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Chemistry Letters Vol.38, No.1 (2009)
Near-infrared Cavity Ring-down Spectroscopic Study of the Reaction
of Methylperoxy Radical with Nitrogen Monoxide
Shinichi Enami,y Takashi Yamanaka, and Masahiro Kawasakiꢀ
Department of Molecular Engineering, Kyoto University, Kyoto 615-8510
(Received October 20, 2008; CL-081006; E-mail: kawasaki@moleng.kyoto-u.ac.jp)
Time-resolved near-infrared cavity ring-down spectroscopy
lution 0.2 cmꢂ1). After the photolysis laser pulse beam traversed
a glass tube reactor, the probe laser pulse beam was injected
nearly collinear to the axis of the photolysis laser through one
of two high-reflectivity mirrors. In the presence of an absorbing
species, the light intensity within the cavity is given by eq 4.
was applied to the kinetics of the gas-phase reaction of CH3O2
with NO at 100 Torr total pressure and 298 K. After flash photol-
ysis of the CH4/Cl2/O2/N2 mixture at 355 nm or the CH3I/O2/
N2 mixture at 266 nm, the CH3O2 absorption at 1.36 mm was
monitored for the kinetic study. The reaction rate constants de-
termined in two different radical sources are essentially the same
and in agreement with the recently proposed values.
IðtÞ ¼ I0 expðꢂt=ꢂÞ ¼ I0 expðꢂt=ꢂ0 ꢂ ꢁncLRt=LCÞ ð4Þ
where I0 and IðtÞ are the light intensities at time 0 and t, ꢂ is the
cavity ring-down time with photolysis beam, ꢂ0 is the cavity
ring-down time without photolysis laser light (typically 5 ms),
LR is the length of the reaction region (0:46 ꢃ 0:02 m), LC is
the cavity length (1.04 m), c is the velocity of light, and n and
ꢁ are the concentration and the absorption cross section of
absorbing species, respectively. By varying the delay between
the photolysis and probe laser pulses, the concentration of
CH3O2 was monitored as a function of delay time. Further ex-
perimental information is available in Supporting Information
(SI).11
Figure 1 shows a typical cavity ring-down spectrum of
CH3O2 in the 7365–7405 cmꢂ1 region measured 5 ms after
355 nm flash photolysis of a CH4/Cl2/O2/N2 mixture at
100 Torr, 298 K with 0.02 nm step. The spectral baseline was
taken in this wavenumber range without photolysis conditions.
The peak at 7376 cmꢂ1 is attributable to the 0–0 band head of
the A2A0–X2A00 transition of CH3O2.6 Cl atom was generated
by 355 nm photolysis of Cl2. Cl atom reacts with excess CH4
([CH4]0 > 104[Cl]0) to form CH3 radical, which rapidly reacts
with O2 to form CH3O2 within a few microseconds.
Peroxy radicals (RO2) are critical intermediates in atmo-
spheric and combustion chemistry, which are formed during
the low-temperature oxidation of organic compounds.1 Recently
high concentrations of RO2 up to 80 pptv in the marine boundary
layer (MBL) have been reported.2 RO2 in the atmosphere con-
verts NO into NO2, results in the formation O3, and then contrib-
utes HOx (OH and HO2 radicals) cycles:1
RO2 þ NO ! RO þ NO2
NO2 þ hꢀ ! NO þ O(3P)
O(3P) þ O2 þ M ! O3 þ M
ð1Þ
ð2Þ
ð3Þ
where M is a third body. Because of its significant roles, the re-
actions of RO2 have been widely studied.3 Typically a UV ab-
sorption centered at ꢁ240 nm that comes from the B2A00–X2A00
transition of RO2 has been used for kinetic study.3 However,
this absorption region is wide (half-width ꢁ40 nm), broad, and
unstructured. Hence, it is essentially difficult to monitor only
RO2 absorption without interferences of unwanted absorbed spe-
cies. RO2 uniquely has a structured A2A0–X2A00 transition at
much longer wavelength, typically in the near-infrared (NIR) re-
gions. Use of the NIR region absorption has great advantages for
the kinetic study because 1) it avoids complications due to the
spectral overlap in the presence of other species, and 2) there
is no effect of UV flash photolysis scattering, which occurs un-
desirably during the generation of RO2 radicals. However, it is
usually difficult to generate tunable NIR light, and the absorption
cross section ꢁ at this region is small 10ꢂ20–10ꢂ22 cm2
moleucleꢂ1. Cavity ring-down spectroscopy (CRDS)4,5 com-
bined with an optical parametric oscillator laser is a powerful ab-
sorption spectroscopic method and has an extremely long effec-
tive optical path under atmospheric relevant conditions. Miller
and co-workers are the first to monitor RO2 absorptions in
the NIR region using a combination of CRDS and a dye-laser-
pumped Raman shifter.6 Here we report a novel application to
a kinetic study of the gas-phase reaction of CH3O2 with NO
using time-resolved NIR-CRDS with two different CH3O2 gen-
eration systems under atmospheric relevant conditions.
Cl2 þ hꢀð355 nmÞ ! 2Cl
CH4 þ Cl ! CH3 þ HCl
CH3 þ O2 þ M ! CH3O2 þ M
ð5Þ
ð6Þ
ð7Þ
The observed structured absorption spectrum of CH3O2 reason-
ably agrees with the previous reported one measured in the
193 nm photolysis of acetone in the presence of O2 at 250 Torr
of Ne/O2 mixture.6
Figure 2 shows a typical decay time profile of CH3O2 in the
presence of NO. The decays were analyzed by considering 1st-
0.16
0.14
0.12
0.10
0.08
0.06
7360
7370
7380
7390
7400
7410
Wavenumber / cm-1
The CRDS apparatus used in the present study is essentially
the same as previous studies.7,8 The system employed a photol-
ysis laser (Spectra Physics, GCR-250, 266 or 355 nm) and a
probe laser (Spectra Physics, MOPO-SL, 1.36 mm, spectral reso-
Figure 1. Absorption spectrum of CH3O2 appearing after
355 nm flash photolysis of CH4/Cl2/O2/N2 mixture at 298 K
with 100 Torr total pressure. OD stands for optical density.
Copyright ꢀ 2009 The Chemical Society of Japan
Enami, Shinichi
