Mid-infrared quantum cascade laser spectroscopy probing of the kinetics of an atmospherically significant radical reaction, CH 3O 2+ NO 2+ M → CH 3O 2NO 2+ M , in the gas phase

Article abstract of DOI:10.1007/s12039-018-1451-2

Abstract: The kinetic parameters of an important atmospheric reaction, CH ₃O ₂+ NO ₂+ M → CH ₃O ₂NO ₂+ M , have been recorded by monitoring directly the changes in concentrations of methylperoxy radicals (CH ₃O ₂) in the gas phase employing a new mid-infrared quantum cascade laser (QCL)-based apparatus. CH ₃O ₂ radicals in our apparatus have been generated by pulsed UV laser (266?nm) photolysis of CH ₃I in a gaseous mixture with oxygen. The absorption band corresponding to the mid-infrared O-O stretching fundamental of the peroxy radical, within a narrow spectral range, 1070–1120?cm ^{- 1}, has been recorded by tuning the wavelength of the QCL operated in CW mode. The kinetics of the aforementioned reaction of CH ₃O ₂ with NO ₂ has been followed by analyzing the changes of the infrared (QCL) decay profile of CH ₃O ₂ at 9.1μm(1098.9cm-1) maintaining a pseudo first order reaction condition. We noticed that the rate constant of the reaction at 298?K varies in the range of (1.21–3.08) × 10 ^{- 12}cm3molecule-1s-1 for changing the total pressure in the range of 75–730?mbar. The absorption cross-section of CH ₃O ₂ at the probe wavelength (1098.9cm-1), has been estimated for the first time to be 8.3±0.4×10-20cm2. Graphical Abstract?: Kinetics measurement of an atmospherically important gas phase reaction, CH ₃O ₂+ NO ₂+ M → CH ₃O ₂NO ₂+ M , is studied using a newly developed apparatus based on pulsed UV laser photolysis time-resolved mid-infrared absorption spectroscopy. CH ₃O ₂ radicals are probed in the mid-infrared by a CW quantum cascade laser (QCL). [Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-018-1451-2

J. Chem. Sci. (2018) 130:54
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1451-2
REGULAR ARTICLE
Mid-infrared quantum cascade laser spectroscopy probing of the
kinetics of an atmospherically significant radical reaction,
CH3O2 + NO2 + M → CH3O2NO2 + M, in the gas phase
APARAJEO CHATTOPADHYAY∗ , MONOJ SAMANTA, KOUSHIK MONDAL and
TAPAS CHAKRABORTY∗
Department of Physical Chemistry, Indian Association for the Cultivation of Science, 2A Raja S C Mullick
Road, Jadavpur, Kolkata, West Bengal 700 032, India
E-mail: pcac@iacs.res.in; pctc@iacs.res.in
MS received 25 December 2017; revised 2 March 2018; accepted 22 March 2018
Abstract. The kinetic parameters of an important atmospheric reaction, CH3O2 +NO2 +M → CH3O2NO2 +
M, have been recorded by monitoring directly the changes in concentrations of methylperoxy radicals (CH3O2)
in the gas phase employing a new mid-infrared quantum cascade laser (QCL)-based apparatus. CH3O2 radicals
in our apparatus have been generated by pulsed UV laser (266 nm) photolysis of CH3I in a gaseous mixture
with oxygen. The absorption band corresponding to the mid-infrared O-O stretching fundamental of the peroxy
radical, within a narrow spectral range, 1070–1120 cm−1, has been recorded by tuning the wavelength of the
QCL operated in CW mode. The kinetics of the aforementioned reaction of CH3O2 with NO2 has been followed
by analyzing the changes of the infrared (QCL) decay profile of CH3O2 at 9.1 μm (1098.9 cm−1) maintaining
a pseudo first order reaction condition. We noticed that the rate constant of the reaction at 298 K varies in the
range of (1.21–3.08) ×10−12 cm3 molecule−1 s−1 for changing the total pressure in the range of 75–730 mbar.
The absorption cross-section of CH3O2 at the probe wavelength (1098.9 cm−1), has been estimated for the first
time to be 8.3 0.4 × 10−20 cm2.
Keywords. Quantum cascade laser; time-resolved mid-infrared spectroscopy; transient absorption; peroxy
radicals; absorption cross-section.
1. Introduction
known to be an extremely sensitive method for detection
of fluorescent radicals, but for non-fluorescent species,
viz. organic peroxy radicals, the method cannot be used
directly.7,8,13,14 Likewise, in spite of being extremely
sensitive, simple mass spectrometric technique cannot
be used in differentiating isomeric species present in
a gas mixture. Organic radicals usually have strong
absorption bands in ultraviolet, and for that reason
UV spectroscopy is being used extensively over a long
period for probing different radical species.9,17–19 How-
ever, an inherent problem of the method is that the UV
absorption bands are usually broad, and in consequence
it can have only limited use when more than one radi-
cal species are present in a gas mixture, which display
overlapping absorption bands.9,17
Investigation of the kinetics of gas phase radical reac-
tions is important in many interrelated research fields
like atmospheric chemistry, combustion chemistry,
chemistry of interstellar medium to name but a few.1–6
Detection of gas phase organic radicals is tricky because
of their transient nature with a typical lifetime of a
few microseconds to several minutes depending on
the environment and nature of the radical. Therefore,
time-resolved spectroscopic techniques are required for
probing the gas phase transient radicals. Several time-
resolved probing methods were developed over the last
few decades and the most popular methods available
in the literature are based on laser induced fluores-
cence (LIF) spectroscopy, UV/ Visible/ NIR absorption
spectroscopy, mass spectrometry, etc.7–16 Each of these
probing techniques has its own merits as well as limita-
tions. For example, laser induced fluorescence (LIF) is
In recent years, near-infrared (NIR) and visible spec-
troscopy, coupled with cavity enhanced methods, e.g.,
cavity ring down spectroscopy (CRDS) have emerged as
powerful tools for probing the kinetics of organic radical
reactions.16,20,21 Although NIR spectroscopy provides
better selectivity in comparison with the methods based
*
For correspondence

54 Page 2 of 10
J. Chem. Sci. (2018) 130:54
on UV absorption, the small NIR absorption cross- by an aircraft campaign (∼100 parts per trillion by vol-
sections of organic species is a serious disadvantage ume (pptv) at higher altitudes and ∼35 pptv at lower
in kinetics measurements at low concentrations of the altitudes).29 Thus, because of the enormous atmospheric
radical species. On the other hand, the characteristic significances, it is essential to have reliable experimental
vibrational bands of organic species, particularly in the data concerning accurate values of the rate constants of
mid-IR fingerprint region are very sharp with typically thisreactionindifferentconditions, andalsotoverifythe
more than on order of magnitude absorption cross- rate constant data employing different probing method-
section values, and the concerned species in a mixture ologies. Although kinetics of this reaction has been
can be selectively probed for measurement of reaction studied before by several methods, we report here the
kinetics.22–24 Therefore, the band selective mid-infrared measurement of the same by direct IR absorption moni-
spectroscopic method should be superior in studies of toring of CH3O2 radical for the first time.30–37 Recently,
kinetics of the radical species produced by laser photol- Lee and co-workers measured vibrational spectrum of
ysis method, because typically more than one radical various peroxy radicals in the mid-infrared by Step-
species are produced in more than one dissociation Scan FTIR method at low spectral resolutions, but no
pathways. However, in the literature, the reports on mid- kinetic data of those radicals were reported using this
infrared (MIR) probing of organic radicals for kinetics method.38–40 We also have measured for the first time the
measurements of fast radical reactions are rare, and this infrared absorption cross-section of the peroxy radical
is mainly due to non-availability of stable commercial near the band maximum of its O-O stretching funda-
lasers in this optical region.22,23 Recently, Quantum Cas- mental.
