2512
J. Phys. Chem. A 2002, 106, 2512-2519
CH3O Yield in the CH3 + O3 Reaction Using the LP/LIF Technique at Room Temperature Jose´ Albaladejo,* Elena Jime´ nez, Alberto Notario, Beatriz Caban˜ as, and Ernesto Martı´nez Departamento de Quı´mica Fı´sica, Facultad de Ciencias Quı´micas, UniVersidad de Castilla-La Mancha, Campus UniVersitario s/n, 13071 Ciudad Real, Spain ReceiVed: June 14, 2001; In Final Form: October 17, 2001
CH3O(X2E) radicals have been detected by laser-induced fluorescence (LIF) for the first time as a product of the reaction between CH3 and O3. CH3 radicals were generated by excimer-laser photolysis of CH3Br at 193 nm in the presence of O3 at room temperature and 100 Torr of He. The rate constant for reaction 1a (CH3 + O3 f CH3O + O2) has been determined by monitoring the CH3O build-up as a function of reaction time and k1a was found to be (9.68 ( 1.10) × 10-14 cm3 molecule-1 s-1. The rate coefficient of the global reaction 1 (CH3 + O3 f products) has been determined by the numerical analysis of the CH3O temporal profiles, yielding a value of k1 ) (2.2 ( 0.3) × 10-12 cm3 molecule-1 s-1. Reaction of CH3 with NO2 has also been studied under the same experimental conditions as reaction 1 and has been used to calibrate the CH3O LIF signal. On the other hand, the rate constant k2 for reaction 2 (CH3O + O3 f products) has been directly determined (k2(T ) 298 K) ) (2.53 ( 0.75) × 10-14 cm3 molecule-1 s-1) using the CH3ONO(λ ) 193 nm)/O3/He system and was included in the chemical model used to describe the kinetics of CH3 in the presence of O3. The branching ratio φCH3O ) k1a/k1 obtained was found to be (0.044 ( 0.013). This low yield of CH3O in reaction 1a can be explained not only by the formation of highly vibrationally excited CH3O and the subsequent prompt dissociation to produce CH2O + H but also by the direct formation of CH2O + HO2 in reaction 1.
1. Introduction Despite the relevance of methyl radical, CH3, in atmospheric chemistry, since this radical is produced in the earth’s atmosphere by CH4 oxidation, the nature of the products of the reaction of CH3 with ozone, O3, k1
CH3 + O3 98 products
(1)
is still a matter of speculation. The kinetics of reaction 1 has been the subject of several direct1,2 and indirect studies.3 Simonaitis and Heicklen3 performed a kinetic study of reaction 1 by measuring the quantum yield of O3 removal as a function of the O3/O2 ratio at +25 and -52 °C and at a pressure of about 1 atm. These authors photolyzed O3 at λ ) 253.7 nm in the presence of CH4 and O2. The chemical complexity of that system and the treatment of the proposed mechanism gave only an estimation of the rate constant of the above reaction. On the other hand, an upper limit for the rate constant of the reaction of CH3O radical with O3 in the same chemical system (O3(λ ) 253.7 nm)/CH4/O2) k2
CH3O + O3 98 products
(2)
was determined indirectly by Simonaitis and Heicklen.3 More recently, Washida et al.1 carried out a room temperature study of the CH3 + O3 reaction at low pressures (pT ) 2-6 Torr of He), using a discharge-flow system coupled to a mass spectrometry detection system. The reaction of O atoms with C2H4 used to generate methyl radicals is thought to be more * Phone: +34 9 26 29 53 00. Fax: +34 9 26 29 53 18. E-mail:
[email protected].
complicated than was initially assumed. Finally, the last kinetic study of the CH3 + O3 reaction was also performed using low total pressures (pT ) 2.2 ( 0.2 Torr in He) by Ogryzlo et al.2 as a function of temperature (T ) 243-384 K). Laser photolysis of nitromethane, CH3NO2, at 193 nm was employed as a source of CH3 radicals, and photoionization mass spectrometry was used to follow the CH3 decay rate at various ozone concentrations.2 Information regarding the products was not given in the above study, and the room temperature rate constant reported is approximately three times greater that those obtained in the two previous studies. In the work described here, kinetic studies of CH3 and CH3O radicals with O3 were carried out at room temperature and with a total pressure of 100 Torr in He. The experiments involved laser-pulsed photolysis (LP) of a suitable precursor to generate the transient species and laser-induced fluorescence (LIF) to monitor CH3O(X2E) as a function of reaction time. A numerical simulation of the CH3O concentration profiles was performed in order to derive k1. Furthermore, we report the first direct determination of the rate constant of the reaction CH3O + O3 (k2) as well as the CH3O yield in reaction 1 for the reaction channel, k1a
CH3 + O3 98 CH3O + O2
(1a)
obtained by fitting the rate data to the simulated profiles and varying only the absolute rate constant for this route, k1a, under the same experimental conditions. Determinations of the rate constant k1a and the branching ratio of CH3O(X2E) formation in reaction 1, k1a/k1, are achieved by comparing the time dependence of the CH3O LIF signal with that recorded, under the same experimental conditions, in the reaction of CH3 with NO2, to calibrate the CH3O LIF signal.
