Crossed-Beams Studies of the Dynamics of the H-Atom Abstraction

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Crossed-Beams Studies of the Dynamics of the H-Atom Abstraction Reaction, O(3P) + CH4 f OH + CH3, at Hyperthermal Collision Energies Jianming Zhang, Sridhar A. Lahankar, Donna J. Garton, and Timothy K. Minton* Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, United States

Weiqing Zhang and Xueming Yang State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, P. R. China ABSTRACT: The H-atom abstraction reaction, O(3P) + CH4 f OH + CH3, has been studied at a hyperthermal collision energy of 64 kcal mol
1 by two crossed-molecular-beams techniques. The OH products were detected with a rotatable mass spectrometer employing electron-impact ionization, and the CH3 products were detected with the combination of resonance-enhanced multiphoton ionization (REMPI) and time-sliced ion velocity-map imaging. The OH products are mainly formed through a stripping mechanism, in which the reagent O atom approaches the CH4 molecule at large impact parameters and the OH product is scattered in the forward direction: roughly the same direction as the reagent O atoms. Most of the available energy is partitioned into product translation. The dominance of the stripping mechanism is a unique feature of such H-atom abstraction reactions at hyperthermal collision energies. In the hyperthermal reaction of O(3P) with CH4, the H-atom abstraction reaction pathway accounts for 70% of the reactive collisions, while the H-atom elimination pathway to produce OCH3 + H accounts for the other 30%.

I. INTRODUCTION H-atom abstraction in the reaction of O(3P) + CH4 is the primary step in methane combustion,1 and it is the only experimentally observed reaction at relatively low collision energies.2
5 The barrier to this reaction, 10.5 kcal mol
1,6 has made the study of H-atom abstraction accessible by a variety of experimental techniques. The first investigation of the dynamics of this reaction was carried out by Hirota et al.,2 who observed the vibrational distributions of the CH3 radical in the “umbrella” mode by a diode-laser absorption technique. McKendrick and co-workers also performed experiments on the O(3P) + CH4 reaction, and they found very cold vibrational and rotational distributions in the OH product using laser-induced fluorescence.3 They were unable to detect vibrational states above the ground state, and the rotational distributions had a maximum population around N = 1 or 2 and extended out to N = 10. The first scattering dynamics study was performed by Zhang and Liu using a crossed-molecular-beams technique and time-sliced ion velocity-map imaging detection of the CH3 products.4 They explored the reactivity of O(3P) with ground-state and bend-excited methane. The angular distribution showed that the products were predominantly backward scattered, which is consistent with a rebound reaction mechanism. In addition, they found that bend-excited methane yielded more vibrational excitation of the OH product, compared to ground-state methane. Wang and Liu further investigated the effects of the C
H stretching vibration of methane on the O(3P) + CHD3 reaction.5 They found that excitation of the C
H stretching vibration in the reactant CHD3 led to vibrationally excited OH products, and the OH products were scattered mainly in the sideways direction. The C
H stretching r 2011 American Chemical Society

vibration of CHD3 enhanced the reactivity for the H-atom abstraction channel, while it hindered the overall reactivity of the D-atom abstraction channel. In contrast to the few experimental dynamics studies on the reaction of O(3P) with CH4, many theoretical investigations have been done on O(3P) reactions with methane as well as larger alkanes.7
10 H-atom abstraction reactions with larger alkanes have lower barriers than that for H-atom abstraction from methane: 7, 4.5, and 3.3 kcal mo1
1 for primary, secondary, and tertiary hydrogen atoms, respectively.7 An early theoretical study by Walch and Dunning suggested that the O(3P) + CH4 reaction proceeds through a collinear O
H
CH3 intermediate.11 Simultaneous to that study, Andresen and Luntz7,8 proposed that the transition state in all reactions of O(3P) with alkanes has a collinear O
H
R geometry, where R is an alkyl fragment. Luntz and Andresen proceeded to develop a triatomic model for the generic reaction of O(3P) + HR where the hydrocarbon radical, R, was considered to be a structureless particle, and the dynamics were dominated by the O(3P) interaction with the C
H bond.8 High-level calculations have confirmed the collinear geometry of the transition state for methane and larger alkanes.6,12 Assuming a collinear transition state, a line-of-centers model can be used to understand many experimental results on the dynamics of O(3P) and analogous Cl(2P) reactions with alkanes at different collision energies.13
15 In the line-of-centers model, Received: July 26, 2011 Revised: August 25, 2011 Published: September 16, 2011 10894