cade Lasers (QCLs) have been introduced as stable and
intense laser sources in the MIR spectral region. We
describe here a QCL-based methodology in reaction
kinetics studies of gaseous radical species. The reaction
2. Experimental
used to demonstrate the potential of the method is the
reaction (Eq. 1), where an adduct between methylperoxy
radical (CH3O2) and nitrogen dioxide (NO2) is pro-
duced via three body collision involving a background
gas molecule (M = N2, Ar, etc.).
2.1 Chemical reactor and gas handling system
A schematic of the experimental setup is depicted in Figure 1.
The reactor is a cylindrical stainless steel tube of length
105 cm and inner diameter 2.5 cm. Two ends of the tube are
closed with two CaF2 windows and sealed by silicon gaskets.
CaF2 has high transmittance with respect to UV (266 nm),
mid-IR (9–10 μm) and visible (532 nm) wavelengths, and
thus the lasers are introduced into the cell through the win-
dows. The gas inlet and outlet ports are placed at the two
opposite ends of the reactor. The distance between these two
ports along the length of the cell is ∼100 cm. A separate mix-
ing chamber of volume ∼4 L has been used to prepare the
reaction gas mixture. The gas mixture (CH3I vapour diluted
in N2/O2 mixture) is introduced into the reactor as a contin-
uous flow through the sample inlet, and drained out through
the outlet port by a root pump (Adixen ACP 15). The flow
of the gas mixture is controlled by a precision metering valve
(Swagelok) and the flow rate is monitored both at the inlet and
outlet by two mechanical flow meters (Rota Yokogawa), and
the mean of the two flow meters reading is typically ∼5−25
L/min. A high pressure gauge (Pfeiffer CMR 361) is attached
with a vacuum port located at the middle of the reactor. The
temperature within the flow reactor remains almost the same
as that of the room temperature (298 K), which has been mea-
sured using a thermocouple temperature sensor attached at
one of the terminal ports.
CH3O2 + NO2 + M → CH3O2NO2 + M
(1)
Methylperoxy radical is an important intermediate in
the oxidation of volatile organic compounds (VOCs)
in the atmosphere and also in combustion systems.25–27
From the perspective of atmospheric chemistry, it is an
important reaction intermediate, especially with respect
to the reactions that occur in polluted environments,
where NO2 concentrations are high. Recently, this
reaction (Eq. 1) has gained much attention, because,
similar to much discussed peroxyacetylnitrate (PAN),
peroxymethylnitrate (CH3O2NO2) can also have a long
lifetime, especially at upper troposphere, and thus the
species can act as a sink as well as a transporting agent of
NO2.28,29 It is important therefore to estimate the accu-
rate kinetic data of the reaction of NO2 with CH3O2 and
other RO2 species, because, atmospheric NO2 concen-
tration has a large effect on the tropospheric abundances
of O3 and OH radical. The lifetime of CH3O2NO2 is
highly sensitive to temperature, and it is less than a sec-
ond at room temperature near sea level, but it could be
as long as several days in the upper troposphere at low
temperatures.28,29 Recently, Nault et al., measured the
2.2 Optical system
2.2a Photolysis system: For generation of radicals, the
concentration of CH3O2NO2 in the upper troposphere precursor compound (CH3I in the present case), is photolyzed

J. Chem. Sci. (2018) 130:54
Page 3 of 10 54
Pellin Broca Prism
Nd:YAG
Laser
Pressure Gauge
Valve
266 nm
Gas lines
Mixing Chamber
Sample flow
Flow meter
Reference cell
FTIR Spectrometer
Pressure Gauge
BB mirror
BB mirror
with hole
Sample Inlet
UV mirror
Reaction Chamber
Pump Out
CaF2 Window
BB mirror
60 cm
105 cm
20 cm
Ge window
Tunable MIR
QCL
(1060 – 1120 cm-1)
MCT Detector
BB mirror
BB mirror
Oscilloscope
Diode Laser
532 nm
Photo diode
BB mirror
Figure 1. Schematic diagram of the pulsed laser photolysis time-resolved IR absorption kinetics measurement
apparatus. Overall length of the reactor is 105 cm, and the distance between inlet and outlet ports of reaction gas
mixture is 100 cm. Distance between UV mirror and input window of the reactor is 60 cm, while the same in the other
side is 20 cm (distances mentioned in the figure are not scaled). The BB (broadband) mirror is coated with protected
aluminum (450 nm−20 μm). UV and IR beams are aligned at ∼2◦ with respect to each other that results in 100 cm
overlap between them within the reactor.
