N(4S) formation following the 193.3-nm ArF laser irradiation of NO and NO2 and its application to kinetic studies of N(4S) reactions with NO and NO2

Article abstract of DOI:10.1021/jp054089c

Formation of the ground-state nitrogen atom, N(⁴S), following 193.3-nm ArF laser irradiation of NO and NO₂ was detected directly by a technique of laser-induced fluorescence (LIF) spectroscopy at 120.07 nm. Tunable vacuum ultraviolet (VUV) laser radiation around 120.07 nm was generated by two-photon resonance four-wave sum frequency mixing in Hg vapor. Photoexcitation processes of NO and NO₂ giving rise to the N( ⁴S) formation are discussed on the basis of the Doppler profiles of the nascent N(⁴S) atoms produced from the photolysis of NO and NO₂ and the photolysis laser-power dependence of the N(⁴S) signal intensities. Using laser flash photolysis and vacuum ultraviolet laser-induced fluorescence detection, the kinetics of the reactions of N( ⁴S) with NO and NO₂ have been investigated at 295 ± 2 K. The rate constants for the reactions of N(⁴S) with NO and NO₂ were determined to be (3.8 ± 0.2) × 10^-11 and (7.3 ± 0.9) × 10^-12 cm³ molecule ^-1 s^-1, respectively, where the quoted uncertainties are 2σ statistical uncertainty including estimated systematic error.

Full text of DOI:10.1021/jp054089c

J. Phys. Chem. A 2005, 109, 10897-10902
10897
4
N( S) Formation following the 193.3-nm ArF Laser Irradiation of NO and NO2 and Its
4
Application to Kinetic Studies of N( S) Reactions with NO and NO2
Tomoki Nakayama, Kenshi Takahashi, and Yutaka Matsumi*
Solar-Terrestrial EnVironment Laboratory and Graduate School of Science, Nagoya UniVersity, Honohara
-13, Toyokawa, Aichi 442-8507, Japan
3
Kazuhiko Shibuya
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1
Ohokayama, Meguro, Tokyo 152-8551, Japan
ReceiVed: July 25, 2005; In Final Form: September 23, 2005
4
Formation of the ground-state nitrogen atom, N( S), following 193.3-nm ArF laser irradiation of NO and
NO
Tunable vacuum ultraviolet (VUV) laser radiation around 120.07 nm was generated by two-photon resonance
four-wave sum frequency mixing in Hg vapor. Photoexcitation processes of NO and NO giving rise to the
N( S) formation are discussed on the basis of the Doppler profiles of the nascent N( S) atoms produced from
2
was detected directly by a technique of laser-induced fluorescence (LIF) spectroscopy at 120.07 nm.
2
4
4
4
the photolysis of NO and NO
2
and the photolysis laser-power dependence of the N( S) signal intensities.
Using laser flash photolysis and vacuum ultraviolet laser-induced fluorescence detection, the kinetics of the
4
reactions of N( S) with NO and NO have been investigated at 295 ( 2 K. The rate constants for the reactions
2
4
-11
-12
3
-1
of N( S) with NO and NO
2
were determined to be (3.8 ( 0.2) × 10 and (7.3 ( 0.9) × 10 cm molecule
-1
s , respectively, where the quoted uncertainties are 2σ statistical uncertainty including estimated systematic
error.
