Reaction of Cl with vibrationally excited CH4 and CHD3: State‐to‐state differential cross sections and steric effects for the HCl product

Mode- and bond-selective reaction of Cl(P3∕22) with CH3D: C–H stretch overtone excitation near 6000cm−1

Experiments explore the influence of different C-H stretching eigenstates of CH3D on the reaction of CH3D with Cl(2P3/2). We prepare the mid |110>|0>(A1,E), mid |200>|>0(E), and mid |100>|0> +nu3 +nu5 eigenstates by direct midinfrared absorption near 6000 cm(-1). The vibrationally excited molecules react with photolytic Cl atoms, and we monitor the vibrational states of the CH2D or CH3 radical products by 2+1 resonance enhanced multiphoton ionization. Initial excitation of the |200>|0>(E) state leads to a twofold increase in CH2D products in the vibrational ground state compared to|100>|0> +nu3 +nu5 excitation, indicating mode-selective chemistry in which the C-H stretch motion couples more effectively to the H-atom abstraction coordinate than bend motion. For two eigenstates that differ only in the symmetry of the vibrational wave function, |110>|0>(A1) and |110>|0>(E), the ratio of reaction cross sections is 1.00 +/- 0.05, showing that there is no difference in enhancement of the H-atom abstraction reaction. Molecules with excited local modes corresponding to one quantum of C-H stretch in each of two distinct oscillators react exclusively to form C-H stretch excited CH2D products. Conversely, eigenstates containing stretch excitation in a single C-H oscillator form predominantly ground vibrational state CH2D products. Analyzing the product state yields for reaction of the |110>|0>(A1) state of CH3D yields an enhancement of 20 +/- 4 over the thermal reaction. A local mode description of the vibrational motion along with a spectator model for the reactivity accounts for all of the observed dynamics.

Quasiclassical trajectory study of the Cl+CH4 reaction dynamics on a quadratic configuration interaction with single and double excitation interpolated potential energy surface

An ab initio interpolated potential energy surface (PES) for the Cl+CH(4) reactive system has been constructed using the interpolation method of Collins and co-workers [J. Chem. Phys. 102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); Theor. Chem. Acc. 108, 313 (2002)]. The ab initio calculations have been performed using quadratic configuration interaction with single and double excitation theory to build the PES. A simple scaling all correlation technique has been used to obtain a PES which yields a barrier height and reaction energy in good agreement with high level ab initio calculations and experimental measurements. Using these interpolated PESs, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions has been carried out for the Cl+CH(4) and Cl+CD(4) reactions, and the theoretical results have been compared with the available experimental data. It has been shown that the calculated total reaction cross sections versus collision energy for the Cl+CH(4) and Cl+CD(4) reactions is very sensitive to the barrier height. Besides, due to the zero-point energy (ZPE) leakage of the CH(4) molecule to the reaction coordinate in the quasiclassical trajectory (QCT) calculations, the reaction threshold falls below the barrier height of the PES. The ZPE leakage leads to CH(3) and HCl coproducts with internal energy below its corresponding ZPEs. We have shown that a Gaussian binning (GB) analysis of the trajectories yields excitation functions in somehow better agreement with the experimental determinations. The HCl(v'=0) and DCl(v'=0) rotational distributions are as well very sensitive to the ZPE problem. The GB correction narrows and shifts the rotational distributions to lower values of the rotational quantum numbers. However, the present QCT rotational distributions are still hotter than the experimental distributions. In both reactions the angular distributions shift from backward peaked to sideways peaked as collision energy increases, as seen in the experiments and other theoretical calculations.

The roaming atom pathway in formaldehyde decomposition

We present a detailed experimental and theoretical investigation of formaldehyde photodissociation to H(2) and CO following excitation to the 2(1)4(1) and 2(1)4(3) transitions in S(1). The CO velocity distributions were obtained using dc slice imaging of single CO rotational states (v=0, j(CO)=5-45). These high-resolution measurements reveal the correlated internal state distribution in the H(2) cofragments. The results show that rotationally hot CO (j(CO) approximately 45) is produced in conjunction with vibrationally "cold" H(2) fragments (v=0-5): these products are formed through the well-known skewed transition state and described in detail in the accompanying paper. After excitation of formaldehyde above the threshold for the radical channel (H(2)CO-->H+HCO) we also find formation of rotationally cold CO (j(CO)=5-28) correlated to highly vibrationally excited H(2) (v=6-8). These products are formed through a novel mechanism that involves near dissociation followed by intramolecular H abstraction [D. Townsend et al., Science 306, 1158 (2004)], and that avoids the region of the transition state entirely. The dynamics of this "roaming" mechanism are the focus of this paper. The correlations between the vibrational states of H(2) and rotational states of CO formed following excitation on the 2(1)4(3) transition allow us to determine the relative contribution to molecular products from the roaming atom channel versus the conventional molecular channel.