by the 4th harmonic UV light pulses (266 nm) of a Q-switched in the effective gap of ∼1 cm between the two laser beams
Nd:YAG laser (Spectra Physics Quanta-Ray Lab Series 150). near UV mirror. The QCL has a visible beam guide of HeNe
The photolysis laser beam is introduced into the reactor laser, which is extremely helpful to monitor the alignment
through center of one of the terminal windows, and the beam for beam overlap. At the exit end, the distance between the
passes through the reactor along its central axis. The beam output window and the broad band (BB) IR mirror is nearly
shape of the photolysis laser is nearly circular with a diam- 20 cm. We have estimated that the overlap length between
eter of 8 mm. The laser is operated at 10 Hz repetition rate, the two lasers within the cell is ∼100 cm, and this has been
and typical pulse energy is ∼10 mJ.
considered as the effective reaction path length for absorp-
tion of probe wavelength by the peroxy radicals. The range of
frequency tunability of the QCL laser for this mode of opera-
tion is ∼60 cm−1 (wavelength maximum, 9.1 μm) and has a
linewidth of ∼0.003 cm−1 with a power of ∼100 mW at the
peak of the laser emission profile. The wavelength calibration
of the QCL is verified by measuring the infrared fundamen-
tal band profile of acetone vapor (at room temperature) near
1091 cm−1 and a comparison of the recorded spectrum with
the one recorded using a FTIR spectrometer is shown in
Figure 2.
2.2b Probe laser: The infrared beam from a QCL laser
(Pranalytica Inc., Model: Omnilux), which was operated in
continuous wave (CW) mode, is introduced into the reactor
in counter propagating direction with respect to the photol-
ysis laser keeping a small angle between the two beams so
that they can have overlap within the reactor to the largest
possible extent. The beam diameter of the IR laser is ∼2 mm
while the same for the UV laser is ∼8 mm and this is par-
ticularly helpful in keeping the two lasers overlapped over
a longer distance. The angle between the two laser beams
is ∼2◦, and for achieving this situation the input windows 2.2c Reactant concentration monitoring by optical
of the cell are kept far away from the folding mirrors that methods: Rate constants of bimolecular reactions are gen-
steer the beams into the reactor. The distance between the erally measured adopting pseudo first order conditions where
UV quartz mirror and starting point of the overlap between the one of the reactants is kept in excess compared to the other,
two lasers is approximately 63 cm. This large distance results and the same approach is adopted here for measuring the

54 Page 4 of 10
J. Chem. Sci. (2018) 130:54
0.4
oscilloscope. Typically, the decay signals were averaged over
50–100 laser pulses before recording. For further improve-
ment of the signal to noise ratio (S/N), the data are treated with
Savitzky-Golay smoothing algorithm.42 This same smooth-
ing algorithm was used in a previous study for measuring the
kinetics of reaction (Eq. 1).31 The output of 532 nm diode
laser is detected by a photodiode and the signal is sent to the
same oscilloscope for records. Since both the beams propa-
gate through the cell along almost the same path, the latter is
separated from the former by placing a Ge window in front of
the MCT detector, which allows the IR beam to pass through
but the wavelength 532 nm is reflected.
0.3
0.2
0.1
0.0
QCL
FTIR
2.4 Materials
1070 1080 1090 1100 1110 1120 1130 1140
Methyliodide (CH3I) was procured from Sigma Aldrich and
used without further purification. The nitrogen and oxygen
gases used were of the purity 99.99%. NO2 was prepared by
reaction of NaNO2 with FeSO4·7H2O in presence of excess
oxygen, and the gas was dried by using a column of Na2CO3
before introducing into the mixing chamber.
Wavenumber (cm-1)
Figure 2. A comparison of the spectral features of the
infrared 1091 cm−1 band of acetone vapor at room tempera-
ture recorded in transmission mode by continuous scanning
of the QCL, with the spectrum obtained using a FTIR spec-
trometer (Model: IFS66S, Bruker Optics). For the latter, the
spectrometer resolution is maintained at 0.25 cm−1. The
depicted comparison ensured the wavelength calibration of
the QCL used.
3. Results and Discussion
3.1 Measurement of the kinetics of
CH3O2 + NO2 + M → CH3O2NO2 + M reaction
rate constant of reaction in Eq. 1. For accurate measurement
of the rate constant under this condition, precise measure-
ments of the concentrations of the reactants within the reactor,
particularly for NO2 which is taken in excess is important
and the following arrangement is made for this purpose. The
reaction gas mixture is first flown through a reference cell
placed directly inside the sample compartment of a FTIR
spectrometer (Bruker IFS 66 s), and this arrangement enables
monitoring absolute concentrations of the different reactants
used. In addition, an in situ arrangement for NO2 concen-
tration monitoring has been made by introducing a CW
beam of wavelength 532 nm from a diode laser which is
aligned with the photolysis laser (Figure 1). The attenua-
3.1a Production of CH3O2 radical: In the presence
of oxygen, methyl radical (CH3) produced via pho-
tolysis of methyliodide (CH3I) by 266 nm UV laser
pulses of a Q-switched Nd:YAG laser (4th harmonic
wavelength), reacts to generate CH3O2 radical. At this
wavelength, the absorption cross-se−ct1ion of CH3I is very
large ( ∼ 1 × 10−18 cm2 molecule ), and photodisso-
σ
ciation quantum yield (ꢁ) for production of CH3 radical
is effectively unity.41,43,44 The concentration of the CH3
radicals produced is determined in the following way:
tion of 532 nm laser intensity was recorded continuously for Methyl radical concentration, [CH3]
monitoring the actual concentration of NO2 inside the main
σ
= [CH3I]x xꢁx F
(2)
σ
reactor. Absorption cross-section of NO2 at 532 nm is
=
1.50 × 10−19 cm2 molecule−1 41
.