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The Journal of Physical Chemistry A if there is enough energy along the reaction coordinate to overcome the barrier to reaction, the reaction will proceed.16 With collision energies near the barrier to reaction, as in the experiments that have been done on O(3P) + CH4,2
5 the line-ofcenters model necessitates a low-impact-parameter collision, where there is enough energy along the O
H
R reaction coordinate for the barrier to be surmounted. The OH products from low-collision-energy reactions are thus backward scattered, consistent with low-impact-parameter collisions. Using a collision energy of 3.7 kcal mol
1, Zare and co-workers observed similar dynamics in the reaction, Cl + CH4 f HCl + CH3, which also has a collinear transition state similar to that of O(3P) reactions.15 In their study of Cl + CH2D2 f HCl/DCl + CHD2/ CH2D with a wide range of collision energies from 2 to 22 kcal mol
1, Wu and Liu17 observed that the reaction proceeds through a rebound mechanism at collision energies near threshold and shifts to a peripheral mechanism at higher collision energies. Peripheral dynamics were first described by Levine et al.,18,19 and they occur for reactions that proceed preferentially with largeimpact-parameter collisions, which form very short-lived complexes and give rise to rotationally cold and forward scattered products. A peripheral reaction mechanism is essentially the same as a stripping mechanism, which is the term that we favor in our description of such dynamics. Blank and co-workers14 described the dynamics of Cl reactions with propane in terms of a stripping mechanism. With low collision energies, the HCl product was mostly backward/sideways scattered; however, as the collision energy was increased, the HCl product angular distributions shifted toward the forward direction. The authors suggested the onset of a stripping mechanism at high collision energies, which corresponds to reaction through larger impact parameter (“glancing” or “peripheral”) collisions, with most of the available energy going into translation. The proposed stripping mechanism is a natural outcome of the line-of-centers model because as the collision energy is increased the “cone of acceptance,” or the range of impact parameters that have sufficient energy along the line-of-centers to lead to reaction, will increase. The HCl products that scattered in the forward direction were believed to be vibrationally cold based on previous studies by Varley and Dagdigian,20 which reinforces the assumption of a weak interaction between the reactants. A stripping mechanism was also observed by Kajimoto et al. in reactions of O(3P) with larger alkanes13 and by Zare and co-workers in reactions of Cl(2P) with vibrationally excited methane.15,21
23 In the reaction of O(3P) with ground-state methane, however, the stripping mechanism has never been observed experimentally. Theoretical calculations show the importance of the stripping mechanism as the collision energy is increased. Troya, Schatz, and co-workers24,25 showed that the OH products in the O(3P) + CH4 reaction shift from backward-scattered at low collision energies to sideways- and forward-scattered at high collision energies, as a result of broader cones of acceptance at higher collision energies, allowing O
H
C angles farther from the collinear minimum energy path. With hyperthermal collision energies, the stripping mechanism is dominant in such H-atom abstraction reactions. The dominance of the stripping mechanism has also been observed in the hyperthermal reactions of O(3P) with other alkanes, such as C2H6.26
28 H-atom abstraction is not the only reaction that is accessible at hyperthermal collision energies. The most significant additional reaction pathway for O(3P) + CH4 is an oxygen-atom addition with subsequent hydrogen-atom elimination that yields H + OCH3.25,29

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This H-atom elimination reaction pathway has a barrier of ∼45 kcal mol
1. Above this barrier, H-atom elimination becomes significant, and the reaction cross section increases with collision energy until it equals or exceeds the cross section for H-atom abstraction. We report here a crossed-molecular-beams study of the dynamics of the O(3P) + CH4 f OH + CH3 reaction, with product detection by two different techniques. For the OH product, we used a rotatable mass spectrometer with electronimpact ionization. For the CH3 product, we employed a time-sliced ion velocity-map imaging detector combined with resonance-enhanced multiphoton ionization (REMPI). The results show the close correspondence of the two techniques by detecting the two momentum-matched products. We have also combined the results from our studies of the H-atom abstraction and H-atom elimination reactions to determine the relative yields of these two reaction pathways with a collision energy of 64 kcal mol
1.

II. EXPERIMENTAL DETAILS A. OH Detection. The OH product was detected in experiments performed with the use of a “universal” crossed-molecularbeams apparatus equipped with a rotatable mass spectrometer detector and a hyperthermal atomic-oxygen beam source, the details of which have been described earlier.30,31 A pulsed, hyperthermal beam of oxygen atoms was crossed at right angles with a pulsed, supersonic beam of CH4 gas (Matheson, UHP grade). Both beams operated at a repetition rate of 2 Hz. Products that scattered from the interaction region were detected with a rotatable mass spectrometer detector that measured number density distributions as a function of arrival time, N(t), which are commonly referred to as time-of-flight (TOF) distributions. TOF distributions collected with different detector angles were integrated to give laboratory angular distributions N(Θ). The laboratory angle, Θ, is the detection direction with respect to the hyperthermal O-atom beam, where 0° is aligned with the O-atom beam and 90° is aligned with the CH4 beam. A forward convolution method was used to derive center-of-mass (c.m.) translational energy, P(ET), and angular, T(θ), distributions from the laboratory TOF and angular distributions.30,32
34 The hyperthermal oxygen beam was produced with a laserdetonation source based on the original design of Caledonia et al.35 and employing a piezoelectric pulsed valve of our own design. Molecular oxygen at a pressure of 500 psig was used as the precursor gas. The beam passed through a 2 mm diameter skimmer located 90 cm from the apex of the conical nozzle into a differential pumping region and then passed through another 2 mm diameter skimmer, positioned 6 cm from the first skimmer, before reaching the main scattering chamber. A relatively narrow range of velocities was selected from the overall hyperthermal beam pulse with the use of a synchronized, 17.8 mm diameter, chopper wheel rotating at 400 Hz and placed 97 cm from the apex of the conical nozzle. The velocity-selected hyperthermal beam arrived at the interaction region after traveling 99 cm from the nozzle apex. The nominal velocity of the O-atom beam used to study the O(3P) reaction with CH4 was 8100 m s
1, corresponding to a c.m. collision energy of 64 kcal mol
1. The velocity width (fwhm) of the oxygen-atom beam was ∼630 m s
1, giving a corresponding width to the collision energy of 10 kcal mol
1. The mole fraction of atomic oxygen in the beam was approximately 70%, with the balance being O2. The velocity 10895