Potential energy surface, kinetics, and dynamics study of the Cl+CH4→HCl+CH3 reaction

A modified and recalibrated potential energy surface for the gas-phase Cl+CH4-->HCl+CH3 reaction is reported and tested. It is completely symmetric with respect to the permutation of the four methane hydrogen atoms and is calibrated with respect to updated experimental and theoretical stationary point properties and experimental forward thermal rate constants. From the kinetics point of view, the forward and reverse thermal rate constants and the activation energies were calculated using the variational transition-state theory with semiclassical transmission coefficients over a wide temperature range of 150-2500 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of tunneling in this hydrogen abstraction reaction. A dynamics study was also performed on this PES using quasiclassical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found a noticeable internal energy in the coproduct methyl radical, both in the ground-state [CH4 (v=0)] and vibrationally excited [CH4 (v=1)] reactions. This CH3 internal energy was directly precluded in some experiments or oversimplified in previous theoretical studies using pseudotriatomic models. Second, our QCT calculations give HCl rotational distributions slightly hotter than those in experiment, but correctly describing the experimental trend of decreasing the HCl product rotation excitation in going from HCl (v'=0) to HCl (v'=1) for the CH4 (v=1) reaction. Third, the state specific scattering distributions present a reasonable agreement with experiment, although they tend to make the reaction more forward and backward scattered than found experimentally probably because of the hotter rotational distribution and the deficiencies of the QCT methods.

Ab initio and direct quasiclassical-trajectory study of the Cl+CH4→HCl+CH3 reaction

We present an electronic structure and dynamics study of the Cl + CH(4)--> HCl + CH(3) reaction. We have characterized the stationary points of the ground-state potential-energy surface using various electronic structure methods and basis sets. Our best calculations, CCSD(T) extrapolated to the complete basis-set limit based on geometries and harmonic frequencies obtained at the CCSD(T)/aug-cc-pvtz level, are in agreement with the experimental reaction energy and indirect measurements of the barrier height. Using ab initio information, we have reparametrized a semiempirical Hamiltonian so that the predictions of the improved Hamiltonian agree with the higher-level calculations in various regions of the potential-energy surface. This improved semiempirical Hamiltonian is then used to propagate quasiclassical trajectories and characterize the reaction dynamics. The good agreement of the calculated HCl rotational and angular distributions with the experiment indicates that reparametrizing semiempirical Hamiltonians is a promising approach to derive accurate potential-energy surfaces for polyatomic reactions. However, excessive energy leakage from the initial vibrational energy of the CH(4) molecule to the reaction coordinate in the trajectory calculations calls into question the suitability of the standard quasiclassical-trajectory method to describe energy partitioning in polyatomic reactions.

Comparing reactions of H and Cl with C–H stretch-excited CHD3

We report the methyl radical product state distributions for the reactions of H and Cl with CHD3(nu1 = 1,2) at collision energies of 1.53 and 0.18 eV, respectively. Both reactions demonstrate mode selectivity. The resulting state distributions from the H+CHD3(nu1 = 1,2) reactions are well described by a spectator model. The reactions Cl + CHD3(nu1 = 1,2) exhibit similar behavior, but in some aspects the spectator model breaks down. We attribute this breakdown to enhanced intramolecular vibrational redistribution in the Cl + CHD3(nu1 = 1,2) reactions compared to the H + CHD3(nu1 = 1,2) reactions, caused by the interaction of the slower Cl atom with the vibrationally excited CHD3, which is promoted either by its longer collision duration, its stronger coupling, or both.

