Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH3, CH2, and CH Fragments.

Under irradiation of a vacuum ultraviolet (VUV) photon, methane dissociates and yields multiple fragments. This photochemical behavior is not only of fundamental importance but also with wide-ranging implications in several branches of science. Despite that and numerous previous investigations, the product channel branching is still under debate, and the underlying dissociation mechanisms remain elusive. In this study, the photofragment imaging technique was exploited for the first time to map out the momentum and anisotropy parameter distributions of the CH3, CH2, and CH fragments at the 118 nm photolysis wavelength (10.48 eV photon energy). In conjunction with previously reported results of the H atom fragment at 121.6 nm (10.2 eV), a complete set of product channel branching in both two-body and three-body fragmentations is accurately determined. We concluded that extensive nonadiabatic transitions partake in the processes with two-body fragmentations accounting for more than 90% of overall photodissociation, for which the channel branching values for CH2 + H2 and CH3 + H are about 0.66 and 0.25, respectively. Careful kinematic analysis enables us to untangle the intertwined triple fragmentations into the CH2(X̃ 3B1 and ã 1A1) + H + H and CH(X2Π) + H + H2 channels and to evidence their underlying sequential (or stepwise) mechanisms. With the aid of electronic correlation and prior theoretical calculations of the potential energy surfaces, the most probable or dominant dissociation pathways are elucidated. Comparisons with fragmentary reports in the literature on various photochemical aspects are also documented, and discrepancies are clarified. This comprehensive study benchmarks the VUV photochemistry of methane and advances our understanding of this important process.

Stretching-mode specificity in the Cl + CH3D(v1-I, v1-II, and v4 = 1; |jK〉) reactions: dependency on the initial |jK〉 selectivity.

The title reactions were studied at a collisional energy of 5.4 kcal mol-1 in a crossed-beam product-imaging experiment. The dynamics attributes of the dominant ground-state CH2D(00) and the accompanied C-D bend-excited CH2D(61) products were imaged in reactions with totally 16 ro-vibrationally selected states of the CH3D(vi, |jK〉) reagents. We found that all three vibrational excitations yielded marked |jK〉-dependent rate-enhancements in forming the (00, 0/1)s product pairs. Furthermore, for a given rotational |jK〉-mode, a vibrational-mode propensity of v4 > v1-I > v1-II in rate promotion and a clear manifestation of the Fermi-phase-induced interference effect of the latter two were observed. Compared to the reactivity of the rotationless state |jK〉 = |00〉, a minute rotational-excitation of all three stretch-excited CH3D(vi = 1) reagents could yield significantly higher reaction rates for the product pair (00, 0)s, but not so for (00, 1)s. The signals in forming the (61, 0)s pair were clearly notable but smaller than that of the ground-state reaction product pair, (00, 0)g. An opposite propensity of v1-II ≈ v1-I > v4 with a milder dependency on the initial |jK〉-states was observed. The angular distributions of the (00, 0)s pairs were nearly identical for all ro-vibrationally excited reagents, displaying the typical trait for a direct abstraction of the rebound mechanism. Similar distributions were found for the (61, 0)s pairs; yet, both pairs deviated substantially from the peripheral feature of the ground-state reaction pair of (00, 0)g. Those of the (00, 1)s pairs in reactions with v4-excitation featured a prominent forward-peaking distribution-suggestive of a time-delayed, resonance-mediated pathway, again with little dependency on the initial |jK〉-states. As for the reactions with the two Fermi-dyads, v1-I and v1-II, albeit showing globally similar distributions to that for v4, a substantial variation with the initial rotational-mode excitation could be discerned in the forward-peaking features. To unravel the impact of the Fermi-phase on the |jK〉-dependent attributes, we adopted a comparative approach by contrasting the observations in reactions with the Fermi-dyad reagents (the superposition states) to those with the pure-state reagents. Remarkable distinctions are unveiled and elucidated with the unexplained results explicitly pointed out, which call for future theoretical investigations for deeper understanding.

Imaging the Mode-Specificity in Cl + CH3D(v1-I, v1-II, v4 = 1; |jK⟩ = |10⟩) → CH2D(41) + HCl(v).