Absolute concentrations where, [CH3I] is the concentration of the precursor
of CH3O2 radicals are evaluated from time-resolved mid-IR
absorption data. The required absorption cross-section value
of the radical at 9.1 μm is also measured for the first time in
the present study.
σ
CH3I, the absorption cross-section of CH3I at 266 nm,
ꢁ the quantum yield of photo-dissociation of CH3I at
266 nm, and F the laser fluence.
In the present experiment for kinetic measurements,
concentration of CH3I being ∼1 × 1016 − 5 × 1016
molecule cm−3 (determined from FTIR absorption data)
and laser fluence ∼2×1016 photons cm−2, the estimated
concentration of the produced CH3 radical is typically
(2−10) × 1014 cm−3. Adduct formation of CH3 radi-
cal with O2 is a termolecular reaction (Eq. 3) and the
following are the values of the rate constants:
2.3 Signal detection and data recording
The intensity of the probe IR beam after exiting the reaction
chamber is monitored by a fast pre-amplified MCT detector
(VIGO PVMI-4TE-10.6). The time-resolved absorption sig-
nalfromthedetectorisviewed, averagedandstoredinadigital
storage oscilloscope (Lecroy wavesurfer 62Xs, 600 MHz,
2.5 GS/s). A 50 ꢀ termination was used at the input of the CH3 + O2 + M → CH3O2 + M.
(3)

J. Chem. Sci. (2018) 130:54
Page 5 of 10 54
2.0
CH3O2 + CH3 → CH3O + CH3O,
Experimental decay profile
Simulated decay curve
k = 9.1 × 10−11 cm3 molecule−1 s−1
CH3O + O2 → HCHO + HO2,
(5)
(6)
1.5
1.0
0.5
0.0
k = 1.9 × 10−15cm3 molecule−1s−1
CH3O2 + HO2 → CH3OOH + O2,
k = 4.68 × 10−12cm3 molecule−1s−1
→ HCHO + H2O + O2,
(7a)
(7b)
k = 5.20 × 10−13cm3 molecule−1s−1
The production and decay of CH3O2 radicals as
obtained from simulation considering reactions 3–7 is
also presented in Figure 3 (blue trace), and this has
been done using a standard computer program called
ACUCHEM.47 The experimental CH3O2 concentration
profile shows satisfactory agreement with the modeled
one. The fate of I atoms formed in the photolysis of CH3I
is as follows. I atoms rapidly combine with CH3O2 to
form CH3O2I, which react very rapidly with I atoms to
give rise to CH3O2 and I2. As a result, there is practi-
cally no loss of CH3O2 radicals in this process.20,48,49
It is worth mentioning here that the concentrations of
the reactants used in our experiments are not too small,
and when the reaction with these concentrations is per-
formed in a static cell, we observe accumulation of
molecular iodine (I2) on the walls of the reactor. How-
ever, in the flowing reactor, with the flow rates that are
typically used, we never noticed I2 accumulation.
0.000
0.001
0.002
0.003
0.004
0.005
Time (s)
Figure 3. A typical decay signal of CH3O2 radicals
(in the absence of NO2) for probing at 9.1 μm.
[CH3I] = 4.9 × 1016 cm−3, [O2] = ∼9.0 × 1017 cm−3 and
total pressure of the reaction cell was 520 mbar. The decay
curve obtained from modeling (blue trace) is also presented
for a comparison with the experimental signal.
Here, k0 = 7.0 × 10−31 [N2] cm3 molecule−1 s−1 and
k
∞
= 1.8 × 10−12 cm3molecule−1 s−1. k0 and k are
∞
low and high pressure rate constants, respectively, at
298 K.43,44 Almost all of CH3 radicals produced by such
means is converted into the peroxy radicals, CH3O2,
within a few microseconds. Thus, at 520 mbar total
pressure and in the absence of NO2, it has been esti-
mated from a reaction modeling study (described later)
that the number of CH3 radicals produced on average
from each laser shot, 5.0 × 1014 cm−3, is converted into
CH3O2 with a number density of 4.7×1014 cm−3 within
10 μs. Depicted in Figure 3 is a typical decaying signal
of CH3O2 radicals (in absence of NO2) probed at 9.1 μm
for a total pressure of 520 mbar. The detection limit of
the methylperoxy radical in the current experimental
setup is ∼2 × 1014 cm−3(S/N = 2). It should be noted
that in Figure 3, the concentration of CH3O2 radical pro-
duced is much higher. However, for kinetic experiments,
lower CH3O2 concentrations have been used.
3.2 Mid-IR absorption spectrum and absorption
cross-section of CH3O2 radical
The mid-infrared spectrum of CH3O2 radical has been
measured previously by Huang et al., using a step-scan
FTIR method at a low spectral resolution of 4 cm−1.38
Using the QCL we have recorded the O-O stretching
absorption profile of the radical within 1070–1120 cm−1
range with a laser resolution of 100 MHz by the follow-
ing approach. Time-resolved absorption signals were
recorded at discrete wavelengths (maintaining a step
size of 1.5 cm−1) operating the laser in CW mode.
Absorbance value at each wavelength is measured using
the standard relation, Absorbance (O.D.) = ln (I0/I),
where I0 is the signal level before introducing photolysis
laser and I, the signal after ∼10 μs of firing of the photol-
ysis laser. The spectrum, as shown in Figure 4, has been
developed by line joining the discrete absorbance values
as a function of laser frequency, and within the depicted
spectral range it looks nearly similar to the one reported
As reported in previous studies, CH3O2 radicals could
decay predominantly via the following self reaction
pathways:16,20,43–45
CH3O2 + CH3O2 → CH3OH + HCHO + O2,
k = 2.2 × 10−13cm3 molecule−1 s−1
→ CH3O + CH3O + O2,
(4a)
(4b)
k = 1.3 × 10−13cm3 molecule−1 s−1
The following reactions also contribute to the decay of by Huang et al.38 The segment of the band profile within
CH3O2 radicals.43,44,46
1070−1120 cm−1 range was assigned to a vibrational

54 Page 6 of 10
J. Chem. Sci. (2018) 130:54
Pushkarsky et al., and Atkinson et al.16,45 In the present
study, as shown in Figure 3, a simulation of the decay
profile (considering reactions, Eqs. 3–7) of CH3O2 rad-
icals is carried out. During the construction of the decay
profile, the initial CH3O2 concentration is iterated so as
to match the simulated decay profile well with the exper-
imental plot as depicted in Figure 3. This initial CH3O2
concentration (1.8 × 1015 molecule cm−3) is then used
to estimate the absorption cross-section. Using this con-
centration data, the absorption cross-section of CH3O2
radical at 9.1 μm (1098.9 cm−1) has been estimated
to be 8.3 × 10−20 cm2. Another source of uncertainty
0.006
0.004
0.002
0.000
1070
σ
to the value of may come from the consideration
1080
1090
1100
1110
1120
of the effective optical path length as 100 cm which
may typically vary within 5%. Considering this uncer-
Wavenumber (cm-1)
Figure 4. The infrared absorption spectrum of CH3O2 rad-
ical in the depicted spectral range permitted by the tunability
of QCL. The detailed method used to record the spectrum has
been discussed in the text.