Direct evidence for nonadiabatic dynamics in atom+polyatom reactions: Crossed-jet laser studies of F+D2O→DF+OD

Quantum-state-resolved reactive-scattering dynamics of F+D(2)O-->DF+OD have been studied at E(c.m.)=5(1) kcal/mol in low-density crossed supersonic jets, exploiting pulsed discharge sources of F atom and laser-induced fluorescence to detect the nascent OD product under single-collision conditions. The product OD is formed exclusively in the v(OD)=0 state with only modest rotational excitation (<E(rot)> =0.50(1) kcal/mol), consistent with the relatively weak coupling of the 18.1(1) kcal/mol reaction exothermicity into "spectator" bond degrees of freedom. The majority of OD products [68(1)%] are found in the ground ((2)Pi(32) (+/-)) spin-orbit state, which adiabatically correlates with reaction over the lowest and only energetically accessible barrier (DeltaE( not equal) approximately 4 kcal/mol). However, 32(1)% of molecules are produced in the excited spin-orbit state ((2)Pi(12) (+/-)), although from a purely adiabatic perspective, this requires passage over a DeltaE( not equal) approximately 25 kcal/mol barrier energetically inaccessible at these collision energies. This provides unambiguous evidence for nonadiabatic surface hopping in F+D(2)O atom abstraction reactions, indicating that reactive-scattering dynamics even in simple atom+polyatom systems is not always isolated on the ground electronic surface. Additionally, the nascent OD rotational states are well fitted by a two-temperature Boltzmann distribution, suggesting correlated branching of the reaction products into the DF(v=2,3) vibrational manifold.

H + CD4 Abstraction Reaction Dynamics: Excitation Function and Angular Distributions

We compare experimental photoloc measurements and quasi-classical trajectory calculations of the integral cross sections, lab-frame speed distributions, and angular distributions associated with the CD3 products of the H + CD4(nu = 0) --> CD3 + HD reaction at collision energies ranging from 0.5 to 3.0 eV. Of the potential energy surfaces (PES) we explored, the direct dynamics calculations using the B3LYP/6-31G** density functional theory PES provide the best agreement with the experimental measurements. This agreement is likely due to the better overall description that B3LYP provides for geometries well removed from the minimum energy path, even though its barrier height is low by approximately 0.2 eV. In contrast to previous theoretical calculations, the angular distributions on this surface show behavior associated with a stripping mechanism, even at collision energies only approximately 0.1 eV above the reaction barrier. Other potential energy surfaces, which include an analytical potential energy surface from Espinosa-García and a direct dynamics calculation based on the MSINDO semiempirical Hamiltonian, are less accurate and predict more rebound dynamics at these energies than is observed. Reparametrization of the MSINDO surface, though yielding better agreement with the experiment, is not sufficient to capture the observed dynamics. The differences between these surfaces are interpreted using an analysis of the opacity functions, where we find that the wider cone of acceptance on the B3LYP surface plays a crucial role in determining the integral cross sections and angular distributions.

Effects of C–H stretch excitation on the H+CH4 reaction

We have investigated the effects of C-H stretching excitation on the H+CH4-->CH3+H2 reaction dynamics using the photo-LOC technique. The CH3 product vibrational state and angular distribution are measured for the reaction of fast H atoms with methane excited in either the antisymmetric stretching fundamental (nu3=1) or first overtone (nu3=2) with a center-of-mass collision energy of Ecoll ranging from 1.52 to 2.20 eV. We find that vibrational excitation of the nu3=1 mode enhances the overall reaction cross section by a factor of 3.0+/-1.5 for Ecoll=1.52 eV, and this enhancement factor is approximately constant over the 1.52-2.20-eV collision energy range. A local-mode description of the CH4 stretching vibration, in which the C-H oscillators are uncoupled, is used to describe the observed state distributions. In this model, the interaction of the incident H atom with either a stretched or an unstretched C-H oscillator determines the vibrational state of the CH3 product. We also compare these results to the similar quantities obtained previously for the Cl+CH4-->CH3+HCl reaction at Ecoll=0.16 eV [Z. H. Kim, H. A. Bechtel, and R. N. Zare, J. Chem. Phys. 117, 3232 (2002); H. A. Bechtel, J. P. Camden, D. J. A. Brown, and R. N. Zare, ibid. 120, 5096 (2004)] in an attempt to elucidate the differences in reactivity for the same initially prepared vibration.