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Figure 1. Velocity distributions of hyperthermal O-atom beams (top panel) and c.m. collision energy distributions (bottom panel) used in the experiments for the detection of OH (A distributions) and CH3 (B distributions). The average velocity of the beam for the experiment with OH detection is 8100 m s
1, corresponding to a collision energy of 64 kcal mol
1. For the experiment with CH3 detection, the moment that the probe laser was fired is indicated, and the O-atom beam velocity and the collision energy to which it corresponds to are 8100 m s
1 and 64 kcal mol
1, respectively. Some CH3 products that were formed before the probe laser was fired can still be detected. The O-atom velocities and the collision energies to which the CH3 products correspond are shown in the shaded portions of the B distributions.

distribution of O atoms in the hyperthermal beam and the resulting O
CH4 collision energy distribution are shown in the upper and lower panels of Figure 1, respectively (A distributions). Pulsed beams of pure CH4 were created using a piezoelectric pulsed valve, with a nozzle diameter of 1.0 mm. The pressure behind the nozzle of the pulsed valve was held at 60 psia. The pulsed beam passed through a 2 mm skimmer into a differential pumping region and then exited to the main scattering chamber (held at ∼10
7 Torr) through a 2.5 mm diameter aperture. The distance from the CH4 pulsed valve nozzle to the interaction region was 12 cm. No dimers or larger clusters could be detected when the CH4 beam was directed into the mass spectrometer for analysis. The nominal CH4 beam velocity was estimated to be ∼1160 m s
1.36 The velocity of the CH4 beam was about 1/7 that of the O-atom beam, and the velocity width of the CH4 beam was so narrow compared to the width of the O-atom beam that it was not considered in the analysis. TOF distributions of OH reaction products were collected at a mass-to-charge ratio (m/z) of 17 (OH+) in 2.5° increments from

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Θ = 5° to 25° and in 5° increments from Θ = 25° to 45°. No signal was observed at m/z = 18 (H2O+). Laboratory angular distributions were obtained by integrating the TOF distributions collected at the various detector angles. Using the forward convolution method,30 c.m. angular and translational energy distributions were obtained by fitting the laboratory TOF and angular distributions. In the forward-convolution method, the general approach is to start with trial distributions of P(ET) and T(θ), followed by a calculation of the lab-frame time-of-flight and angular distributions and then a comparison of the calculated distributions with the experimental data. The input P(ET) and T(θ) distributions are iteratively adjusted until optimum fits to all laboratory distributions are obtained. Uncertainties in the derived P(ET) and T(θ) distributions can be determined by observing the maximum variation in these distributions that can still produce reasonable calculated fits. The uncertainties in the P(ET) and T(θ) distributions are estimated to be (5%. B. CH3 Detection. The CH3 product was detected in experiments that used another crossed-beams apparatus equipped with a time-sliced velocity-map imaging detector. A pictorial diagram of this apparatus is shown in Figure 2. The two beams were generated in the same way as described above. The hyperthermal O-atom beam passed through a 3 mm diameter skimmer located 82 cm from the apex of the conical nozzle before reaching the main scattering chamber. The chopper wheel, which rotated at 300 Hz, was placed 3 cm downstream from the skimmer. The chopper wheel not only selected a narrow velocity range from the hyperthermal beam velocity distribution but also blocked the UV and VUV light generated by the laser-detonation plasma in the source. An ion deflector plate was placed in the source chamber just before the skimmer to remove any residual ions that might exist in the beam. The CH4 beam passed through a 1.8 mm skimmer into the main scattering chamber. The distance from the CH4 pulsed valve nozzle to the skimmer was 10 cm. The crossing region of the two beams was 94 cm from the apex of the laser-detonation nozzle cone and 19 cm from the CH4 pulsed valve nozzle. The CH3 product was ionized by (2 + 1) REMPI via the 3pz intermediate state at about 333.6 nm37
39 and detected by a time-sliced ion velocity-map imaging detector. The imaging detector is based on the design of Yang and co-workers.40 A set of multiplate ion optics was designed so that both velocity focusing and temporal extension of the ion packet could be achieved. Twenty-three stainless steel circular plates and a shielding cylinder constitute the ion optics assembly. As the reaction was studied at hyperthermal collision energies, both the laboratoryframe velocity of the c.m. and the c.m. recoil velocities of the products are typically a few thousand meters per second. Therefore, a relatively high ion energy was needed to prevent the ion packet from expanding beyond the size of the microchannel plate (MCP) at the top of the detector. For the experiment described herein, the total voltage applied to the ion optics was 4000 V. With this voltage, the turnaround time is about 140 ns for a CH3+ ion with a velocity of 3500 m s
1. From the end of the ion optics assembly, there is a 36 cm long, field-free drift region to the detector. The distance from the crossing region of the two beams to the detector is about 60 cm. The ion detector consists of a pair of 75 mm diameter MCPs coupled to a phosphor screen (Photonis, model 75 FM P47 CT). A high voltage pulse generator module (DEI, model PVM-4210) was used to supply a pulsed voltage of 950 V to the first plate of the MCP detector for time slicing the ion signals. The sliced time width was about 10896