1
. Introduction
Photochemical processes involving the N( S) atom and NOx
NO and NO2) molecules play a crucial rule in the terrestrial
TABLE 1: Summary of the Present and Previous Studies on
4
the Reaction Kinetics of N( S) with NO and NO
2
at Room
4
Temperature
(
reactant rate coefficienta
NO
methode
DF/MS
references
Herron (1961) [6]
Philips and Schiff (1962) [7]
Clyne and McDermid (1975) [9]
Lee et al. (1978) [10]
1-4
4
and planetary atmospheres. For instance, the reaction of N( S)
with nitric oxide
1.7 ( 0.8b
2
2
.2 ( 0.6
DF/MS
DF/MS
DF/RF
FP/RF
PR/RA
DF/RF
FP/RF
DF/RF
DF/MS
DF/MS
DF/RF
.2 ( 0.2c
4
3
N( S) + NO f N + O( P)
(1)
2
2.7 ( 0.4
4
1
.0 ( 0.2c
.9 ( 0.2c
Lee et al. (1978) [10]
Sugawara et al. (1980) [16]
Cheah and Clyne (1980) [11]
Husain and Slater (1980) [15]
Clyne and Ono (1982) [12]
Brunning and Clyne (1984) [13]
Jeoung et al. (1991) [14]
Wennberg et al. (1994) [5]
NASA/JPL (2003) [30]
has been thought to act as a sink of odd nitrogen in the middle
and upper terrestrial atmosphere.1,2 Possible significance of
reaction 1 in the Martian and Venusian atmosphere has also
3.4 ( 0.3
4
2
.5 ( 0.2
.77 ( 0.04
c
2
4
been suggested. The reaction of N( S) with nitrogen dioxide
2.03 ( 0.17c
2
3
3
3
.4 ( 0.2d
4
3
.6 ( 0.4
N( S) + NO f N O + O( P)
(2)
2
2
.0
.8 ( 0.2d
LP/VUV-LIF This work
plays a less significant role as a sink of odd nitrogen in the
middle terrestrial atmosphere because of the small concentration
of NO2. A small amount of N2O may be formed by reaction 2
c
NO2 1.85 ( 0.22
0.14 ( 0.02
DF/MS
DF/MS
FP/RF
Philips and Schiff (1965) [8]
Clyne and McDermid (1975) [9]
Husain and Slater (1980) [15]
Clyne and Ono (1982) [12]
Wennberg et al. (1994) [5]
NASA/JPL (2003) [30]
5
3.8 ( 0.1
0.301 ( 0.033 DF/RF
1
1
0
c
4
in the Martian atmosphere. For the N + NO2 reaction, other
d
.2 ( 0.1
.2
DF/RF
channels forming 2NO, N2 + O2, and N2 + O + O are
energetically possible. However, those channels are suggested
to be of minor importance as related to products.5
.73 ( 0.09 d
LP/VUV-LIF This work
a
In units of 10-11 cm molecule s-1. b The error limits are estimates
3
-1
Although a number of kinetic studies of reactions 1 and 2
have been reported at room temperature, as listed in Table 1,
agreement among them is relatively poor. In previous kinetic
studies, several techniques such as microwave discharge of
of 3σ. c The error limits are estimates of 1σ. The error limits are
estimates of 2σ. Experimental techniques. DF: discharge flow, MS:
d
e
mass spectrometry, KS: kinetic spectroscopy, RF: resonance fluores-
cence, FP: flash photolysis, PR: pulsed radiolysis, RA: resonance
absorption, LP: laser photolysis, VUV-LIF: vacuum ultraviolet laser-
induced fluorescence.
5
-14
10,15
N2,
vacuum ultraviolet (VUV) flash photolysis of N2O,
16
and pulsed radiolysis of N2 were employed to generate the
N( S) atoms. For N( S) detection, mass spectrometry techniques
4
4
were used in several studies.6
-9,13,14
With nitrogen atom lamps,
5,10-12,15
resonance fluorescence detection
and resonance absorp-
*
Corresponding author. Fax: +81-533-89-5593. E-mail: matsumi@
1
6
4
stelab.nagoya-u.ac.jp.
tion detection of N( S) atoms were also used.
1
0.1021/jp054089c CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/12/2005

10898 J. Phys. Chem. A, Vol. 109, No. 48, 2005
Nakayama et al.
Very recently, Nakayama et al.17 reported a new detection
technique for the N( S) atom using vacuum ultraviolet laser-
experimental observation of channel 4c with the irradiation of
an ArF laser. In our present paper, we report a first experimental
observation of N( S) production from NO2 with high-power ArF
4
4
induced fluorescence (VUV-LIF) spectroscopy at 120.07 nm,
2
4
3 4
which is resonant to the N(2p 3s P1/2-2p S3/2) transition. They
laser irradiation at 193.3 nm. The photoexcitation processes of
4
4
photolyzed N2O molecules at 193.3 nm, and the N( S) atoms
NO2 giving rise to N( S) formation have been discussed.
produced were detected using the VUV-LIF technique. In the
present study, we demonstrate that the laser flash photolysis/
VUV-LIF detection technique is a powerful tool for the study
2. Experiments
4
The experimental setup used in the present study is similar
to that in our previous work using the VUV-LIF technique,17
and therefore only a brief description related to this study is
given here. Unpolarized output of an ArF excimer laser (Lambda
Physik, COMPex 102) was employed for photoexcitation of NO
and NO2. Typical pulse energy of the 193.3-nm laser was 0.5-8
of the kinetics of reactions 1 and 2, in which the N( S) formation
following 193.3-nm laser irradiation of NO and NO2 has been
4
utilized as a photolytic source of N( S) atoms.