σ
tainty, the value of absorption cross-section is taken
as 8.3 0.4 × 10−20 cm2.
3.3 Kinetics of CH3O2 + NO2 reaction
fundamental corresponding to O-O stretch with a con- InpresenceofNO2, CH3O2 reactswithNO2 (Eq. 1)apart
tribution from CH3 rocking, and this band was shown to from the self reactions. The kinetics experiments were
have a maximum at 1117 cm−1. The same peak is also performed under pseudo-first order condition where
present in the spectrum shown in Figure 4, although the [NO2] > [CH3O2].
power of QCL is low at this wavelength. There is another
Before presenting our results, first we review here
peak at ∼1094 cm−1, and although no assignment for the studies performed earlier. In the decade of 1980 s,
it was discussed in reference 38, on the basis of the- four different research groups carried out measurements
oretical prediction it can be assigned to CH3 wagging independently by probing CH3O2 radical at an ultra-
fundamental of the radical38. In measuring the decay violet wavelength near 250 nm.30–33 Using the same
kinetics, the radical has been probed at the maximum of method, Bridier et al., studied the reaction in the tem-
the laser emission wavelength, 9.1 μm (1098.9 cm−1), perature range of 333–373 K at 730 Torr pressure, and
and it is seen that the radical has a considerably large Wallington et al., measured the rate constant values
absorbance at this wavelength, which results in a good in the pressure range of 0.5–14 atm at 295 K.34,35 As
signal-to-noise ratio.
pointed out before, although the UV absorption cross-
σ
The absorption cross-section ( ) at the probe wave- section is higher, but UV absorption band is very broad
length has been evaluated in the following manner. The andthatlimitsselectivity. Later, Bacaketal., usedchem-
σ
σ
cross-section ( ) is defined as, = Absorbance/(n l), ical ionization mass spectrometry as a probe to study the
n denotes the concentration of CH3O2 radical and l, reaction at low temperatures.36 Very recently, McKee et
the optical path length. Here, absorbance is determined al., studied the kinetics of the reaction as well as equi-
from the time-resolved absorption signal in the way librium parameters for the process CH3O2 + NO2 ↔
mentioned before. Optical path length l is 100 cm. Pre- CH3O2NO2 by probing CH3O2 radicals indirectly.37
cise measurement of n, i.e., the concentration of CH3O2 CH3O2 was photolyzed to produce CH3O and OH radi-
radical is important for accurate measurement of the cals, and these secondary radicals were probed by laser
σ
cross-section ( ). A course measurement of the con- induced fluorescence spectroscopic method.
centration of CH3O2 radical, i.e., n, is evaluated from Figure 5 represents a typical decay profile for IR
laser fluence and CH3I concentration following the pro- absorption of CH3O2 radical at the probing wavelength
cedure as mentioned previously. For fine measurement (9.1 μm) in presence of NO2 under the condition of a
of the initial CH3O2 concentration, following approach fast flow of the reaction gas mixture within the reactor.
has been taken. Recently, Farago et al., measured the An important merit of the method, in spite of smaller
absorption cross-section of CH3O2radical at NIR region infrared absorption cross-section of the radical at this
using a reaction model to accurately quantify the ini- wavelength in comparison with the previously used UV
tial radical concentration and its time evolution.20 The probing wavelength, is that in the absence of photolysis
same approach has also been adopted previously by laser the background absorption of the gas mixture is

J. Chem. Sci. (2018) 130:54
Page 7 of 10 54
8
30000
25000
20000
15000
10000
5000
0
Experimental decay profile
Exponential fit
6
4
2
0
0.0000
0.0005
0.0010
0.0015
0
2
4
6
8
10
[NO2] (1015 x molecule cm -3
)
Time (s)
Figure 5. DecayingIRabsorptionsignalofCH3O2 radicals
in presence of NO2 at 75 mbar total pressure. The initial con-
centration of CH3O2 radical and NO2 are 7.30 × 1014 cm−3
and 5.30 × 1015 cm−3, respectively. It is seen that the decay
can be nicely fitted with a single exponential decay curve.
Figure 6. Plotdepictingthechangesoftheestimatedvalues
of the decay constant (A) for different concentrations of NO2
(total pressure 305 mbar).
A = k1 + Ad = k[NO2] + Ad
(8)
zero and the adduct formed CH3O2NO2 also does not
absorb at this wavelength.50
where, A = decay constant, k1 = pseudo first order
rate constant, k = bimolecular rate constant, [NO2] =
concentration of NO2 (molecule cm−3), Ad = decay
constant of CH3O2 in the absence of NO2. The slope
of the plot of A vs [NO2] gives the bimolecular rate
constant for the reaction. A representative plot of the
decay constant (A) vs concentration of NO2 is presented
in Figure 6. The experiment has been performed in N2
buffer gas with a total pressure of 305 mbar. The slope
of the linear plot, i.e. the rate constant of the reaction
It is notable that for maintaining pseudo first order
reaction conditions the concentration ratio, [NO2]:
[CH3O2], has been varied within the range of ∼8−40.