Imaging photon-initiated reactions: A study of the Cl(P3∕22)+CH4→HCl+CH3 reaction

The hydrogen or deuterium atom abstraction reactions between Cl((2)P(3/2)) and methane, or its deuterated analogues CD(4) and CH(2)D(2), have been studied at mean collision energies around 0.34 eV. The experiments were performed in a coexpansion of molecular chlorine and methane in helium, with the atomic Cl reactants generated by polarized laser photodissociation of Cl(2) at 308 nm. The Cl-atom reactants and the methyl radical products were detected using (2+1) resonantly enhanced multiphoton ionization, coupled with velocity-map ion imaging. Analysis of the ion images reveals that in single-beam experiments of this type, careful consideration must be given to the spread of reagent velocities and collision energies. Using the reactions of Cl with CH(4), CD(4), and CH(2)D(2), as examples, it is shown that the data can be fitted well if the reagent motion is correctly described, and the angular scattering distributions can be obtained with confidence. New evidence is also provided that the CD(3) radicals from the Cl+CD(4) reaction possess significant rotational alignment under the conditions of the present study. The results are compared with previous experimental and theoretical works, where these are available.

Imaging a reactive resonance in the Cl+CH4 reaction

The title reaction has been under extensive experimental and theoretical investigations. Presented here is the experimental evidence suggesting a reactive resonance in this reaction-an intriguing possibility that has been hitherto unsuspected. The initial speculation was inferred from theoretical works in the literature, and subsequent confirmation came from the observed pattern of the angular distributions of the HCl(nu'=1)+CH(3)(upsilon=0) product pair, when plotted in the angle-collision energy plane. This characteristic pattern proves particularly incisive and universal in revealing the imprint of reactive resonance in experimental observable. The nature of the proposed resonance in this reaction is also elucidated.

Nonadiabatic dynamics in the CH3+HCl→CH4+Cl(PJ2) reaction

Nonadiabatic dynamics in the title reaction have been investigated by 2+1 REMPI detection of the Cl(2P(3/2)) and Cl*(2P(1/2)) products. Reaction was initiated by photodissociation of CH(3)I at 266 nm within a single expansion of a dilute mixture of CH(3)I and HCl in argon, giving a mean collision energy of 7800 cm(-1) in the center-of-mass frame. Significant production of Cl* was observed, with careful checks made to ensure that no additional photochemical or inelastic scattering sources of Cl* perturbed the measurements. The fraction of the total yield of Cl(2P(J)) atoms formed in the J=1/2 level at this collision energy was 0.150+/-0.024, and must arise from nonadiabatic dynamics because the ground potential energy surface correlates to CH(4)+Cl(2P(3/2)) products.

Vibrationally Controlled Chemistry: Mode- and Bond-Selected Reaction of CH3D with Cl

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH(3)D with Cl, which forms either HCl + CH(2)D or DCl + CH(3). The reaction of CH(3)D molecules with the first overtone of the C-D stretch (2nu(2)) excited selectively breaks the C-D bond, producing CH(3) exclusively. In contrast, excitation of either the symmetric C-H stretch (nu(1)), the antisymmetric C-H stretch (nu(4)), or a combination of antisymmetric stretch and CH(3) umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH(2)D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH(3)D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.

Effect of bending and torsional mode excitation on the reaction Cl+CH4→HCl+CH3

A beam containing CH(4), Cl(2), and He is expanded into a vacuum chamber where CH(4) is prepared via infrared excitation in a combination band consisting of one quantum of excitation each in the bending and torsional modes (nu(2)+nu(4)). The reaction is initiated by fast Cl atoms generated by photolysis of Cl(2) at 355 nm, and the resulting CH(3) and HCl products are detected in a state-specific manner using resonance-enhanced multiphoton ionization (REMPI). By comparing the relative amplitudes of the action spectra of Cl+CH(4)(nu(2)+nu(4)) and Cl+CH(4)(nu(3)) reactions, we determine that the nu(2)+nu(4) mode-driven reaction is at least 15% as reactive as the nu(3) (antisymmetric stretch) mode-driven reaction. The REMPI spectrum of the CH(3) products shows no propensity toward the formation of umbrella bend mode excited methyl radical, CH(3)(nu(2)=1), which is in sharp distinction to the theoretical expectation based on adiabatic correlations between CH(4) and CH(3). The rotational distribution of HCl(v=1) products from the Cl+CH(4)(nu(2)+nu(4)) reaction is hotter than the corresponding distribution from the Cl+CH(4)(nu(3)) reaction, even though the total energies of the two reactions are the same within 4%. An explanation for this enhanced rotational excitation of the HCl product from the Cl+CH(4)(nu(2)+nu(4)) reaction is offered in terms of the projection of the bending motion of the CH(4) reagent onto the rotational motion of the HCl product. The angular distributions of the HCl(nu=0) products from the Cl+CH(4)(nu(2)+nu(4)) reaction are backward scattered, which is in qualitative agreement with theoretical calculation. Overall, nonadiabatic product vibrational correlation and mode specificity of the reaction indicate that either the bending mode or the torsional mode or both modes are strongly coupled to the reaction coordinate.