4
N( S) formation following 193.3-nm ArF laser irradiation of
NO under shock-heated conditions (1400-3500 K) was reported
18,19
4
by several groups,
in which the N( S) atoms were monitored
4
mJ. The N( S) atoms produced following 193.3-nm laser
by atomic resonance absorption spectroscopy at 119.9 nm.
irradiation of NO and NO2 were directly detected by the VUV-
2
4
3
LIF technique at 120.07 nm, which corresponds to the N(2p 3s
NO + hν(193.3 nm) f N( S) + O( P)
(3)
4
3 4
P1/2-2p S3/2) transition. The tunable VUV laser around 120.07
nm was generated by four-wave sum frequency mixing (ω
2
+
2
Three doublet states of NOsA Σ (V′ ) 3), B Π(V′ ) 7),
VUV
2
) 2ω + ω ) in Hg vapor,28 in which Kr gas was added for
and C Π(V′ ) 3)slie near the dissociation limit of NO. They
suggested that N atoms are produced by direct excitation of
vibrationally excited states of NO(X Π, V g 1) to the D Σ
state, following conversion from the D Σ state to the predis-
sociating C Π state. In our present paper, we report the first
observation of N( S) formation following 193.3-nm irradiation
1
2
phase matching. Two dye lasers were simultaneously pumped
by a single XeCl excimer laser (Lambda Physik, COMPex 201,
Scanmate2E and FL3002). One dye laser operating with
2
2 +
2
+
2
Rhodamin 101 dye in CH OH solvent generated 4-5 mJ/pulse
around 625 nm. The visible output was frequency-doubled in a
3
4
of NO at 295 K. Photoexcitation processes of NO giving rise
KD*P crystal to obtain 312.76 nm (in air), which was two-
4
photon resonant with the Hg electronic transition (6s7s 1S r
to the N( S) atom formation at 295 K have been discussed.
0
2
1
Photoexcitation processes of NO2 in the UV region have been
thoroughly investigated at a number of wavelengths other than
6s S0). The other dye laser operating with Coumarin 307 dye
in CH3OH solvent produced 2-4 mJ/pulse around 517 nm. The
two laser beams were carefully overlapped using a dichroic
mirror and focused into the Hg vapor cell with a fused silica
lens (f ) 250 mm). Temperature in the Hg vapor cell was around
450-470 K, which corresponds to the equilibrium Hg vapor
2
0
1
93.3 nm. At 193.3 nm, there are only a limited number of
experimental studies on the photoexcitation processes. The
following two channels are thought to be dominant as photo-
dissociation pathways at 193.3 nm,
2
9
pressure of 8-14 Torr. The VUV laser line width was
estimated to be 0.40 cm full-width-at-half-maximum (fwhm)
with a Gaussian shape, which was estimated from the Doppler
profile of thermalized N( S) atoms produced from 193.3-nm
laser irradiation of NO2 in the presence of 3.6 Torr of helium
at a delay time of 5 µs between the photolysis and probe laser
pulses. The repetition rate of the photolysis and probe laser
pulses was 10 Hz. The delay time between the photolysis and
probe laser pulses was controlled by a digital delay generator
(Stanford Research Systems, DG535).
2
3
-1
NO + hν(193.3 nm) f NO(X Π) + O( P)
(4a)
(4b)
2
4
2
1
f NO(X Π) + O( D)
1
3
and the branching ratio for O( D) and O( P) productions was
1
1
3
21
estimated to be [O( D)]/([O( D)] + [O( P)]) ) 0.55 ( 0.03.
Gradd and Slanger observed the vibrationally excited NO-
X Π, V ) 4-10) in the photodissociation of NO2 at 193.3 nm.
Gong et al. reported that the nascent vibrational state popula-
2
2
2
(
23
2
tions of NO(X Π, V ) 1-7) in the photodissociation of NO2 at
The photolysis and probe laser beams crossed at right angles
in the reaction chamber. The VUV-LIF signal of N( S) atoms
93.3 nm have a maximum at V ) 5. Tsuji et al.24 studied the
4
1
decomposition of NO2 in N2 atmosphere at 193.3 nm by ArF
laser irradiation and observed a lower NO yield than that
predicted from simple NO2 photolysis (reactions 4a and 4b) at
a low NO2 concentration of 200 ppm. They suggested that the
lower NO yield was attributable to the photolytic loss of
vibrationally excited NO which was produced primarily from
was detected using a solar-blind photomultiplier tube (EMR,
542J-09-17) mounted at right angles to the propagation direc-
tions of both the photolysis and probe laser beams. The PMT
has an MgF2 window and a KBr photocathode that is sensitive
only between 115 and 165 nm. A band-pass filter was installed
(Acton Research, model 122-VN, λ ) 122 nm, ∆λ ) 12 nm).