Although this ratio is relatively small, but the single
exponential behavior of the plot in Figure 5 indicates
that the pseudo first order condition is indeed achieved
around this value of the concentration ratio. At still
higher concentrations of NO2we have noticed occur-
rence of dimer (N2O4) formation (N2O4 was probed at
750 cm−1 by FTIR). Since the reaction rate of N2O4 with
CH3O2is not known, to avoid possible source of errors
for such dimer formation, NO2 concentration was kept
below ∼1 × 1016 cm−3, and no prominent sign of dimer
formation was observed at this concentration of NO2.
We would like to point out here that in a number of pre-
vious studies, pseudo first order condition was invoked
upon use of the same concentration ratio between NO2
andCH3O2.30,31 Inaddition, thelinearplotofvariationof
the decay constant with [NO2] in the reactor as depicted
in Figure 6 (discussed below) is a direct support in favor
of pseudo 1st order behavior of the reaction. The rate
constant of the reaction (Eq. 1) has been measured in
the following way.
is k = 2.45
0.47 × 10−12 cm3 molecule−1 s−1 (error
σ
limit: 2 ).
It is worth mentioning about a possible error source
concerning depletion in concentration of NO2 as a result
of UV photolysis. The UV absorption cross-section of
NO2 at 266 nm is typically low, ∼2.2 × 10−20 cm2,
and although the quantum yield for photodissociation
of NO2 at this wavelength is not known, but for the sake
of discussion we have considered it to be 1. For NO2
concentration of 5.30×1015 cm−3 (Figure 5), the extent
of its loss for each laser pulse has been calculated (fol-
lowing Eq. 2) and the value is 4.10 × 1012 cm−3, which
is quite low. Moreover, the actual concentration of NO2
inside the reactor during the laser photolysis is probed
by a 532 nm CW diode laser, and the change of its sig-
nal in the presence and absence of the photolysis laser
is also not significant.
τ
The associated time constant ( ) for decay of the indi-
vidual CH3O2 absorption trace is measured by fitting the
decay curve with an exponential function as stated ear-
lier. The decay rate constant (A) is the reciprocal of the
3.4 Effect of total pressure on rate constant
τ
corresponding time constant (A = 1/ ). The reaction
rate constant (k) is calculated from decay constant A The reaction was studied at different total pressures
using the following relation:
within the range of 75–730 mbar at 298 K by varying

54 Page 8 of 10
J. Chem. Sci. (2018) 130:54
Table 1. Bimolecular rate constant values estimated as a 4. Conclusions
function of total pressure using N2 as the buffer gas (error is
σ
2 ).
We have demonstrated here that a quantum cascade
laser-based time-resolved mid-infrared absorption spec-
troscopic method can be used as an effective means for
measuring the kinetic parameters of radical reactions
in the gas phase. The performance of the apparatus
has been demonstrated measuring the reaction kinet-
ics of a tropospherically important reaction between
CH3O2 radical and NO2. Our estimated rate constant of
the reaction for the range of total pressure between 75
and 730 mbar varies between (1.21−3.08) × 10−12 cm3
molecule−1 s−1 which agree quite satisfactorily with the
previous reports. The major merit of the method, in com-
parison to much used UV probing of the radicals, is
selectivity with respect to specific reactant or product of
the reaction, and this is possible due to narrower spectral
widths of the infrared bands in comparison with typi-
cally broad UV spectral bands. On the other hand, larger
absorption cross-sections of the UV bands of the radi-
cals in comparison with mid-infrared bands, and also
easy availability of intense and narrow bandwidth UV
light source are the primary factors for the popularity
of the UV-based spectroscopic methods. We would also
like to make a remark that for having only a single pass
of optical path (path length ∼100 cm), the detection
sensitivity of this apparatus appears smaller compared
to other much used methods, e.g. cavity ring down spec-
troscopy (CRDS), and partly also due to relatively lower
absorption cross-sections of radicals in mid-IR spectral
range. For methylperoxy radical, the detection limit of
our apparatus has been shown to be ∼2 × 1014 cm−3.
On the other hand, typical optical path length for cav-
ity ring down spectroscopy (CRDS) is ∼1 km, which
results in detection limit of ∼1011−1012 cm−3. The
laser induced fluorescence (LIF) or mass spectromet-
ric techniques also have similar detection limit. In near
future, we plan to replace the linear reactor by a Herriott
type multi-pass absorption cell, which will enable us in
detecting the peroxy radicals with concentration as low
as ∼5×1012 cm−3. Furthermore, we intend to investigate
the reactions of NO and NO2 with other atmospherically
important peroxy radicals.
Pressure (mbar)
Bimolecular rate constant k
(10−12cm3 molecule−1 s−1
)
730
545
305
255
75
3.08 0.36
2.75 0.30
2.45 0.47
2.19 0.39
1.21 0.27
Pressure (mbar)
200
400
600
800
1000
6
Present Study
Blitz (2015)
Bacak (2005)
5
4
3
2
1
0
Sander (1980)
Ravishankara (1980)
0
5
10
15
20
25
[N ] (1018 x molecule cm-3)
2
Figure 7. Plot depicting the variation of the rate constant
for reaction between NO2 and CH3O2 due to change in N2
buffer gas pressure. The observed results [ꢀ] are compared
with similar studies performed earlier.
the partial pressure of N2 buffer gas, and the rate con-
stant values estimated are shown in Figure 7 as well as
in Table 1. As expected for an adduct formation reac-
tion, the rate constant decreases steadily with decrease
in total pressure.
In Figure 7, a comparison of the changes of the esti-
mated values of the rate constants with total pressure in
the present study with those reported by other work-
ers by use of different methods is shown. It is seen
that the agreements are very much satisfactory in lower
pressure range. At higher pressures, although the rate
constant values estimated here by direct infrared prob-
ing of CH3O2 radical is a little smaller compared to the
UV absorption probing, the values fall within the error
range.
Acknowledgements
The authors sincerely acknowledge the financial support
received from the Department of Science and Technol-
ogy, Govt. of India, under the Scheme Number SB/S1/PC-
027/2013. AC thanks CSIR, Govt. of India and IACS for the
Senior Research Fellowship, MS and KM thank UGC, Govt.
of India for Junior Research Fellowships.