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Figure 2. Schematic diagram of the crossed-molecular beams apparatus and the time-sliced ion velocity-map imaging detector which employs resonance enhanced multiphoton ionization (REMPI).

30 ns. A constant high voltage of 1350 V was applied to the second plate of the MCP detector. The phosphor screen was held at 4500 V. The ion signals were converted to photon signals through the phosphor screen, which then were recorded by a charge-coupled device (CCD) camera (Stemmer Imaging, model JAI CM-040 GE, 776  582 pixels) and sent to a computer for storage and further analysis. During the recording, an ion event counting method was used,41 which greatly improved the image resolution. In addition, a photomultiplier tube (Beijing Hamamatsu, model CR125) was used on the side of the CCD camera to collect the integral photon signals from the phosphor screen. The signal from the photomultiplier tube was averaged and displayed on an oscilloscope (Tektronix, model TDS3044B) and then sent to a computer for storage and further analysis. The detector may be used to record REMPI spectra of products as well as velocity-map images of products that are ionized by REMPI at a particular wavelength. The probe laser for REMPI-based detection of CH3 was generated by doubling the output of an Nd:YAG (Continuum Surelite III) pumped dye laser (Continuum ND 6000). The dye solution was DCM in methanol. The laser energy was 1.3 mJ per pulse, and the pulse width was 5 ns. Higher laser energy can increase the signal, but it also increases the background caused from the O/O2 beam or CH4 beam. Moreover, a lower laser energy also reduces Coulomb repulsion which can cause distortion to a velocity-mapped image. The laser energy and focusing conditions were chosen to optimize the signal-to-noise ratio while achieving minimum image distortion. A spherical planoconvex lens with a focal length of 750 mm was used to focus the laser beam at the interaction region. The spread of the laser spot at the interaction region was 0.3 mm. The product detection sensitivity depends on the product laboratory-frame velocity. The products with higher laboratory-frame velocity can fly away from the detection zone, while the slow products will stay and accumulate. Thus, the slower products have a higher probability of being ionized by the probe laser beam. To reduce this effect on the detection sensitivity, the laser beam was scanned when collecting the ion images via a servo motor repeatedly over (5 mm

perpendicular to the laser propagation direction and in the plane defined by the two molecular beams. The laser beam was scanned at a speed in the range 2
9 μm per second and therefore formed a uniform artificial laser sheet, which provided less detection bias and greatly reduced the distortion in the density-to-flux conversion.42 The CH3 product was only detected in its ground vibrational state. No excited vibrational states of the umbrella mode (for example, 211, 222, and 213)39,43 or other modes (111, 311, and 411)39 were detected within our detection sensitivity. During the collection of images, the laser wavelength was also scanned in a range that covered the whole 000 Q branch to compensate for the Doppler shift. Therefore, the images were effectively summed over all the rotational states. Within the detection sensitivity, the images obtained with a scanned range of laser wavelengths are identical to the images obtained with the laser wavelength fixed at the center of the 000 Q branch. The probe laser was synchronized with the oxygen beam and the CH4 beam in such a way that the nominal collision energy at which the CH3 product was formed could be controlled with high precision. For the current study, the probe laser passed the interaction region at the precise moment that O atoms with a velocity of 8100 m s
1 in the laboratory frame arrived at the interaction region. Given the range of possible CH3 product velocities in the laboratory frame, it is possible to detect the CH3 products that were formed at earlier times corresponding to higher O-atom beam velocities. Therefore, the relevant O-atom beam velocity distribution for this experiment starts at 8100 m s
1 and extends to the maximum velocity allowed by the chopper wheel (which in this case means the maximum velocity in the overall hyperthermal beam pulse), as illustrated in the shaded portion of distribution B in the upper panel of Figure 1. The B distribution in the lower panel shows the corresponding O
CH4 c.m. collision energy distribution. With this collision energy distribution, three CH3 images were collected, and each was accumulated for 17 857 beam pulses. Since both the hyperthermal O/O2 beam and the CH4 beam created background during the REMPI detection of CH3, background images of the O/O2 beam alone and the CH4 beam alone were also collected for half of the beam pulses used for the signal collection. After background 10897