We report a crossed-beam imaging experiment on the title reactions at two collisional energies (Ec) of 5.3 and 10 kcal mol-1. Both the integral cross sections relative to the ground-state reactivity and the differential cross sections were measured and compared. We found that one-quantum excitations of the CH3-stretching vibrations of the CH3D reagent exerted profound mode-specificity in forming the umbrella-mode-excited CH2D(41) products with the vibrational efficacy of v4 > v1-I > v1-II at both Ec values. The concomitantly formed HCl(v) coproducts were vibrationally cold. Interestingly, the branching ratios of (v = 1)/(v = 0) appeared invariant to the initial stretch-modes of excitation at Ec = 5.3 kcal mol-1, yet exhibited a pronounced mode-specific dependency in the order of v1-II > v1-I > v4 at Ec = 10.3 kcal mol-1. This large and Ec-dependent disparity between the two Fermi-coupled reagents, v1-I and v1-II, is particularly significant and could be another facet─in addition to that in the recently reported vibrational enhancement factors─of the Fermi-phase-induced interference effect manifested in the product vibrational branching ratio. The pair-correlated angular distributions (vCH2D, vHCl)s = (41, 0)s in the three stretch-excited reactions were globally alike and resembled that of the ground-state reaction pair (00, 0)g, suggestive of a direct abstraction mechanism of the peripheral type. This is in sharp contrast to all other vibrationally excited pairs of (11, 0)s, (31, 0)s, and (61, 0)s previously reported in the CH2D + HCl isotopic channel, for which both the direct abstraction and a time-delayed resonance pathway partake.

Pair-Correlated Imaging of Cl + CH3D(v4, v1-I, v1-II = 1, |jK⟩) → CH2D(vi) + HCl(v).

The title reactions were studied at a collisional energy of 10.0 kcal mol-1 in a crossed-beam, product-imaging experiment. In terms of integral cross sections, all three CH3-stretching excited CH3D(vCH3 = 1) reagents promote the reactivity in forming the predominant product pair of (vCH2D, vHCl)s = (00, 0/1)s with a prominent mode-propensity of v4 > v1-I > v1-II, where v4 denotes the degenerate mode of CH3 asymmetric stretch and v1-I and v1-II are a pair of Fermi-coupled, symmetric-stretch states. The vibrationally excited CH2D product pairs of (61, 0)s, (11, 0)s, and (31, 0)s appear to be minor channels and display a reverse propensity of v4 < v1-I ≈ v1-II for (61, 0)s, while v4 > v1-I for (11, 0)s. Based on the observed angular distributions, we conjecture that, irrespective of the initial mode of excitation, the (00, 0)s product pair proceeds by a direct abstraction of the peripheral type, whereas the (00,1)s pair is mediated via a resonance pathway. Intriguingly, the angular distributions of the excited product pairs-(61, 0)s, (11, 0)s, and (31, 0)s-are remarkably similar and comprise the traits of both the peripheral mechanism and resonance pathway. Possible interpretation and implication are suggested. In addition, due to the spectral overlap of the REMPI bands and heavily congested image features, a robust data analysis method is developed, which enables us to extract the dynamics attributes of a weak feature buried in the proximate, more intense ones with high fidelity.

Time-Resolved Pair-Correlated Imaging of the Photodissociation of Acetaldehyde at 267 nm: Pathway Partitioning.

Photodissociation of acetaldehyde (CH3CHO) by UV excitation involves interwoven multiple reaction pathways, including nonradiative decay, isomerization, transition-state pathway, roaming, and other dissociation mechanisms. Recently, we employed picosecond time-resolved, pair-correlated product imaging in a study of acetaldehyde photodissociation at 267 nm to disentangle those competing mechanisms and to elucidate the possible roaming pathways (Yang, C. H.; Chem. Sci. 2020, 11, 6423-6430). Here, we complement the pair-correlated product speed distribution of CO(v = 0) at the high-j side of the CO rotational state distribution in the CO + CH4 channel and detail the two-dimensional data analysis of the time-resolved images. As a result, extensive comparisons with other studies can be made and the branching fractions of the previously assigned TScc(S0), non-TScc(S0), and CI(S1/S0) pathways for the CO(v = 0) + CH4 molecular channel are evaluated to be 0.74 ± 0.08, 0.15 ± 0.02, and 0.11 ± 0.02, respectively. Together with the macroscopic branching ratio between the molecular (CO + CH4) and radical (CH3 + HCO) channels at 267 nm from the literature, a global view of the microscopic pathways can then be delineated, which provides invaluable insights and should pave the way for further studies of this interesting system.

Active stereo-control of the Cl + CH4(ν3 = 1) reaction: a three-dimensional perspective.

The transition state in Cl + CH4 is of Cl-H-C collinear geometry. As the reactant CH4 is vibrationally excited by a linearly polarized infrared (IR) light to the antisymmetric-stretching state of ν3 = 1, all four C-H bonds are collectively excited and any one of the H-atoms can be reactive. Yet, a strong alignment of the excited CH4(ν3 = 1), as evidenced from the striking stereo-specificity in the Cl + CH4 reaction, was clearly revealed in a previous, exploratory study. Reported here is the full account of that investigation at two collisional energies of Ec = 4.8 and 2.7 kcal mol-1, using a crossed molecular-beam, product-imaging approach. By active control of the polarization direction of an IR laser under judiciously chosen beam-geometries, a complete set of polarization-dependent differential cross sections is disentangled from the CH3(00) product images. To our surprise, the quantitative results appear nearly identical to those obtained for the isotope-substituted reaction of Cl + CHD3(ν1 = 1) → HCl(ν) + CD3(00). A detailed discussion is presented to elucidate the underlying physics for such an intriguing similarity in stereo-reactivity between a spherical-top and a symmetric-top reactant.