J. Chem. Sci. (2018) 130:54
Page 9 of 10 54
References
17. Wallington T J, Dagaut P and Kurylo M J 1992 UV
absorptioncrosssectionsandreactionkineticsandmech-
anisms for peroxy radicals in the gas phase Chem. Rev.
92 667
1. Cox R A 2003 Chemical Kinetics and Atmospheric
Chemistry:RoleofDataEvaluationChem. Rev. 1034533
2. Jacob D J 1999 Introduction to Atmospheric Chemistry 18. Krasnoperov L N and Mehta K 1999 Kinetic Study of
(Princeton University Press: Princeton) p. 157
3. Orlando J J and Tyndall G S 2012 Laboratory studies
of organic peroxy radical chemistry: an overview with
CH3 + HBr and CH3 + Br Reactions by Laser Photolysis
– Transient Absorption over 1-100 Bar Pressure Range
J. Phys. Chem. A 103 8008
emphasis on recent issues of atmospheric significance 19. Krasnoperov L N, Chesnokov E N, Stark H and Ravis-
Chem. Soc. Rev. 41 6294
hankara A R 2004 Unimolecular dissociation of formyl
radical, HCO → H+CO, studied over 1 – 100 bar pres-
sure range J. Phys. Chem. A 108 11526
4. Miller J A, Kee R J and Westbrook C K 1990 Chemi-
cal kinetics and combustion modeling Annu. Rev. Phys.
Chem. 41 345
5. Westbrook C K 2000 Chemical kinetics of hydrocarbon
ignition in practical combustion systems Proc. Combust.
Inst. 28 1563
20. Faragó E P, Viskolcz B, Schoemaecker C and Fittschen
C 2013 Absorption Spectrum and Absolute Absorption
Cross Sections of CH3O2 Radicals and CH3I Molecules
in the Wavelength Range 7473 − 7497 cm−1 J. Phys.
Chem. A 117 12802
6. Smith I W M and Rowe B R 2000 Reaction Kinetics at
Very Low Temperatures: Laboratory Studies and Inter- 21. Rajakumar B, Gericzak T, Flad J E, Ravishankara A R
stellar Chemistry Acc. Chem. Res. 33 261
7. Miller T A 2006 Spectroscopic probing and diagnostic
and Burkholder J B 2007 Visible Absorption Spectrum
of the CH3CO Radical J. Phys. Chem. A 111 8950
of the geometric structure of the alkoxy and alkyl peroxy 22. Miyano S and Tonokura K 2011 Measurements of
radical intermediates Mol. Phys. 104 2581
8. DengW, WangC, KatzDR, GawinskiGR, DavisAJand
Dibble T S 2000 Direct kinetic studies of the reactions
nitrogen-broadening coefficients in the υ3 band of the
hydroperoxyl radical using a continuous wave quantum
cascade laser J. Mol. Spectrosc. 265 47
of 2-butoxy radicals with NO and O2 Chem. Phys. Lett. 23. Sakamoto Y and Tonokura K 2012 Measurements of the
330 541
Absorption Line Strength of Hydroperoxyl Radical in
the υ3 Band using a Continuous Wave Quantum Cascade
Laser J. Phys. Chem. A 116 215
9. Lightfoot P D, Cox R A, Crowley J N, Destriau M, Hay-
man G D, Jenkin M E, Moortgat G K and Zabel F 1992
Organic peroxy radicals: Kinetics, spectroscopy and tro- 24. Mah D A, Cabrera J, Nation H, Ramos M, Sharma S and
pospheric chemistry Atmos. Environ. 26 1805
10. Krasnoperov L N, Chesnokov E N, Stark H and Ravis-
hankaraAR2005Elementaryreactionsofformyl(HCO)
Nickolaisen S L 2003 Mid-infrared Spectrum of the Gas
Phase Ethyl Peroxy Radical: C2H5OO J. Phys. Chem. A
107 4354
radical studies by laser photolysis-transient absorption 25. Salisbury G, Rickard A R, Monks P S, Allan B
spectroscopy Proc. Combust. Inst. 30 935
11. Raventós-Duran M T, Mcgillen M, Percival C J, Hamer
P D and Shallcross D E 2007 Kinetics of the CH3O2 +
HO2 reaction: A temperature and pressure dependence
study using chemical ionization mass spectrometry Int.
J. Chem. Kinet. 39 571
J, Bauguitte S, Penkett S A, Carslaw N, Lewis A
C, Creasey D J, Heard D E, Jacobs P J and Lee
J D 2001 Production of peroxy radicals at night
via reactions of ozone and the nitrate radical in the
marine boundary layer J. Geophys. Res. [Atmos]. 106
12669
12. Elg A -P, Andersson M and Rosen A 1997 REMPI as 26. Tyndall G S, Cox R A, Granier C, Lesclaux R, Moortgat
a tool for studies of OH radicals in catalytic reactions
Appl. Phys. B 64 573
13. Rajakumar B, Portmann R W, Burkholder J B and
G K, Pilling M J Ravishankara A R and Wallington T
J 2001 Atmospheric chemistry of small organic peroxy
radicals J. Geophys. Res. [Atmos]. 106 12157
Ravishankara A R 2006 Rate Coefficients for the 27. Wang S, Miller D L, Cernansky N P, Curran H J, Pitz
Reactions of OH with CF3CH2CH3 (HFC-263fb),
CF3CHFCH2F (HFC-245eb), and CHF2CHFCHF2
(HFC-245ea) between 238 and 375 K J. Phys. Chem.
A 110 6724
W J and Westbrook C K 1999 A flow reactor study of
neopentane oxidation at 8 atmospheres: experiments and
modeling Combust. Flame 118 415
28. Murphy J G, Thornton J A, Wooldridge P J, Day D A,
Rosen R S, Cantrell C, Shetter R E, Lefer B and Cohen
R C 2004 Measurements of the sum of HO2NO2 and
CH3O2NO2 in the remote troposphere Atmos. Chem.
