5Br ͑0.47 eV͒,33 and C6F5 ͑3.4 eV͒.34
The Rc values for each reaction are given in Table VI.
The ion–ion neutralization reactions proceed through an
*
electron is located on the C–Cl or C–Br ␴ orbital. Since the
equilibrium geometries of C6F5XϪ ͑XϭF,Cl,Br͒ are different
from those of C6F5X, some energy will be released as the
vibrational energy of C6F5X in the neutralization reaction.
Consequently, almost all excess energy will be transformed
into the relative kinetic energy of the neutral products and
the vibrational energy of C6F5X.
approach of the NOϩ(X ⌺ϩ) and C6F5XϪ or C6FϪ5 ion pair
1
under their mutual Coulombic field followed by an electron
transfer from C6F5XϪ or C6FϪ5 to NOϩ. Since the Born–
Oppenheimer approximation holds during a fast electron
transfer, the relative motion of an ion pair is unchanged after
the neutralization. Therefore, a large kinetic energy resulting
from the strong mutual Coulombic force is conserved at the
instant of electron transfer. The electron transfer occurs at
relatively large intermolecular separations of 4.3–38 Å,
For the production of NO(A,C,D), the symmetry of the
reactant and product potentials must be matched. Although
2
2
NO(A ⌺ϩ) and NO(D ⌺ϩ) are formed, NO(E) with the
same ⌺ϩ symmetry is not produced in the NOϩ/C6F5XϪ
2
͑XϭF,Cl,Br,CF3͒ reactions. It is therefore reasonable to as-
sume that the symmetry of the product potential is not a
TABLE VI. Crossing points in the ion–ion neutralization reactions of NOϩ
with C6F5ClϪ, C6F5BrϪ, and C6FϪ5 .
*
significant factor for the electronic state selectivity of NO in
the NOϩ/C6F5XϪ ͑XϭF,Cl,Br,CF3͒ reactions. Here, it was
Rc ͑Å͒
*
found that the electronic state selectivity of NO is the same
NO
NOϩ/C6F5ClϪ
4.35
NOϩ/C6F5BrϪ
NOϩ/C6FϪ5
for the reactions with C6FϪ6 , C6F5ClϪ, and C6F5CFϪ3 with
D6h, D2h, and Cs molecular symmetries, respectively. On
the other hand, the electronic selectivity of C6F5BrϪ is dif-
ferent from that of C6F5ClϪ, even though the molecular sym-
metry is the same. These facts led us to conclude that mo-
lecular symmetry of negative ion is not a significant factor
*
NO(A)
NO(B)
NO(C)
NO(D)
NO(E)
4.34
͑4.56͒
͑6.24͒
͑6.65͒
͑11.6͒
37.8
͑65.2͒
a
͑4.57͒
6.27
6.61
͑11.7͒
aRc values for unobserved NO states are given in parentheses.
for assessing the electronic selectivity of NO .
*
*
J. Chem. Phys., Vol. 105, No. 7, 15 August 1996
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*
Tsuji et al.: NO formation by ion–ion neutralization reactions
2708
There exist two types for the electronic state selectivity
fore, significant stabilization by the field effect occurs for the
in the ion–ion neutralization reactions of NOϩ with negative
␴
*
*
orbital, while the diffuse ␲ orbital is less strongly sta-
2
2
2
ions. One is the formation of the A ⌺ϩ, B ⌸r , and E ⌺ϩ
bilized by the field effect and destabilized by the ␲ dona-
*
tion. A similar ␴ –␲ crossover also occurs for C6F5ClϪ
and C6F5BrϪ on the basis of the ESR data in the condensed
phase.38 Therefore, the excess electron is dominantly located
*
*
states, which is independent of the negative ion. The other is
2
2
the formation of the C ⌸r and D ⌺ϩ states, which de-
pends strongly on the negative ion. For the former type of
reactions, the A state is formed in all reactions, even though
there are significant differences in the crossing points
͑4.3–38 Å͒ as shown in Table VI and in the nature of the
highest occupied molecular orbital ͑HOMO͒ in the negative
ions. The formation of the A state from the NOϩ/C6F5Ϫ reac-
tion occurs at an especially long intermolecular distance of
38 Å. A similar near-resonant ion–ion neutralization process
via curve crossings at long intermolecular distances has re-
cently been found in the Heϩ/C6F6Ϫ reaction39
*
on the C–Cl or C–Br ␴ orbital. Although the formation of
C6F5ClϪ, ClϪ, C6F5BrϪ, and BrϪ have been observed at low
energy electron attachment, the C6FϪ5 formation has not been
observed due to the higher endothermicity.8,41 This result is
consistent with the fact that the excess electron has ClϪ or
BrϪ
C6F5BrϪ.