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Figure 5. Center-of-mass translational energy distributions for the O(3P) + CH4 f OH + CH3 reaction at Ecoll = 64 kcal mol
1 derived from the TOF and laboratory angular distributions for m/z = 17 (OH+) through the forward convolution method (distribution A) and from the CH3 image shown in the left panel in Figure 7 (distributions B and C). Distributions B and C are derived from two ranges of the image, θcm = 90° to 180° and 0° to 90°, respectively.

Figure 3. Representative time-of-flight distributions of reactively scattered OH following reaction of O(3P) with CH4 at Ecoll = 64 kcal mol
1. Laboratory detection angles with respect to the direction of the hyperthermal O-atom beam are indicated in each panel. The open circles are the experimental data. The solid lines are the forward convolution simulation to the data derived from the c.m. distributions labeled “A” in Figures 5 and 6.

Figure 6. Center-of-mass angular distributions of the OH product (distribution A) and the CH3 product (distribution B) from the O(3P) + CH4 f OH + CH3 reaction at Ecoll = 64 kcal mol
1. Distribution A is derived from the TOF and laboratory angular distributions for m/z = 17 (OH+) through the forward convolution method, and distribution B is derived from the top half of the CH3 image shown in the left panel in Figure 7.

Figure 4. Laboratory angular distribution of the OH product from the reaction of O(3P) with CH4 at Ecoll = 64 kcal mol
1. The open circles are the experimental data, and the solid line is the forward convolution simulation to the data derived from the c.m. distributions labeled “A” in Figures 5 and 6. The error bars represent (1σ uncertainty in the integrated experimental time-of-flight distributions.

subtraction, the three CH3 images were summed to give one image for further analyses.

III. RESULTS AND ANALYSIS A. OH Product. The OH products scatter with large center-ofmass (c.m.) velocities, resulting in kinematics that yield relatively low signal levels. Representative time-of-flight (TOF) distributions

collected at six different laboratory angles for m/z = 17 (OH+) are shown in Figure 3. The solid-line curves come from the forward convolution simulation of the data using optimized c.m. translational energy and angular distributions (A distributions in Figures 5 and 6, respectively). The poor signal-to-noise levels in the TOF distributions for m/z = 17 are exacerbated by the background signal resulting from dissociative ionization of residual H2O in the detector. Any possibility of a contribution to the signal at m/z = 17 from mass leakage from the large inelastic scattering signal at m/z = 16 (O+) was eliminated by increasing the resolution of the quadrupole mass filter until no signal was observed for m/z = 16.5. This high-resolution operation, which was needed for clean detection of OH+, further reduced the signal levels and degraded the signal-to-noise ratio of the TOF distributions collected for m/z = 17. The corresponding experimental laboratory angular distribution and the calculated distribution from the forward convolution simulation of the data 10898

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Figure 7. Center-of-mass velocity-flux maps of the CH3 product (left panel) and the OH product (right panel) from the reaction, O(3P) + CH4 f OH + CH3, at Ecoll = 64 kcal mol
1. Superimposed are Newton diagrams with circles representing the maximum velocities corresponding to ground vibrational and rotational states of OH and CH3. Both images display the desired physical quantity d2σ/sin θdudθ.

are shown in Figure 4. The angular distribution, which has a maximum at a laboratory angle of ∼20°, indicates that the OH radical is scattered predominantly in the forward direction (where a laboratory angle of 0° is defined as the direction of the O-atom beam). The optimized c.m. translational energy and angular distributions, obtained from the OH data, are shown in Figures 5 and 6 (A distributions), respectively. The translational energy distribution has a maximum near 80% of the available energy, which is the sum of the collision energy and the reaction exoergicity (Eavl = 64
1.65 = 62.35 kcal mol
1). Thus, most of the energy available for the reaction goes into translation of the OH and CH3 fragments, with little going into internal energy. The c.m. angular distribution, although broad, shows forward scattering of the OH, with a peak in the distribution near 38°. The right panel in Figure 7 displays a c.m. velocity-flux map for the OH product, which was derived from the c.m. translational energy and angular distributions (A distributions) in Figures 5 and 6. B. CH3 Product. The signal detected via the REMPI technique is proportional to the number density of the CH3 product. Compared to the products with fast laboratory-frame velocities, the products with slower laboratory-frame velocities are more likely to be ionized by the probe laser because the slow products remain in the irradiated volume longer. Therefore, a density-toflux transformation is needed to account for this velocity-dependent detection sensitivity.42 In addition, the physical quantity that a raw image collects corresponds to d3σ/u2 sin θdudθdj, where σ is the reaction cross section, u the c.m. velocity, θ the c. m. polar angle, and j the c.m. azimuthal angle. For a system with cylindrical symmetry, the desired differential cross section d2σ/ sin θdudθ, i.e., the velocity-flux map, can readily be obtained from the raw image by multiplying it by a weighting factor of u2. The left panel in Figure 7 shows the velocity-flux map image for the ground vibrational state of the CH3 product (all product rotational states are included). The density-to-flux correction has been made, in addition to the u2 weighting. Superimposed on the image is the corresponding Newton diagram for O and CH4 velocities of 8100 and 1160 m s
1, respectively. The radius of the white circle gives the maximum possible speed of the probed CH3, which occurs when both the CH3 and OH momentummatched products are formed in their ground vibrational and rotational states. The lack of observation of multiple rings, even with a fixed probe laser wavelength, corresponding to different