The output of the photomultiplier was preamplified and averaged
over 10 laser pulses, using a gated integrator (Stanford Research,
SR-250). The reaction chamber was continuously evacuated by
4
NO2 photolysis at 193.3 nm. The N( S) and O2 productions are
energetically possible in the UV region below 272 nm:
4
-1
NO + hν f N( S) + O
(4c)
a rotary pump (Alcatel, 300 L min ) through a liquid N2 trap.
2
2
The total gas pressure in the chamber was measured by a
capacitance manometer (MKS Baratron, 122A). All experiments
were performed at 295 ( 2 K.
NO gas (Nihon Sanso Co., >99%) purchased was used after
purification by passing it through a cold trap that was immersed
in a methanol/liquid N2 slush bath at 175 K. Helium gas (Iwatani
Gas Co., >99.99%) was used without further purification in
the kinetic experiments. Much attention was paid to the handling
of NO2 gas reagent in this study, since purified NO2 gas is easily
degraded to NO mixture and the reaction rate constant of NO
Matsumi et al.25 detected laser-induced fluorescence from the
vibrationally excited O2 molecules around 220-300 nm fol-
lowing the irradiation of focused visible laser light (470-580
nm) on NO2 gas, and suggested that the vibrationally excited
O2 molecules were directly produced from a multiphoton
absorption process of NO2. However, further experimental
studies revealed that the O2 molecules were produced from the
reaction of O* + NO2 where O* was generated from the
multiphoton dissociation of NO2.26,27 There has been no

4
N( S) Formation from Laser-Irradiated NO and NO2
J. Phys. Chem. A, Vol. 109, No. 48, 2005 10899
4
Figure 1. Fluorescence excitation spectra of N( S) produced from the
1
93.3-nm laser irradiation of (a) NO and (b) NO
2
, respectively, which
were recorded by scanning the probe laser wavelength across the
4
4
4
resonance center of the P1/2- S3/2 transition for the N( S) atom at
20.07 nm. The solid curves indicate Gaussian shapes that fit the
1
observed spectrum. Spectrum (a) was measured at an 80-ns delay time
between the photolysis and probe laser pulses at the pressure of NO
Figure 2. Plots of the photolysis laser power dependence of the LIF
intensity of N( S) produced following 193.3-nm laser irradiation of (a)
4
1
45 mTorr. Spectrum (b) was measured at an 80-ns delay time at
pressures of NO 34 mTorr and O 82 mTorr. The broken curve shows
the fluorescence excitation spectrum of thermalized N( S), in which
2
NO and (b) NO , respectively. The photolysis laser power was varied
2
2
while monitoring the N( S) LIF signal at 120.07 nm. The probe laser
power was kept constant during the measurements. Solid lines indicate
the results of least-squares fit analysis in order to determine the order
n of laser power dependence of the LIF signals. The values obtained
for n are (a) 0.9 ( 0.1 and (b) 1.7 ( 0.2, respectively, which are
described in the figure. The quoted uncertainties indicate the 2σ errors
of the fit.
4
4
4
2
the N( S) atom was produced from 193.3-nm irradiation of NO in the
presence of 3.6 Torr of He. The delay time was 5 µs.
4
with N( S) has been reported to be about 5 times larger than
that of NO2 (Table 1). The NO2 gas was synthesized by mixing
the purified NO gas with excess O2 (Nihon Sanso Co.,
4
excitation spectrum of the nascent N( S) atoms produced
>99.99%) in a glass vessel (2 L) and stored in the vessel for
following 193.3-nm laser irradiation of NO, which was obtained
more than several days prior to use in the experiments. The
total pressure of the NO2/O2 vessel was about 700 Torr with
the mixing ratio [NO2]/[O2] ≈ 1:3. For the NO2 photolysis and
kinetics experiments, the gas mixture of NO2/O2 was supplied
to the reaction cell directly from the vessel through a mass flow
controller. N2O4 equilibrates with NO2 gas. The equilibrium
between NO2 and N2O4 in the reactant gas which was flowed
2
4
by scanning the probe laser wavelength across the 2p 3s P1/2-
3
4
2
p
S3/2 transition at 120.07 nm. The spectrum was taken at an
8
0-ns delay time with 145 mTorr of NO, and no buffer gas
was added. It should be noted that the dissociation limit of
4
3
rovibrationally cold NO giving rise to the N( S) + O( P) channel
-
1 34
is 52397 ( 2 cm , which is slightly larger than the photon
energy of the 193.3-nm laser radiation of 51730 cm . To reveal
the photodissociation processes of NO producing the N( S)
-
1
-
6
into the reaction cell was estimated to be attained within 10
4
20
s
before the laser irradiation. When the partial pressure of NO2
atom, photolysis laser power dependence of the LIF signal was
measured. As shown in Figure 2a, we found that the LIF
intensity was linearly dependent on the photolysis laser power.
