Imaging the O(1D) + CD4 → OD + CD

Letter pubs.acs.org/JPCL

Imaging the O(1D) + CD4 → OD + CD3 Reaction Dynamics: The Threshold of Abstraction Pathway Quan Shuai, Huilin Pan, Jiayue Yang, Dong Zhang, Bo Jiang, Dongxu Dai, 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 S Supporting Information *

ABSTRACT: The O(1D) + CD4 → OD + CD3 reaction was investigated using the crossed molecular beam technique with sliced velocity map imaging at four different collision energies: 1.6, 2.8, 4.6, and 6.8 kcal/mol. The vibrational ground state product CD3 was detected using a (2 + 1) resonance-enhanced multiphoton ionization (REMPI). Remarkably different features were found in the forward and backward scatterings, and gradually changed with the collision energy. These features were attributed to two distinctive reaction mechanismsinsertion and abstractionthat occur on the ground and excited state surfaces, respectively. Contributions from the two mechanisms were extracted from the experiment results, and a positive correlation was found between the abstraction proportion and the collision energy. The threshold for the abstraction pathway was determined and compared with results from calculations. SECTION: Kinetics and Dynamics inserts into the C−H bond and forms a “hot” methanol intermediate species. Additionally, it was suggested that O(1D) might also attack the C−H bond collinearly and immediately separate into OH + CH3 (abstraction). In the crossed beam study by Yang and co-workers in 2000, a large forward peak and a relatively smaller backward peak (OH product respected to the O(1D) beam direction) in the OH + CH3 reaction channel were observed, implying that the reaction mechanism for this channel might not be simply due to an insertion mechanism. In 2001, Miller et al. also investigated this reaction using ultrafast pump−probe technique and suggested a three-mechanism model.34 Very recently, Suzuki and co-workers have investigated the O(1D) + CD4 → OD + CD3 reaction using the nonsliced velocity map ion imaging method19,20 and clearly showed that the OD + CD3 channel seems to proceed with both insertion and abstraction mechanisms. Theoretically, both insertion and abstraction reaction pathways have been predicted previously by Hernando et al.37 It was believed that the insertion reaction occurs on the ground state potential energy surface (PES), while the abstraction mechanism proceeds on an excited PES. Although experimental evidence has been given for the dual reaction mechanisms, some important dynamical details remain ambiguous. For example, the change of the relative importance of these two fundamentally different mechanisms with collision energy, the change of OH vibration excitation with collision

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eaction dynamics of polyatomic reactions have received much attention in recent years, for instance, the F + CH4 system1−9 by Liu and co-workers, and the F (Cl) + SiH4 reactions10,11 and H + CD4 → HD + CD3 by Yang and coworkers.12 Many interesting dynamical behaviors have been observed. In recent years, theoretical studies on these polyatomic reactions have also made significant progresses.13−16 These previous studies are mostly focused on the HX formation channel in these reactions. One of the challenges in the study of polyatomic reaction dynamics is multiple channels, as shown theoretically in the H + SiH4 reaction.17 Methane is the most abundant hydrocarbon in the atmosphere. The reaction of methane with O(1D) is a source for stratospheric OH, and oxidation also provides a portion of stratospheric H2O, which itself can produce OH through reaction with O(1D). The OH radical generated from this source partly determines the chemistry of the earth’s ozone layer through the HOx cycles.18 Furthermore, this reaction also plays a significant role in tropospheric chemistry. The O(1D) reaction with methane is a typical multichannel chemical reaction. Extensive experimental and theoretical studies have been carried out in order to elucidate the dynamics of this reaction (and its isotope variants).19−43 Three main product channels have been observed previously, OH + CH3, CH2OH + H, and CHOH/CH2O + H2, by Yang and coworkers, using the universal crossed molecular beam technique.33 The OH formation channel was found to be the major reaction channel, with its branching ratio of 77%. Unlike the F + CH4 reaction, the O(1D) + CH4 reaction was regarded as a prototype of an insertion reaction,21 i.e., the O(1D) atom © 2012 American Chemical Society