OH vibrational states, indicates that the OH products are vibrationally cold and/or highly rotationally excited. The CH3 products are mainly backward scattered with respect to the reagent O-atom direction, while some sideways and weak forward scattering are also evident. The c.m. angular distribution of CH3 was obtained by integrating the differential cross section at different angles, with an angular binning size of 1°, and the distribution from the top half of the image is presented in Figure 6 (distribution B). The angular distribution obtained from the bottom half of the image is essentially identical and is not presented. The angular distribution of scattered CH3 products has a maximum at about 150°, equivalent to 30° for the angular distribution of OH products. Although the angular distributions of both products have maxima that correspond to momentum-matched counter fragments, the OH angular distribution is significantly broader than that for CH3. The resolution in the OH angular distribution is estimated to be about 5 to 10°, which cannot account for this discrepancy. An attempt was made to use the angular distribution from the CH3 data to fit the OH data, and the fitting results are obviously unacceptable. As discussed below, this difference is a consequence of the different collision velocity distributions used in the two experiments. In addition to the c.m. angular distribution, the c.m. translational energy distribution for the two recoiling products can be obtained from the velocity-flux map for CH3. As can be seen in Figure 7, the velocity of the CH3 product is higher when it scatters in the same direction as the reagent CH4 than when it scatters in the direction of the reagent O atoms. This result indicates a coupling between the c.m. translational energy and angular distributions. This coupling is manifested in the translational energy distributions shown in Figure 5. Distributions B (red) and C (green) in this figure are the translational energy distributions averaged from θcm = 90° to 180° and from θcm = 0° to 90°, respectively. The translational energy distribution for the backward-scattered CH3 (distribution B) is very similar to that obtained from the OH product (distribution A). The translational energy distribution for OH was derived by the forwardconvolution method under the assumption that the translational energy and angular distributions, P(ET) and T(θ), respectively, are not coupled. This assumption is valid over the range of forward-scattered angles where OH products were detected. A small portion of distribution B is above the nominal available 10899

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1. It should be noted that the observed CH3 products are the result of reactions corresponding to a range of collision energies from 64 to ∼76 kcal mol
1, whose collision energy distribution is shown as the shaded region in the lower panel of Figure 1. Although the most probable available energy in the experiment where CH3 was detected was ∼62 kcal mol
1, the range of available energies extended to ∼74 kcal mol
1. Therefore, some pairs of products may recoil with translational energies that are above the most probable available energy. In the analysis procedure30 used for the experiment where OH was detected, a canonical translational energy distribution was derived that corresponds to the nominal available energy of 62.35 kcal mol
1. The slightly different analysis procedures thus yield translational energy distributions that have somewhat different meanings. Nevertheless, the two distributions (distributions A and B in Figure 5) are remarkably similar for translational energies less than the available energy. Distributions A and B pertain to the case when the OH and CH3 products tend to scatter approximately in the same directions that the reagent O and CH4, respectively, were initially traveling. Highimpact-parameter (“grazing-type”) collisions would lead to this result, assuming a direct reaction without a transient intermediate complex. On the other hand, distribution C in Figure 5 is the translational energy distribution for the case when OH and CH3 scatter in the direction opposite to the initial direction of the reagent O and CH4, respectively. These “rebound” collisions lead to a product translational energy distribution that is broader and of lower average energy than do the former, grazing-type, collisions. Therefore, rebound collisions, on average, channel less of the available energy into translation and more energy into internal degrees of freedom than grazing collisions. The OH abstraction reaction pathway is not the only one that is accessible at high collision energies for the reaction of O(3P) with CH4. The most significant additional reaction pathway is hydrogen-atom elimination that yields H + OCH3. During the study of the OH products, the OCH3 products were also detected,29 and the relative yield of each reaction pathway was determined. The relative product yields of the two reaction pathways are estimated to be Oð3 PÞ þ CH4

fOH þ CH3 fOCH3 þ H

ð70 ( 10%Þ ð30 ( 10%Þ

The relative product yields were obtained from the c.m. translational energy and angular distributions, taking into account the electron-impact ionization cross sections, ionizer fragmentation patterns, and quadrupole transmission function. The precise relative yield is elusive as there is relatively large uncertainty in the c.m. translational energy and angular distributions used for fitting the OCH3 data. However, we believe that the uncertainties are very conservative, so the values obtained still provide a good indication of the relative importance of each reaction pathway. Even though the barrier and endoergicity for the H-atom elimination reaction are ∼35 and ∼12 kcal mol
1, respectively, higher than those for the H-atom abstraction reaction, the relative yield of the H-atom elimination reaction becomes significant at hyperthermal collision energies.