When we turned the ArF excimer laser off, no obvious LIF
16
-3
is 1 × 10 molecule cm , the partial pressure of N2O4 is about
1
/400 of NO2 at 298 K using the equilibrium constant presented
3
0
by NASA/JPL.
The concentration of NO2 in the reaction chamber was
determined directly by absorption measurements. The collimated
output of a tungsten lamp was passed through the reaction
chamber and focused onto the entrance slit of a spectrograph
with a 2048-element diode array detector (Oceans Optics,
HR2000, f ) 101 mm). A 10-µm entrance slit was used for
absorption measurements at a resolution of ∼0.7 nm. The
absorption features of the NO2 spectrum recorded between 300
and 650 nm are in good agreement with previous measure-
4
signal was observed. This indicates that N( S) production from
the NO photodissociation at 120 nm was negligible under our
4
experimental conditions. These results suggest that the N( S)
formation observed in this study was attributable to the one-
photon excitation process of NO:
4
3
NO + hν(193.3 nm) f N( S) + O( P)
(3)
To check the interference in the measurements because of
impurities, we recorded the fluorescence excitation spectra of
3
1-33
ments.
The wavelength of the spectrometer was calibrated
at 404.66 and 435.84 nm using an Hg pen ray lamp. The
concentration of NO2 was determined by absorption measure-
4
the nascent N( S) atoms produced from purified and nonpurified
NO. No significant change was observed in the signal intensities
ments around 413.7 nm on the basis of the reported absorption
4
and spectral shapes of N( S).
cross-section.3
1-33
The contribution of N2O4, if present, is
2
+
2
Three doublet states of NOsA Σ (V ) 3), B Π(V ) 7), and
3
3
minimized at this wavelength, as described by Gierczak et al.
2
C Π(V ) 3)slie near the dissociation limit of NO. Davidson
The uncertainty associated with the measurements of NO2
concentration was estimated to be ∼10%.
18
19
4
and Hanson and Koshi et al. reported the N( S) formation
following 193.3-nm laser irradiation of NO under shock-heated
4
conditions. The N( S) atom formation was attributed to direct
3
. Results and Discussion
2
excitation of vibrationally excited states of NO X Π(V g 1) to
4
2
+
18
3
.1. N( S) Production following the 193.3-nm Laser Ir-
the D Σ state. In our experiments which were performed at
radiation of NO. Figure 1a shows a typical fluorescence
room temperature, it is unlikely that the direct photoexcitation

10900 J. Phys. Chem. A, Vol. 109, No. 48, 2005
Nakayama et al.
2
2
+
2
from the NO X Π(V ) 1) to the D Σ (V ) 0) state is
NO + hν(193.3 nm) f NO(X Π, V) + O
(4)
2
4
significantly responsible for N( S) formation, because the
population of vibrationally excited NO X Π(V ) 1) is negligibly
small at room temperature.
Shibuya and Stuhl3
ics of NO through the measurements of emission spectra and
fluorescence lifetimes following 193.3-nm laser irradiation. They
2
2
4
NO(X Π, V) + hν(193.3 nm) f N( S) + O
(5)
5-37
Gradd and Slanger39 observed the vibrationally excited NO
X Π(V ) 4-10) in the photodissociation of NO at 193.3 nm.
Gong et al. reported that the nascent vibrational state popula-
tions of NO X Π(V ) 1-7) in the photodissociation of NO at
193.3 nm have a maximum at V ) 5. The average translational
investigated the excited-states dynam-
2
2
23
2
2
2
observed the fluorescence from B Π(V ) 7), C Π(V ) 0), and
2
2
A Σ(V ) 3) states. The 193.3-nm laser light can be resonant
2
2
4
with the absorption of the NO B Π(V ) 7) r X Π(V ) 0) and
energy of N( S) estimated in the present study (∼21 kcal/mol
2
2
A Σ(V ) 3) r X Π(V ) 0) transitions. From the available
in the laboratory frame) is consistent with the sequential two-
38
2
2
spectroscopic data, NO X Π (V ) 0) is expected to be excited
photon dissociation process in which NO(X Π, V ) 7) is formed
2
to the B Π(V ) 7) state via the absorption lines R11(30.5-32.5),
through channel 4.