Ab initiopotential energy surface, variational transition state theory, and quasiclassical trajectory studies of the F+CH4→HF+CH3 reaction

In this work we present a study of the F+CH(4)-->HF+CH(3) reaction (DeltaHdegrees(298 K)=-32.0 kcal mol(-1)) using different methods of the chemical reaction theory. The ground potential energy surface (PES) is characterized using several ab initio methods. Full-dimensional rate constants have been calculated employing the variational transition state theory and using directly ab initio data. A triatomic analytical representation of the ground PES was derived from ab initio points calculated at the second- and fourth-order Møller-Plesset levels with the 6-311+G(2df,2pd) basis set, assuming the CH(3) fragment to be a 15 a.m.u. pseudoatom in the fitting process. This is suggested from experiments that indicate that the methyl group is uncoupled to the reaction coordinate. A dynamics study by means of the quasiclassical trajectory (QCT) method and employing this analytical surface was also carried out. The experimental data available on the HF internal states distributions are reproduced by the QCT results. Very recent experimental information about the reaction stereodynamics is also borne out by our QCT calculations. Comparisons with the benchmark F+H(2) and analogous Cl+CH(4) reactions are established throughout.

Comparing the dynamical effects of symmetric and antisymmetric stretch excitation of methane in the Cl+CH4 reaction

The effects of two nearly isoenergetic C-H stretching motions on the gas-phase reaction of atomic chlorine with methane are examined. First, a 1:4:9 mixture of Cl(2), CH(4), and He is coexpanded into a vacuum chamber. Then, either the antisymmetric stretch (nu(3)=3019 cm(-1)) of CH(4) is prepared by direct infrared absorption or the infrared-inactive symmetric stretch (nu(1)=2917 cm(-1)) of CH(4) is prepared by stimulated Raman pumping. Photolysis of Cl(2) at 355 nm generates fast Cl atoms that initiate the reaction with a collision energy of 1290+/-175 cm(-1) (0.16+/-0.02 eV). Finally, the nascent HCl or CH(3) products are detected state-specifically via resonance enhanced multiphoton ionization and separated by mass in a time-of-flight spectrometer. We find that the rovibrational distributions and state-selected differential cross sections of the HCl and CH(3) products from the two vibrationally excited reactions are nearly indistinguishable. Although Yoon et al. [J. Chem. Phys. 119, 9568 (2003)] report that the reactivities of these two different types of vibrational excitation are quite different, the present results indicate that the reactions of symmetric-stretch excited or antisymmetric-stretch excited methane with atomic chlorine follow closely related product pathways. Approximately 37% of the reaction products are formed in HCl(v=1,J) states with little rotational excitation. At low J states these products are sharply forward scattered, but become almost equally forward and backward scattered at higher J states. The remaining reaction products are formed in HCl(v=0,J) and have more rotational excitation. The HCl(v=0,J) products are predominantly back and side scattered. Measurements of the CH(3) products indicate production of a non-negligible amount of umbrella bend excited methyl radicals primarily in coincidence with the HCl(v=0,J) products. The data are consistent with a model in which the impact parameter governs the scattering dynamics.

State-to-state dynamics of the Cl+CH3OH→HCl+CH2OH reaction

Molecular chlorine, methanol, and helium are co-expanded into a vacuum chamber using a custom designed "late-mixing" nozzle. The title reaction is initiated by photolysis of Cl2 at 355 nm, which generates monoenergetic Cl atoms that react with CH3OH at a collision energy of 1960 +/- 170 cm(-1) (0.24 +/- 0.02 eV). Rovibrational state distributions of the nascent HCl products are obtained via 2 + 1 resonance enhanced multiphoton ionization, center-of-mass scattering distributions are measured by the core-extraction technique, and the average internal energy of the CH3OH co-products is deduced by measuring the spatial anisotropy of the HCl products. The majority (84 +/- 7%) of the HCl reaction products are formed in HCl(v = 0) with an average rotational energy of [Erot] = 390 +/- 70 cm(-1). The remaining 16 +/- 7% are formed in HCl(v = 1) and have an average rotational energy of [Erot] = 190 +/- 30 cm(-1). The HCl(v = 1) products are primarily forward scattered, and they are formed in coincidence with CH2OH products that have little internal energy. In contrast, the HCl(v = 0) products are formed in coincidence with CH2OH products that have significant internal energy. These results indicate that two or more different mechanisms are responsible for the dynamics in the Cl + CH3OH reaction. We suggest that (1) the HCl(v = 1) products are formed primarily from collisions at high impact parameter via a stripping mechanism in which the CH2OH co-products act as spectators, and (2) the HCl(v = 0) products are formed from collisions over a wide range of impact parameters, resulting in both a stripping mechanism and a rebound mechanism in which the CH2OH co-products are active participants. In all cases, the reaction of fast Cl atoms with CH3OH is with the hydrogen atoms on the methyl group, not the hydrogen on the hydroxyl group.