Received: April 15, 2012 Accepted: April 30, 2012 Published: April 30, 2012 1310

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is symmetric to the relative velocity, the ion image observed within the laboratory frame usually appears asymmetric. This asymmetry results from the density-to-flux problem,47 which depends on the product laboratory velocity and the spatial/ temporal overlaps of molecular and laser beams. Due to the asymmetry, the mathematical inversion process usually applied to recover the desired 3D velocity distribution from a compressed 2D image, such as inverse Abel transformation, can be inapplicable. By the time-sliced technique, we can obtain a direct slicing image, which would be more reliable than transforming a 2D projection image. Clearly, there are two reaction mechanisms involved here, as suggested by Suzuki and co-workers. The broad feature in the images were assigned to the insertion mechanism that occurs on the ground state PES, while the sharp ring structures were attributed to the abstraction pathway on the electronically excited PES.37 Since the sharp structure feature clearly matches the prototype of a colinear abstraction feature, this assignment is very reasonable. The assignment of the broad feature to the insertion mechanism is quite clear. However, the angular distribution for this feature seems to be clearly forward scattered, which is not very typical of an insertion mechanism with a sufficiently long-lived intermediate. This is probably due to the fact that the intermediate does not live sufficiently long. To illustrate the distinct dynamics of two contributions more explicitly, the CD3 products from the two obvious contributions are separated (Figure 2). Since the two features are very different, this can be achieved without too much difficulty. In all cases, CD3 products from the broad feature, largely forward scattered, correspond to OD products with considerable internal excitations, most likely high rotational excitation as has been observed previously.34 On the other hand, CD3 products from the abstraction contribution, mostly backward scattered, correlate to OD products with little rotational excitation. It seems that the broad feature does not change very much, while the abstraction contribution changes quite significantly. At the lowest collision energy studied here (1.6 kcal/mol), only OD product at v′ = 5 was observed. As the collision energy increases, v′ = 4 OD product starts to appear and becomes more important. At the highest collision energy studied (6.8 kcal/mol), v′ = 6 product also was observed, even though the contribution is small. From Figure 2, it can be easily found that the abstraction mechanism becomes more and more important as collision energy increases. This is also consistent with a typical colinear abstraction mechanism with a barrier. Figure 3 shows the percentage of the abstraction contribution over the insertion contribution (the entire board feature) as a function of the collision energy. The relative contribution is considerably larger than the theoretical prediction36 for the O(1D) + CH4 reaction, which indicated that abstraction is almost negligible under our experiment conditions. As shown in Figure 3, the contribution from the abstraction increases considerably as the collision energy increases. From the four experimental data points, we have made a line plot that goes through all four points and also extended to zero. From this plot, we have estimated the reaction threshold to be about 0.8 kcal/mol. This suggests that the theoretical barrier (1.2 kcal/mol) calculated using the CASPT2 (complete active space, second order perturbation theory) ab initio method37 is quite reasonable. The dual reaction mechanisms in the title reaction is similar to the O(1D) + D2 reaction.49 In that case, the insertion pathway yields OD products with forward and backward

energy, and the threshold of the abstraction reaction mechanism, are all not clearly known. An especially interesting issue is the barrier height for the abstraction reaction, since theoretically predicted barriers obtained using different methods vary in a very wide range.37 In this work, we have investigated the O(1D) + CD4 → CD3 + OD reaction by time-sliced ion velocity imaging44 at a series of collision energies. Vibronic ground state CD3 products were detected by (2 + 1) resonance-enhanced multiple photon ionization (REMPI) through a Rydberg state,45 and relative contributions from abstraction and insertion mechanisms have been determined at these collision energies. We can learn how the OH vibrational excitation varies with collision energy. The barrier for the abstraction mechanism is also derived. In this case, the answers to the above-mentioned issues can be clearly unraveled. The (2 + 1) REMPI spectrum of the CD3 product in the ground vibrational state from the O(1D) + CD4 reaction have been measured (see Supporting Information). The Q(000)branch was not rotationally resolved. Crossed beam images were measured in this experiment by ionizing the CD3 product via the Q branch. Figure 1 shows the raw images of the CD3 (v

Figure 1. Raw images of product CD3 (v = 0) at the collision energy of (A) 1.6, (B) 2.8, (C) 4.6, and (D) 6.8 kcal/mol. The reactant beam direction in center-of-mass frame is given in (A) and omitted in the other images for the sameness. On some images along the beam direction, there were minor background spots probably arising from nonresonant multiphoton ionization of the reactant molecules, which could be easily eliminated for their convergence.

= 0) products at EC = 1.6, 2.8, 4.6, and 6.8 kcal/mol, respectively. These images were taken when the REMPI detection laser frequency was fixed to detect the low-N states of CD3 (N ≈ 5). The CD3 products show up in all scattering angles. It is quite interesting that forward scattered CD3 (relative to the CD4 beam direction) has no sharp structures, while sideways- and backward-scattered CD3 products exhibit discrete structures. This observation is quite consistent with the recent experimental results carried out by Suzuki and coworkers,19,20 despite some noticeable differences. The differences might arise from the time-sliced technique46 we use in the experiment. Briefly speaking, in a crossed molecular beam experiment, although the product flux in center-of-mass frame 1311

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small rotational excitation. A noticeable difference is that in the O(1D) + CD4 reaction, the OD products from the insertion pathway is clearly forward scattered, probably due to the formation of some short-lived complex.34 In summary, the O(1D) + CD4 → OD + CD3 reaction is investigated by a time-sliced velocity map imaging technique at collision energies between 1.6 and 6.8 kcal/mol. This reaction provides a good example of a polyatomic reaction proceeding on ground and excited states surfaces via two fundamentally different mechanismsinsertion and abstractionleading to the same product channel. Distinctive dynamics of the two reaction pathways are observed and analyzed. Relative contributions of the insertion and abstraction pathways at different collision energies are determined. From the relative contribution, we have estimated the threshold of the abstraction reaction to be 0.8 kcal/mol. Even though the insertion pathway is the predominant pathway at low collision energies, the abstraction pathway at high collision energies can be quite significant, because the insertion reaction is likely barrierless, and its reaction cross section should decrease as collision energy increases, while that for the insertion pathway will increase as collision energy increases. Also very interesting is the variation, with EC, of the vibrational excitation of the hydroxyl radical coproduct correlated to the methyl radical produced via the abstraction mechanism. These data will represent an overall benchmark for future dynamical calculations on multiple PESs for this important reaction. This study may also aid in the understanding of the related process in atmospheric chemistry and cometary chemistry.