P11(27.5-29.5), Q11(28.5-31.5), R22(28.5-30.5), P22(25.5-
For the simultaneous two-photon absorption mechanism, it
involves the direct formation of N( S) from an electronically
excited state(s) of NO , where the following channels are
2
4
2
7.5), and Q22(26.5-29.5). The A Σ(V ) 3) state can also be
2
prepared via the absorption of X Π(V ) 0, J ∼ 50.5). The
2
2
Boltzmann population of the X Π(V ) 0, J ∼ 50.5) state is
available thermochemically:
2
much smaller than that of X Π(V ) 0, J ∼ 28.5) at 295 K.
Shibuya and Stuhl3
5-37
suggested that the rotational levels of J
4
NO + 2hν(193.3 nm) f N( S) + 2O
(6)
2
)
21.5-34.5 for the F1 component and J ) 20.5-33.5 for the
2
F2 component of the B Π(V ) 7) state were prepared following
4
NO + 2hν(193.3 nm) f NO* + O f N( S) + O + O (7)
2
2
1
)
93.3-nm excitation at room temperature, and that the C Π(V
2
0) and A Σ(V ) 3) states were produced by the collisional
4
2
NO
2
+ 2hν(193.3 nm) f N( S) + O
2
(8)
relaxation from the B Π(V ) 7) state. They also indicated that
2
the photoexcited levels of the B Π(V ) 7) state are predisso-
40
Haak and Stuhl observed the NO emissions from its several
excited states (A Σ , D Σ , and E Σ , and so on) following
the multiphoton absorption of NO2 at 193.3 nm. Further studies
are required to elucidate the mechanism of N( S) formation
ciative and that the fluorescence quantum yield is about 0.15
2
+
2
+
2 +
-
3
and <6 × 10 for J ) 20.5-29.5 and J > 29.5, respectively.
4
Therefore, it is likely that the N( S) atom formation observed
4
in the 193.3-nm laser irradiation of NO in this study is due to
the photoexcitation to the high rotational levels of the B Π(V
7) state, which is followed by the predissociation to N( S) +
2
following 193.3-nm irradiation of NO2.
4
4
4
3.3. Reaction Kinetics of N( S) + NO and N( S) + NO2.
The N( S) formation following 193.3-nm irradiation of NO and
NO2 was applied to the kinetic studies of reactions 1 and 2.
The pulsed ArF laser light was used to irradiate 150-570 mTorr
of NO in the presence of 1.8 Torr of He diluent, and the
chemical loss of N( S) was measured to determine the rate
coefficients for the N( S) + NO reaction at 295 ( 2 K. Figure
shows a typical example of the temporal profile of the N( S)
)
4
3
O( P).
4
The Doppler profile of the nascent N( S) atoms produced
from the 193.3-nm irradiation of NO (Figure 1a) is slightly
wider than that of the thermalized N( S) atoms (broken curve
4
4
in Figure 1b). The kinetic energy release in channel 3 is
4
-
1
estimated to be ∼1000 cm , from the Doppler profile of the
4
3
4
nascent N( S) atoms. This is consistent with the fact that the
LIF intensity following pulsed ArF laser irradiation of a mixture
of 415 mTorr of NO and 1.8 Torr of He diluent, in which the
VUV laser wavelength was fixed at the resonance center of the
high rotational levels (F1 J ) 21.5-34.5, F2 J ) 20.5-33.5) in
the B Π(V ) 7) state lie above the dissociation limit by 500-
2
-
1
1
100 cm .
2
4
3 4
N(2p 3s P1/2-2p S3/2) transition (120.07 nm). The time-
4
3
.2. N( S) Production following the 193.3-nm Laser Ir-
radiation of NO2. Figure 1b shows a typical example of the
fluorescence excitation spectrum of the nascent N( S) atoms
produced from the 193.3-nm laser irradiation of NO2, in which
gas pressure in the chamber was 34 mTorr and 82 mTorr for
NO2 and O2, respectively, without buffer gas. The spectrum was
taken at an 80-ns delay time. The fwhm of the spectrum was
estimated to be 1.38 cm with a Gaussian shape (Figure 1b).