For an efficient electron transfer leading to NO(C,D), a
character in the ground states of C6F5ClϪ and
*
␴
*
favorable overlap is necessary between the ␴ orbital of
Ϫ
ϩ
1
ϩ
˜
C6F5X (X) and the 3p␲ or 3p␴ orbital of NO (X ⌺ ) at
Ϫ
6
3
3
1
HeϩϩC F →He 4p P,4d D,4d D͒ϩC F .
͑19͒
͑
the crossing points. In the case of C6FϪ6 , a negative charge is
6
6 6
*
equally delocalized over the six C–F ␴ orbitals, while the
The B and E states are absent in all cases. The neutralization
reactions leading to the A, C, D, and E states proceed
through an electron jump from the HOMO orbital of the
negative ion to the 3s␴, 3p␲, 3p␴, and 4s␴ orbitals of
NOϩ, respectively.22 The preferential formation of the A
state and the lack of the high Rydberg E state in all cases
imply that an electron jump from the HOMO orbital of the
negative ion to the NOϩ(3s␴) orbital occurs efficiently,
while that to the upper NOϩ(4s␴) orbital does not take
place. One reason for the lack of the E state may be curve
crossings at long intermolecular distances ͑у11.6 Å͒, where
interactions between the entrance and exit potentials are very
weak. The lack of the B state is attributed to a low probabil-
ity of two-electron transfer in the neutralization process and
small Franck–Condon factors between the NOϩ(X) and
NO(B) states, as discussed previously.14 In the
NOϩ/C6F5XϪ ͑XϭF,Cl,Br͒ reactions, the branching ratios of
the C and D states decrease with increasing the mass of the
halogen atom in the anion and becomes zero for the heaviest
C6F5BrϪ molecule. Although the Rc values for the formation
of the C and D states are nearly the same between the
NOϩ/C6F5ClϪ and NOϩ/C6F5BrϪ reactions, there is a sig-
nificant difference in the formation of the C and D states
between the two reactions. This implies that the Rc value is
not a critical factor in assessing the electronic state selectiv-
ity of the C and D states.
negative charge is dominantly located on one C–Cl or C–Br
Ϫ
␴
orbital for the cases of C6F5Cl and C6F5BrϪ. The high
*
branching ratios of the C and D states in the NOϩ/C6F6Ϫ
reaction in comparison with those in the NOϩ/C6F5ClϪ and
NOϩ/C6F5BrϪ reactions imply that a favorable overlap oc-
Ϫ
*
curs between the delocalized ␴ orbital of C6F6 and the 3p␲
or 3p␴ orbital of NOϩ, while such an overlap is inefficient
for the cases of localized ␴ orbitals of C6F5ClϪ and
*
C6F5BrϪ. The lack of the C and D states in the
NOϩ/C6F5BrϪ reaction suggests that the ␴ –3p␲ and
*
*
␴ –3p␴ interactions become negligibly weak by the substi-
tution of Br with a lower electronegativity from Cl. On the
basis of the above findings, different nature of the HOMO
orbitals of C6F5XϪ will be an important factor for assessing
the electronic state selectivity in the ion–ion neutralization
reactions. In order to confirm this prediction, further detailed
experimental and theoretical studies will be required.
IV. SUMMARY
1
Ion–ion neutralization reactions of NOϩ(X ⌺ϩ: Љϭ0)
v
with C6F5XϪ ͑XϭCl,Br͒ and C6FϪ5 have been studied by
using a flowing-afterglow method. The results obtained are
compared with those of our previous data for the NOϩ/C6F6Ϫ
and NOϩ/C6F5CFϪ3 reactions in order to examine the depen-
dence of the electronic state selectivity on the negative ion
with a C6F5 group. It was demonstrated that there exist two
types in the electronic state selectivity. One is the formation
There is a possibility that the electronic state selectivity
arises from the different electronic structure of the negative
ions. Gant and Christophorou40 have predicted the electronic
configuration of the ground state of C6F6 is
2
of the NO(A ⌺ϩ) state, which is independent of the nega-
2
2
2
2
2
2
͑␲ ͒ ͑␲ ͒ ͑␲ ͒ ͑␲ ͒ or ͑␲ ͒ ͑␲ ͒ ͑␲ ͒ ͑␲ ͒. According to
tive ion. The other is the formation of the
1
2
3
4
1
2
3
5
2
2
their results, the HOMO of C6FϪ6 is ␲4 or ␲5. However, later
NO(C ⌺ϩ,D ⌸r) states, which depends on the negative
ion. By the substitution of a heavier halogen atom to C6F6,
the branching ratios of the C and D states decreased: 0.27
͑C6FϪ6 ͒, 0.045 ͑C6F5ClϪ͒, and 0 ͑C6F5BrϪ͒. The significant
decrease in the branching ratios of the C and D states for
C6F5ClϪ and C6F5BrϪ was explained by the weaker interac-
ESR data in the condensed phase demonstrated that an ex-
cess electron is captured into the ␴ orbital.35–37 The fact
*
Ϫ
*
that the HOMO of C6F6 has ␴ character has been explained
*
*
by the ␴ –␲ crossover. The high electronegativity of F
atom leads to a lowering of the lowest unoccupied molecular
orbital ͑LUMO͒ energy. However, the energy lowering oc-
*
tions between the localized C–Cl or C–Br ␴ orbital and the
curs to a much lesser extent for ␲ relative to the ␴ LUMO.