10.1021/jp012249o CCC: $22.00 © 2002 American Chemical Society Published on Web 02/13/2002
CH3O Yield in the CH3 + O3 Reaction
J. Phys. Chem. A, Vol. 106, No. 11, 2002 2513
The reaction of CH3 with NO2 is known to proceed via two channels, a pressure independent channel,
CH3 + NO2 f CH3O + NO
(3a)
and a pressure-dependent channel:
CH3 + NO2 + He f products + He
due to reaction 1a was also monitored by LIF. The same experimental conditions and concentrations were used in the study of the calibration process CH3 + NO2. The diffusion of CH3 and CH3O radicals out of the detection zone constitutes only a minor channel at the experimental pressure used. However, this possibility was also considered:
(3b)
CH3O radicals formed in reaction 3a react rapidly with NO2 present in the reaction cell via the following processes:
CH3O + NO2 f CH2O + HONO
(4a)
CH3O + NO2 + He f products + He
(4b)
For this reason, kinetic data for both reactions 3 and 4 are required to analyze CH3O temporal profiles obtained for the reference reaction. Analysis of the CH3O temporal profiles yields the rate constant for the recombination reaction 3b as a function of total pressure (pT ) 20-311 Torr in He) at room temperature. 2. Experimental Section All experiments were carried out using pulsed laser photolysis in combination with a pulsed laser-induced fluorescence technique (LP-LIF). Details of the experimental setup were previously described in recent publications regarding CH3S4,5 and CH3O6 radicals. A brief description of the experimental system and conditions is given below. Photolysis of an appropriate precursor (commercial CH3Br or synthesized CH3ONO) using the pulsed 193-nm radiation of an ArF-excimer laser (OPTex, Lambda Physik), with fluences in the range 0.4-3 mJ pulse-1 cm-2, was used as a source of CH3 and CH3O radicals, respectively:
CH3Br + hν(λ ) 193 nm) f CH3 + Br
(5)
CH3ONO + hν(λ ) 193 nm) f CH3O+ NO
(6)
Because the CH3 radical cannot be detected by LIF, the conversion of this radical into CH3O in the presence of NO2 or O3 was used to monitor its kinetics in the reactions described here. A Nd:YAG-pumped frequency-doubled dye laser (Continuum ND60 and NY81CS-10) was used to monitor the CH3O concentration decay or build up, generated either directly or indirectly, by exciting CH3O radicals at λ ) 292.4 nm:7
CH3O(X2E) + hν (λ ) 292.4 nm) f CH3O(A2A1) (7) The resulting red-shifted fluorescence was subsequently collected at a right angle with respect to the excitation and photolysis beams in a photomultiplier tube (Thorn EMI, 9813B) coupled to the Pyrex reaction cell. A band-pass filter (Schott, BG5) or an interference filter centered at λ ) 360 nm (Andover) was used to monitor the CH3O LIF signal. The kinetic studies of CH3 and CH3O radicals were carried out at room temperature under a total pressure of 100 Torr, and always under pseudo-first-order conditions with respect to the initial concentration of the radical generated in the photolysis. In the study of CH3O formation by reaction 1a, the photolysis of CH3Br ([CH3Br] ) (1.9-9.8) × 1014 molecule cm-3) generated CH3 radicals ([CH3]0 ) (0.7 -3.1) × 1012 radical cm-3) in the presence of an excess of O3 ([O3] ) (0.5-5.8) × 1014 molecule cm-3). The absorption cross section of CH3Br at 193 nm used8 was 5.75 × 10-19 cm2, and the quantum yield for CH3 formation was taken to be unity.9 The CH3O buildup
CH3 f diffusion
(8)
CH3O f diffusion
(9)
In the kinetic study of the reaction CH3O + O3, the typical concentrations of the photochemical precursor used were [CH3ONO] ) (0.35-2.14) × 1015 molecule cm-3, giving [CH3O]0 ) (1.3-9.8) × 1011 radical cm-3 (σCH3ONO(λ ) 193 nm) ∼ 2 × 10-18 cm2).10 The ozone concentrations employed in these experiments were in the range (0.11-4.47) × 1015 molecule cm-3. Photolysis of O3 present in the system at 193 nm (σO3(λ ) 193 nm) ) 4.28 × 10-19 cm2)11 generates ground- and excitedstate O atoms, O3P and O1D, with quantum yields of 0.57 ( 0.14 and 0.46 ( 0.29, respectively.12 Typical total initial concentrations of atomic oxygen ([O3P] + [O1D]) were in the range (0.053-4.00) × 1012 atoms cm-3. The influence of these concentrations of atomic oxygen in the kinetic measurements is considered in the analysis of the results. Reagents. Helium carrier gas (Carburos Meta´licos, 99.999%) was used without further purification. NO2 and CH3Br were taken from cylinders (Praxair, >99.5%) and were degassed by repeated distillation at low temperatures. The ArF gas mixture for the excimer laser was prepared by Praxair with the required specifications of Lambda Physik. Ozone was synthesized in a commercial ozonizer (Ozogas, TRCE-5000) and trapped in a liquid nitrogen trap, which was continuously pumped to remove any O2 present and then was highly diluted in He (