Assuming the Maxwell-Boltzmann velocity distribution, the
average translational energy of the nascent N( S) atoms in the
laboratory frame was calculated to be ∼7500 cm (∼21 kcal
4
resolved VUV-LIF signal of N( S) atoms exhibits an initial
4
jump due to photolytic formation of N( S), followed by a slow
4
decay due to its chemical removal in collisions with NO
4
molecules. For the kinetic study of the N( S) + NO2 reaction,
4
the time profiles of the N( S) LIF were measured under the
conditions of NO2 60-570 mTorr, O2 200-1700 mTorr, and
He 3.0 Torr. As described in the previous section, the transla-
tionally hot N( S) atoms are produced following 193.3-nm
irradiation of NO and NO2. The translationally hot N( S) atoms
are thermalized within 2 µs under our experimental conditions
in collisions with helium, which was confirmed by measuring
the Doppler profiles of N( S) atoms. The chemical loss of N( S)
by reaction with O can safely be ignored under our experi-
-
1
4
4
4
-
1
-
1
mol ).
4
4
Figure 2b shows a photolysis laser power dependence of the
LIF signal intensity of N( S) atoms produced from the 193.3-
nm laser irradiation of NO2. A quadratic photolysis laser power
dependence of the N( S) LIF signal intensity suggests that N( S)
formation is attributable to the sequential two-photon dissocia-
tion process or the simultaneous absorption of two photons by
NO2.
2
4
mental conditions ([O ] ) 200-1700 mTorr, delay time 0-20
2
-
17
3
µs) because of its small rate constant of 8.5 × 10
cm
4
4
-1 -1
30
molecule
s
at 298 K. For both reaction systems 1 and 2,
single-exponential decay was observed for temporal profiles of
4
N( S) as typically shown in Figure 3.
4
4
For the sequential two-photon dissociation process, it is likely
[N( S)] ) [N( S)] × exp(-k′t)
(9)
t
0
2
that the vibrationally excited NO(X Π, V) is produced from the
2
photolysis of NO2 at 193.3 nm, and then NO(X Π, V) is
The pseudo-first-order rate constant k′ for a particular reactant
pressure was derived by a nonlinear least-squares fit analysis.
4
photolyzed to produce the N( S) atom.

4
N( S) Formation from Laser-Irradiated NO and NO2
J. Phys. Chem. A, Vol. 109, No. 48, 2005 10901
3
4
A′ PES was mainly responsible for the N( S) + NO reaction
over the temperature range of 200-5000 K, and their kNO value
-11
3
-1 -1
at 300 K was 4.68 × 10 cm molecule s . The theoretical
value is larger than our experimental and previous determina-
tions.
The kNO values reported previously range from 0.14 to 3.8
2
-
11
3
-1 -1
×
10 cm molecule
s
as listed in Table 1. The kNO value
2
determined in this study is larger than the result of Clyne and
Ono, and is smaller than that presented by Wennberg et al.5
The experimental technique of laser flash photolysis combined
with VUV laser-induced fluorescence detection has been utilized
12
12
for the first time in this study, while Clyne and Ono and
Wennberg et al. used the technique of discharge flow and
5
4
Figure 3. A typical example of the temporal decay curve of N( S)
LIF intensity following the 193.3-nm pulsed laser irradiation of the
gas mixture containing 415 mTorr of NO and 1.8 Torr of He diluent
at 295 ( 2 K. The solid curve is a first-order decay fit to the data in
the time domain after 2 µs.
resonance fluorescence detection with a nitrogen lamp. Clyne
9
12
and McDermid and Clyne and Ono reported that the measured
4
rate constant kNO2 decreased as the ratio of [NO2]/[N( S)]0
4
increased at lower ratios of [NO2]/[N( S)]0 (<80). They argued
that the change of the observed rate constant was due to catalytic
2
3
interferences involving both H( S) + NO2 and O( P) + NO2
2
3
reactions, in which H( S) and O( P) atoms can be produced as
impurities in the microwave discharge process and O( P) atoms
3
can also be produced through reaction 2. In the present study,
4
N( S) atoms were produced from the 193.3-nm pulsed laser
4
irradiation of NO2, and the ratio of [NO2]/[N( S)]0 was (5-20)
5
4
×
10 as estimated by considering the N( S) detection sensitivity
1
7
reported in our recent paper by Nakayama et al.