IV. DISCUSSION Because the endoergicity for the O(3P) + CH4 f OH + CH3 reaction is 1.65 kcal mol
1 and the experimental collision energy

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is 64 kcal mol
1, there is a large excess of energy available for large-impact-parameter collisions to lead to reaction, and therefore a stripping mechanism would be predicted.16 The experimental c.m. translational energy distributions (see distributions A and B in Figure 5) are relatively narrow and show a peak corresponding to 80% of the available energy, indicating that most of the available energy goes into translation. This result is consistent with calculations done by Troya et al. for O(3P) + CH4 reactions at high collision energies.25 At the collision energy used in our experiment, direct dynamics calculations by Troya et al. (employing MSINDO) suggested that ∼75% of the available energy is channeled into translation. Most of the internal energy, ∼24% of the total available energy, is partitioned into rotation of OH. The small fraction of energy remaining goes into vibration of OH and CH3. This energy partitioning is consistent with our lack of observation of vibrationally excited CH3 and distinct rings in the velocity map image corresponding to different vibrational states of OH. The lack of distinct rings may be the result of vibrationally cold OH products and OH products that are rotationally hot. This explanation is likely, as the calculations by Troya et al. suggested that the OH (v = 0) rotational distribution peaked around j = 15. This is consistent with a simple calculation using the difference in the orbital angular momentum before and after the collision, which indicates a rotational excitation of OH of more than j = 10. In addition, a preliminary measurement in our laboratory of the OD product of the O(3P) + CD4 reaction by laser-induced fluorescence indicates a similar level of rotational excitation. Thus, both the OH and CH3 products appear to be formed vibrationally cold, with significant rotational excitation. The c.m. angular distributions, which show forward scattering of OH and backward scattering of CH3 (where forward is defined as the direction of the reagent O atoms), are further evidence of a stripping mechanism. The peaks in the angular distributions for OH and CH3 suggest that scattering is generally not directly forward but about 30° to 40° toward the sideways direction (see Figure 6). Semiempirical (MSINDO) calculations by Troya et al.25 are in basic agreement with this result, predicting a peak at ∼45°. The preference for slightly sideways scattering may be the result of a steric effect, where trajectories that would lead to directly forward-scattered OH are blocked by neighboring hydrogen atoms. Although the cone of acceptance apparently does not allow a near perpendicular angle between the reagent O-atom direction and a H
C bond in CH4, it clearly increases with increasing collision energy. MSINDO semiempirical calculations25 showed an opacity function that favored low impact parameters, near zero, at a collision energy of 15 kcal mol
1 and shifted to most probable impact parameters near 3 au at a collision energy of 90 kcal mol
1. The calculations predict that the OH products are predominantly backward scattered at a collision energy of 15 kcal mol
1 and become increasingly forward scattered as the collision energy increases to 90 kcal mol
1. Thus, the cone of acceptance broadens at higher collision energies, allowing O
H
C angles farther from the collinear minimum energy path.24,25 The dominance of a stripping mechanism at higher collision energies is not the sole result of an increasing cone of acceptance. At high collision energies, above the barrier of 46 kcal mol
1,29 the H-atom elimination reaction, O(3P) + CH4 f OCH3 + H, competes effectively with the H-atom abstraction mechanism at small impact parameters and leads to a relatively small reaction cross section for the production of OH in the backward direction.25 10900