Phys. 4 377
14. Chai J, Hu H, Dibble T S, Tyndall G S and Orlando
J J 2014 Rate Constants and Kinetic Isotope Effects for
Methoxy Radical Reacting with NO2 and O2 J. Phys.
Chem. A 118 3552
15. Osborn D L, Zou P, Johnsen H, Hayden C C, Taatjes C 29. Nault B A, Garland C, Pusede S E, Wooldridge P J, Ull-
A, Knyazev V D, North S W, Peterka D S, Ahmed M
and Leone S R 2008 The multiplexed chemical kinetic
photoionization mass spectrometer: A new approach to
mann K, Hall S R and Cohen R C 2015 Measurements of
CH3O2NO2 in the upper trosphere Atmos. Meas. Tech.
8 987
isomer-resolved chemical kinetics Rev. Sci. Instrum. 79 30. Ravishankara A R, Eisele F L and Wine P H 1980 Pulsed
104103
laser photolysis-long path laser absorption kinetics study
of the reaction of methylperoxy radicals with NO2 J.
Chem. Phys. 73 3743
16. Atkinson D B and Spillman J L 2002 Alkyl Peroxy
Radical Kinetics Measured Using Near-infrared CW-
CavityRing Down Spectrometer J. Phys. Chem. A 106 31. Sander S P and Watson R T 1980 Kinetics studies of
8891 the reactions of methylperoxy with nitric oxide, nitrogen

54 Page 10 of 10
J. Chem. Sci. (2018) 130:54
dioxide, and methyldioxy at 298 K J. Phys. Chem. 84 43. Atkinson R, Baulch D L, Cox R A, Crowley J N, Hamp-
1664
son R F, Hynes R G, Jenkin M E, Rossi M J and Troe
J 2006 Evaluated kinetic and photochemical data for
atmospheric chemistry: Volume II-gas phase reactions
of organic species Atmos. Chem. Phys. 6 3625. IUPAC
Subcommittee for Gas Kinetic Data Evaluation, (http://
iupac.pole-ether.fr) (Accessed on 1 March 2018)
44. Atkinson R, Baulch D L, Cox R A, Crowley J N, Hamp-
son R F, Hynes R G, Jenkin M E, Rossi M J and Troe
J 2004 Evaluated kinetic and photochemical data for
atmospheric chemistry: Volume I-gas phase reactions of
Ox , HOx , NOx and SOx species Atmos. Chem. Phys.
4 1461. IUPAC Subcommittee for Gas Kinetic Data
Evaluation, (http://iupac.pole-ether.fr) (Accessed on
1 March 2018)
32. Adachi H and Basco N 1980 The reaction of CH3O2
radicals with NO2 Int. J. Chem. Kinet. XII 1
33. Cox R A and Tyndall G S 1980 Rate Constants for
the reactions of CH3O2 with HO2, NO and NO2 using
Molecular Modulation Spectrometry J.C.S. Faraday II
76 153
34. Bridier I, Lesclaux R and Veyret B 1992 Flash potoly-
sis kinetic study of the equilibrium CH3O2 + NO2
=
CH3O2NO2 Chem. Phys. Lett. 191 259
35. Wallington T J, Nielsen O J and Sehested K Kinetics of
the reaction of CH3O2 radicals with NO2 1999 Chem.
Phys. Lett. 313 456
36. Bacak A, Bardwell M W, Ravento’s-Duran M T, Perci-
val C J, Hamer P D and Shallcross D E 2006 Kinetics of 45. Pushkarsky M B, Zalyubovsky S J and Miller T A 2000
the CH3O2 + NO2 reaction: A temperature and pressure
dependence study using chemical ionistion mass spec-
trometry Chem. Phys. Lett. 419 125
Detection and characterization of alkyl peroxy radicals
using cavity ring down spectroscopy J. Chem. Phys. 112
10695
37. McKee K, Blitz M A and Pilling M J 2016 Temperature 46. Keiffer M, Miscampbell A J and Pilling M J 1988 A
and Pressure Studies of The Reactions of CH3O2, HO2
and 1,2-C4H9O2 with NO2 J. Phys. Chem. A 120 1408
38. Huang D-R, Chu L-K and Lee Y-P 2007 Infrared
global technique for analyzing multiple decay curves.
Application to the CH3 + O2 system J. Chem. Soc. Fara-
day Trans. 84 505
absorption of gaseous CH3OO detected with a step- 47. Braun W, Herron J T and Kahaner D K 1988 Acuchem:
scan Fourier-transform spectrometer J. Chem. Phys. 127
234318
A computer program for modeling complex chemical
reaction systems Int. J. Chem. Kinet. 20 51
39. Chu L-K and Lee Y-P 2009 Infrared absorption of 48. Jenkin M E and Cox R A 1991 Kinetics of reactions
gaseousc-ClCOOHandt-ClCOOHrecordedwithastep-
scan Fourier-transform spectrometer J. Chem. Phys. 130
174304
of CH3O2 and HOCH2CH2O2 radicals produced by the
photolysis of iodomethane and 2-iodoethanol J. Phys.
Chem. 95 3229
40. Chu L-K and Lee Y-P 2010 Transient infrared spectra of 49. Dillon T J, Tucceri M E and Crowley J N 2006 Laser
CH3SOO and CH3SO observed with a step-scan Fourier-
transform spectrometer J. Chem. Phys. 133 184303
41. Keller-Rudek H, Moortgat G K, Sander R and Sörensen
induced fluorescence studies of iodine oxide chemistry
part II. The reactions of IO with CH3O2, CF3O2 and O3.
Phys. Chem. Chem. Phys. 8 5185
R 2013 The MPI-Mainz UV/VIS Spectral atlas of 50. Niki H, Maker P D, Savage C M and Breitenbach L P
gaseous molecules of atmospheric interest Earth Syst.
Sci. Data 5 365
42. Savitsky A and Golay M J E 1964 Smoothing and differ-
entiation of data by simplified least squares procedures
Anal. Chem. 36 1627
1978 FTIR spectroscopic observation of peroxymethyl
nitrate formed via ROO + NO2 → ROONO2 Chem.
Phys. Lett. 55 289