It should be noted that, in general, the literature data of the
4
rate coefficients for N( S) reactions with simple molecules are
more scattered than those of other atmospheric reactions
1
2
involving atoms such as O( D) and Cl( Pj) atoms. This might
4
be attributable to the relatively low reactivity of N( S) toward
simple molecules and the lack of applicable experimental
technique. The newly presented technique for investigation of
4
N( S) reactions with NO and NO2 will be applicable to examine
4
some other N( S) reactions as a function of temperature and
pressure. The pulsed laser photolysis/VUV-LIF technique has
been utilized as a powerful tool to study the kinetics and
Figure 4. Plot of pseudo-first-order decay rates (k′) versus the
1
2
2
concentration of the reactants (a) NO and (b) NO . Results of the least-
dynamics of atmospheric reactions involving O( D) and Cl( Pj)
4
2,43
squares fit analysis of the data are drawn by straight lines, which yield
atoms.
-
11
-12
3
the rate coefficients (3.8 ( 0.2) × 10 and (7.3 ( 0.9) × 10 cm
2
at 295 ( 2 K for N( S) reactions with NO and NO ,
-
1
-1
4
molecule
s
4
. Summary
respectively.
In this work, the new technique of VUV-LIF spectroscopy
The resultant dependences of the k′ values on the number
was demonstrated as a powerful tool to study the photochemical
4
4
densities of NO and NO2 for the N( S) + NO and N( S) +
NO2 reactions, respectively, are shown in Figure 4. Linear least-
squares fit analysis of the data in Figure 4 yielded the
processes of NO and NO molecules at room temperature
2
4
following 193.3-nm laser irradiation. Production of the N( S)
atom from NO and NO was directly detected at 120.07 nm
2
-
11
2
4
3 4
bimolecular rate constants of kNO ) (3.8 ( 0.2) × 10
and
which was resonant with the 2p 3s P -2p S3/2 transition.
Doppler spectroscopy of the nascent N( S) atoms from NO
molecules and the photolysis laser power dependence suggested
1
/2
-
12
3
-1 -1
4
kNO ) (7.3 ( 0.9) × 10
cm molecule
s
at 295 ( 2 K.
2
The quoted uncertainties of the rate constants include 2σ
statistical uncertainties and estimated systematic errors.
The rate constants for reactions 1 and 2 at 295 ( 2 K
determined in the present study are listed in Table 1 with
available literature data. The kNO value determined in this study
is in excellent agreement with the latest data reported by
4
that the N( S) formation process is attributed to the one-photon
2
excitation of rotationally excited NO X Π(V ) 0, J ∼ 28.5) to
2
the rotationally excited levels of NO B Π(V ) 7) followed by
4
3
4
decomposition into N( S) + O( P). N( S) formation from NO2
following 193.3-nm laser irradiation was suggested to be due
to the sequential or simultaneous two-photon dissociation
5
Wennberg et al. within the quoted uncertainties. Our result is
1
1
4
also in good agreement with that of Cheah and Clyne and
process. N( S) formation following 193.3-nm pulsed laser
10
one of the values reported by Lee et al. The latest NASA/JPL
irradiation of NO and NO2 was utilized to kinetic studies of
recommended kNO value at 298 K30 is smaller than both the
the N( S) reactions with NO and NO2 at 295 ( 2 K. The
4
5
values recently reported by Wennberg et al. and obtained in
the present study. For the reaction of N( S) + NO, the ground
potential energy surface (PES) A′′ and the first excited PES
experimental technique of laser flash photolysis combined with
the VUV laser-induced fluorescence detection was applied for
4
3
the first time to the kinetic studies of the reactions. The rate
3
-11
A′ were theoretically investigated by means of the CASPT2
coefficients were determined to be (3.8 ( 0.2) × 10
and
-
12
3
-1 -1
method, and the variational transition state theory provided the
(7.3 ( 0.9) × 10
cm molecule
s
for NO and NO2
temperature-dependent rate coefficients.41 It was shown that the
reactions, respectively.

10902 J. Phys. Chem. A, Vol. 109, No. 48, 2005
Nakayama et al.
Acknowledgment. This work was partly supported by the
(20) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson,
R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. Atmos. Chem.
Phys. 2004, 4, 1461.
Grants-in-Aid from the Ministry of Education, Sports, Culture
and Science, Japan. The research grant for Dynamics of Sun-
Earth-Life Interactive System (The 21st Century COE program
from Ministry) is also acknowledged. T.N. thanks the Japan
Society for Promotion of Science for a fellowship for a young
scientist. This work was also supported in part by the Steel
Industrial Foundation for the Advancement of Environmental
Protection Technology (Y.M.) and Espec Foundation for Earth
Environmental Research and Technologies (K.T.).
(
(
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