Bond and mode selectivity in the reaction of atomic chlorine with vibrationally excited CH2D2

The title reaction is investigated by co-expanding a mixture of Cl2 and CH2D2 into a vacuum chamber and initiating the reaction by photolyzing Cl2 with linearly polarized 355 nm light. Excitation of the first C-H overtone of CH2D2 leads to a preference for hydrogen abstraction over deuterium abstraction by at least a factor of 20, whereas excitation of the first C-D overtone of CH2D2 reverses this preference by at least a factor of 10. Reactions with CH2D2 prepared in a local mode containing two quanta in one C-H oscillator /2000>- or in a local mode containing one quantum each in two C-H oscillators /1100> lead to products with significantly different rotational, vibrational, and angular distributions, although the vibrational energy for each mode is nearly identical. The Cl+CH2D2/2000>- reaction yields methyl radical products primarily in their ground state, whereas the Cl+CH2D2/1100> reaction yields methyl radical products that are C-H stretch excited. The HCl(v=1) rotational distribution from the Cl+CH2D2/2000>- reaction is significantly hotter than the HCl(v=1) rotational distribution from the Cl+CH2D2/1100> reaction, and the HCl(v=1) differential cross-section (DCS) of the Cl+CH2D2/2000>- reaction is more broadly side scattered than the HCl(v=1) DCS of the Cl+CH2D2/1100> reaction. The results can be explained by a simple spectator model and by noting that the /2000>- mode leads to a wider cone of acceptance for the reaction than the /1100> mode. These measurements represent the first example of mode selectivity observed in a differential cross section, and they demonstrate that vibrational excitation can be used to direct the reaction pathway of the Cl+CH2D2 reaction.

State-specific correlation of coincident product pairs in the F + CD4 reaction

When a chemical reaction forms two molecular products, even if the state-resolved differential cross section (DCS) for each product is obtained individually, the coincident attributes of the coproducts are still lacking. We exploit a method that provides coincidence information by measuring the state-resolved, pair-correlated DCS. Exemplified by the reaction F + CD4 --> DF + CD3, a time-sliced ion velocity imaging technique was used to measure the velocity distribution of a state-selected CD3 product and to reveal the information of the coincident DF in a state-correlated manner. The correlation of different product state pairs shows a striking difference, which opens up a new way to unravel the complexity of a polyatomic reaction.

Crossed-beam studies of neutral reactions: state-specific differential cross sections

Crossed-molecular-beam and laser techniques have enabled experimentalists to measure the state-resolved differential cross sections of elementary chemical reactions. This article reviews recent progress in this area. Particular emphasis is placed on some intriguing physical phenomena associated with a few benchmark reactions and how these measurements help in answering fundamental questions about reaction dynamics. We examine specifically the geometric phase effects in the reaction H + D2, the dynamical resonance phenomenon in F + HD, the unusually large spin-orbit reactivity in Cl((2)P) + H2, the insertion reaction O((1)D) + H2, and the mode-specific reactivity in Cl + CH4(nu). The give-and-take between experiment and theory in unraveling the physical picture of the dynamics is illustrated throughout this review.

State-to-state chemical reaction dynamics in polyatomic systems: case studies

This review illustrates the experimental study of chemical reaction dynamics using methods that select the quantum states and energy of the reactants and determine the quantum states and energy of the products. The focus is reaction dynamics in systems in which at least one of the reactants or products is a polyatomic molecule. The approach taken is to select four prototype reaction systems as case studies to demonstrate the detail of information and insight that can come from such experiments. Thus, the review is selective and neither claims nor attempts to be comprehensive. Reference to and discussion of theoretical reaction dynamics are included where computational results directly connect with the experiments.