3p␲ and 3p␴ orbitals of NOϩ than those between the delo-
*
*
*
Fluorine is strongly electron withdrawing by the field effect
calized C–F ␴ orbital and the 3p␲ and 3p␴ orbitals of
but is weakly electron pair donating ͑␲ donation͒. There-
NOϩ. Only the A state was produced in the NOϩ/C6FϪ5 re-
*
J. Chem. Phys., Vol. 105, No. 7, 15 August 1996
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*
Tsuji et al.: NO formation by ion–ion neutralization reactions
2709
9
action, because the formation of the upper C and D states is
͑a͒ B. Peart and D. A. Hayton, J. Phys. B 25, 5109 ͑1992͒; ͑b͒ D. A.
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energetically closed. Only the Јϭ0 level was excited in the
v
10 J. Weiner, W. B. Peatman, and R. S. Berry, Phys. Rev. A 4, 1824 ͑1971͒.
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tion, Trivandrum, India, 1995͒, Vol. 5, p. 25.
formation of the C and D states by the NOϩ/C6F5ClϪ and
NOϩ/C6F5BrϪ reactions and in the formation of the A state
in the NOϩ/C FϪ reaction. On the other hand, the Јϭ0 and
v
6
5
1 levels of NO(A) were produced in the NOϩ/C6F5ClϪ and
13 D. Smith, N. G. Adams, and M. J. Church, J. Phys. B 11, 4041 ͑1978͒.
14 M. Tsuji, H. Ishimi, M. Nakamura, Y. Nishimura, and H. Obase, J. Chem.
Phys. 102, 2479 ͑1995͒.
NOϩ/C6F5BrϪ reactions, though the relative populations of
ϭ1 are small. The small vibrational excitation of NO(A)
vЈ
15 M. Tsuji, H. Ishimi, Y. Nishimura, and H. Obase, J. Chem. Phys. 102,
6013 ͑1995͒.
in the NOϩ/C6F5ClϪ and NOϩ/C6F5BrϪ reactions can be ex-
plained as due to the perturbation of the NOϩ(X) potential
by an access of the negative ion. The rotational temperatures
16 M. Tsuji, H. Ishimi, Y. Nishimura, and H. Obase, Int. J. Mass Spectrom.
Ion Process. 145, 165 ͑1995͒.
17 M. Tsuji, H. Ishimi, and Y. Nishimura, Chem. Lett. 1995, 873.
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of NO(A: Јϭ0,1), NO(C: Јϭ0), and NO(D: Јϭ0) were
v
v
v
expressed by single Boltzmann rotational temperatures of
450Ϯ100, 300Ϯ50, and 400Ϯ50 K, respectively. The vibra-
tional and rotational distributions indicated that only 1%–
11% of the total excess energy is partitioned into the rovi-
20 L. G. Piper and L. M. Cowles, J. Chem. Phys. 85, 2419 ͑1986͒, and
references therein.
*
brational energy of NO . It was therefore concluded that a
21 L. G. Piper, T. R. Tucker, and W. P. Cummings, J. Chem. Phys. 94, 7667
͑1991͒.
major part of the excess energy is partitioned into the relative
translational energy of the products and the vibrational en-
ergy of C6F5X. The observed vibrational and rotational dis-
tributions disagreed from statistical prior ones, indicating
that the reaction dynamics is not governed by the simple
statistical theory because of the large impact parameter.
22 K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Struc-
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1979͒.
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ACKNOWLEDGMENTS
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The authors are grateful to Professor Hiroshi Shimamori
at Fukui Institute of Technology and Dr. Shuji Kato at
Himeji Institute of Technology for their helpful discussion.
This work has been supported by the Morino Foundation for
molecular science ͑1992͒, the Iwatani Naoji Memorial Foun-
dation ͑1993͒, Showa Shell Sekiyu Foundation for environ-
mental research ͑1995͒, and a Grant-in-Aid for Scientific Re-
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J. Chem. Phys., Vol. 105, No. 7, 15 August 1996
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