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Figure 2. Separated flux-velocity contour maps of product CD3 (v = 0) from insertion and abstraction pathways at the collision energy of (A) 1.6, (B) 2.8, (C) 4.6, and (D) 6.8 kcal/mol. Density-to-flux simulation48 and symmetrization are required to transform the raw images into these maps. The intensity of each contour has been weighted by u2 (u is the center-of-mass velocity of CD3) in accordance with conventional representation of differential cross section. The contour intensities of the same collision energy are normalized to one another, but not for different collision energies. The abstraction image intensity is multiplied by a factor of 3 for convenience. The vibrational quantum number of the counterpart product OD is shown as v′. The direction of the CD4 beam defines the forward scattering direction, denoted as 0 in panel A, and the same in the others.

EXPERIMENTAL METHODS The experiments were conducted on a time-sliced ion velocity imaging crossed-beam apparatus described in detail previously.48 In brief, the O(1D) atomic beam, generated using the photodissociation of O2 at 157.6 nm in a skimmed O2 pulse beam, was crossed with a skimmed CD4 beam from a rotating source. The O2 beam was obtained by expanding a 10% O2 (diluted in He or Ar) sample at about 80 psig stagnation pressure through a commercial pulsed valve (General Valve) with a rise time of about 50 μs and the pulse width of about 100 μs, and then intercepted at the nozzle tip of the pulsed valve by a 157.6 nm laser beam, generated by a Lambda Physik LPF202I F2 laser, with a pulse energy of about 40 mJ at a repetition rate of 10 Hz. The nascent O(1D) atoms were then coexpanded together with the molecular beam and skimmed by a sharp edged skimmer with a 2-mm-diam orifice before entering the reaction chamber. The reactivity of O(3P) is negligible for this reaction at the collision energies studied in this experiment, because the energy barrier (estimated to be 9.8 kcal/mol)50 for the O(3P) + CH4 reaction is considerably higher than the largest EC of 6.8 kcal/mol investigated here. The F2 laser beam was focused to a spot of 3 mm (w) × 1 mm (h) in the interaction region by a spherical MgF2 lens. Using this focusing condition and laser power, the O2 transition (cross section σ = 6.8 × 10−18 cm2) at 157.6 nm could be easily saturated. The CD4 molecular beam was generated by expanding a neat CD4 (99%, Spectra Gases) sample (or 10% diluted in Ar) at a stagnation pressure of 80 psig through a carefully adjusted pulsed valve (General Valve) with a rise time of about 50 μs, and then skimmed once by a 1 mm orifice skimmer before entering the reaction chamber. Because of the F2 laser path, the crossing angle between the two sources could only be set between 125° and 155°. Therefore, we used differently mixed

Figure 3. The percentage (A/I) of the product CD3 (v = 0) population from the abstraction mechanism (A) over the insertion mechanism (I) at different collision energies. The solid squares are our experimental data, and the solid line is drawn as a simple fit to estimate the threshold of the abstraction pathway, which should be the xintercept of the line on the graph.

symmetry, while the OD products from a colinear abstraction is mainly backward scattered and highly vibrationally excited, with 1312

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gas samples to obtain different beam velocities for both O(1D) and CD4 beams, and thus different collision energies. The O(1D) in He (or Ar) beam velocity was determined to be about 1600 m/s (or 700 m/s), while the CD4 beam (or in Ar) velocity was 1050 m/s (or 700 m/s), respectively. The reaction product, CD3, was detected by a (2 + 1) REMPI scheme via the 3p2A2″ Rydberg state.45 The UV photon of around 334 nm was generated by a YAG-pumped dye laser output. The typical UV pulse energy used in the experiment was 5−6 mJ, and a spherical lens with a focal length of 48 cm was used to focus the laser beam into the crossing region. Under this condition, the two-photon resonant transition (Q(000)) from the vibrational ground state CD3 products can be nearly saturated in this experiment. The total voltage of ion optics was held at 1500 V, at which the whole image could be recorded. During the experiments, the vacuum was maintained at about 1 × 10−5 Torr in both source regions and at about 7 × 10−7 Torr in the reaction and detection regions.

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ASSOCIATED CONTENT

S Supporting Information *

(2 + 1) REMPI spectrum of the CD3 product in the ground vibrational state from the O(1D) + CD4 reaction. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: +86-411-84695174; Fax: +86-411-84675584. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Chinese Academy of Sciences, the National Natural Science Foundation of China, and the Ministry of Science and Technology of China. We would also like to thank Prof. Kopin Liu for many helpful discussions.

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