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The Journal of Physical Chemistry A Nevertheless, a small fraction of low-impact-parameter collisions lead to OH + CH3, as evidenced by the forward and sideways signal in the velocity-flux map for CH3 (see the left panel in Figure 7), which corresponds to backward- and sideways-scattered OH. The presence of backward-scattered OH could only be inferred from the velocity-map imaging experiment, as it enabled the detection of the entire angular range of CH3 products. In contrast, only forward-scattered OH could be detected in the experiment with mass spectrometric detection. As can be inferred from the velocity-flux map in Figure 7 and observed clearly in the translational energy distributions in Figure 5, more energy is channeled into translation when the products react through high-impact-parameter (grazing) collisions than when they react through low-impact-parameter (head-on) collisions. Not surprisingly, the stripping reactions lead to less internal excitation than the rebound reactions. We can conclude from both experiments that most of the H-atom abstraction trajectories proceed through a stripping mechanism, where O(3P) picks off a hydrogen atom during a glancing blow with the methane. Some “harder” collisions can still result in the formation of OH + CH3, and these lead to more energy transfer and to sideways and backward scattering. These results seem to reinforce the validity of the triatomic model for hydrogen abstraction from CH4 by O(3P). Note that analogous dynamics were observed for the H-atom abstraction with ethane: O(3P) + CH3CH3 f OH + CH3CH2.26 This study marks the first simultaneous use of a universal crossed-molecular-beams apparatus and a time-sliced ion velocity-map imaging machine to investigate the dynamics of the same reaction under approximately the same conditions in the same laboratory. For the stripping reaction, we observed excellent agreement between the two different experimental techniques. The c.m. translational energy distributions are essentially identical (distributions A and B in Figure 5), except for a small discrepancy in the high-energy tail which is an artifact of the methods of analysis and is discussed in the previous section. There is a slight difference in the c.m. angular distributions; i.e., the distribution obtained from the OH measurement by mass spectrometry is broader and has a maximum at a slightly larger angle than would be predicted from detection of the momentummatched CH3 product by REMPI and velocity-map imaging (Figure 6). The difference between the angular distributions lies in the velocity distribution of the reagent O atoms. As seen in the lower panel of Figure 1, the collision energy distribution is at least twice as broad for the experiment that detected OH than for the experiment that detected CH3. The use of a laser to probe the CH3 product greatly reduced the effective velocity spread of the O-atom beam, as the products being detected must be formed no later than the moment the probe laser irradiated the products. Therefore, the angular distribution from the OH measurement has more contributions from lower collision energies, which broadens the angular distribution and tends to shift the scattering toward more sideways angles. The combination of the measurements of the OH product and the OCH3 product, which was reported earlier,29 has allowed an experimental determination of the relative yields of the two major reaction pathways for O(3P) + CH4. At a collision energy of 64 kcal mol
1, the H-atom abstraction reaction pathway accounts for about 70% of the reactive collisions, while the H-atom elimination reaction pathway accounts for about 30%. The relative yields show that the hydrogen elimination is a nonnegligible and important reactive channel for hyperthermal O(3P)

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reactions with CH4. The relative yields of the two reaction channels and the corresponding B3LYP/6-31G* calculations by Troya et al. are in good agreement.25 At collision energies below 46 kcal mol
1, H-atom abstraction is the exclusive reaction, but as the collision energy increases, the cross section for OCH3 formation increases relative to that for OH and becomes dominant above 100 kcal mol
1. The H-atom elimination reaction also competes with H-atom abstraction in the reaction of O(3P) with ethane.26 The OCH3 + H product channel therefore seems to be very important when an O(3P) atom encounters an alkane at high collision energies. Such reactions might be important in the hyperthermal reactions of oxygen atoms with hydrocarbon polymers on spacecraft in low Earth orbit.44 Formation of an oxy radical on a hydrocarbon polymer surface would provide a direct pathway to the formation of CO and CO2, thereby eroding the polymer.

V. CONCLUSION In the reaction, O(3P) + CH4 f OH + CH3, at a hyperthermal collision energy of 64 kcal mol
1, the OH product is formed mainly through a stripping mechanism where the O(3P) reacts through large-impact-parameter collisions with CH4, and little energy is transferred into internal modes. Some of the OH products are formed through low-impact-parameter collisions, leading to more energy transfer and to sideways and backward scattering. However, most low-impact-parameter reactions likely follow an H-atom elimination pathway to form OCH3 + H. At the collision energy of the experiments reported here, the relative yields of H-atom abstraction and H-atom elimination are about 70% and 30%, respectively. The widely accepted triatomic model for H-atom abstraction from an alkane by an O(3P) atom is still applicable at the collision energies used in the experiments. The CH3 products are in the ground vibrational state, while the OH products are vibrationally cold and rotationally hot. The experimental results are in good agreement with earlier direct dynamics calculations,25 and they show the strong similarity between the dynamics of O(3P) reactions with methane and ethane. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been supported by grants from the Department of Defense Experimental Program for the Stimulation of Competitive Research (DEPSCoR), administered by the Air Force Office of Scientific Research (Grant No. F49620-01-1-0276), from the Air Force Office of Scientific Research through a Multidisciplinary University Research Initiative (Grant No. F4962001-1-0335), and from the Missile Defense Agency under Cooperative Agreement HQ0006-05-2-0001. We are grateful to Professor Kopin Liu for helpful discussions. ’ REFERENCES (1) Warnatz, J. Combustion Chemistry; Springer-Verlag: Berlin, 1984. (2) Suzuki, T.; Hirota, E. J. Chem. Phys. 1993, 98, 2387–2398. (3) Sweeney, G. M.; Watson, A.; McKendrick, K. G. J. Chem. Phys. 1997, 106, 9172–9181. (4) Zhang, B.; Liu, K. J. Phys. Chem. A 2005, 109, 6791–6795. (5) Wang, F.; Liu, K. Chem. Sci. 2010, 1, 126–133. 10901

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