Multifaceted Stereoselectivity in Polyatomic Reactions.

Stereoselectivity or stereorequirement refers to the enhancement of chemical reactivity resulting from the preferential alignment/orientation of the colliding reactants. This concept is deeply embodied in the pre-exponential A-factor of the Arrhenius rate expression or the entropy effect of thermal kinetics in physical chemistry textbooks. To understand its dynamical consequence and seek for its mechanistic origin, two different approaches of either selecting the rotational states of the reactant or aligning/orienting the reactant in the laboratory have traditionally been taken. Due to the experimental challenges, theory is far more advanced than experiment. However, even for the simple atom + diatom reaction, the physical interpretations of the calculated results can sometimes be ambiguous or controversial because of the entangled potential and kinematic factors. In this Feature Article, we try to experimentally tackle the problem for reactions with polyatomic reactants by adopting both approaches in parallel for the same reaction. By comparing the results from the two approaches as well as contrasting them with the analogous reactants-here, a symmetric-top CHD3 versus a spherical-top CH4, deeper physical insights are gained, which paves the road for future studies of complex systems and for establishing a more complete conceptual framework.

Imaging pair-correlated reaction cross sections in F + CH3D(νb = 0, 1) → CH2D(ν4 = 1) + HF(ν).

The title reactions were studied in a crossed-beam experiment at collisional energies (Ec) from 0.5 to 4.7 kcal mol-1. The νb (ν4) vibrational mode denotes the bending (umbrella) motion of the CH3D reactant (CH2D product). Using a time-sliced, velocity-map imaging technique, we extracted the state-specific, pair-correlated integral and differential cross sections. As with other isotopically analogous ground-state reactions, an inverted vibrational population of the HF coproduct was observed. Both the step-like excitation function near the threshold and the oscillatory forward-backward peakings in the Ec-evolution of the two dominant pair-correlated angular distributions at lower Ec suggest a resonance-mediated, time-delay mechanism. As Ec increases, the angular distribution of the HF(ν = 2) product evolves into a smooth and broad swath in the backward hemisphere, indicative of a direct rebound mechanism. One quantum excitation of the bending modes of CH3D(νb = 1) promotes the reaction rate by two- to three-fold up to Ec = 2.1 kcal mol-1. Broadly speaking, all major findings are qualitatively in line with previous results in the reactions of the F atom with other isotopologues. However, the rainbow feature recently observed in the CH2D(00) + HF(ν = 3) product channel is entirely absent. A possible rationale is put forward, which reinforces the previous reactive rainbow conjecture and calls for future theoretical investigations.

Effects of Stretching Excitations in Cl + CH3D( v4, v1-I, v1-II = 1): As the Cl Atom Attacks the Unexcited C-D Bond.

The title reactions were studied at two collisional energies ( Ec) in a crossed-beam product-imaging experiment. We found that all three initial CH stretching excitations suppress the reactivity toward the abstraction of the unexcited D atom. In terms of vibrational suppression factor, σs/σg, the product channels of CH3(00/41) + DCl and CH3(11/31) + DCl show opposite mode-specific trends. However, the angular distributions of both channels are nearly identical to that of the ground-state reaction at the same Ec, regardless of the initial reactant states. Tentatively, we ascribed these two observations to a vibrationally induced narrowing effect of the attack angle near the barrier to reaction. As for the DCl coproduct state distributions, the two channels are distinct but show little mode-specific difference. When CH3(00) is probed, the DCl coproduct peaks at v = 1 are accompanied by substantial rotational excitation, whereas the DCl products associated with CH3(11/31) are both vibrationally and rotationally cold. We attributed the different (correlated) energy disposals to a manifestation of trajectory bifurcations in the postbarrier region, with a vibrationally nonadiabatic pathway leading to CH3(00) + DCl( v = 1) and the other adiabatically to the CH3(11/31) + DCl( v = 0) channel. For both pathways the initial CH stretching excitation acts as a conserved mode by preferentially retaining one quantum of vibrational excitation in the reaction.

Imaging spectroscopy of the missing REMPI bands of methyl radicals: Final touches on all vibrational frequencies of the 3p Rydberg states.

(2 + 1) resonance-enhanced multiphoton ionization (REMPI) detection of methyl radicals, in particular that via the intermediate 3p Rydberg states, has shown to be a powerful method and thus enjoyed a wide range of applications. Methyl has six vibrational modes. Among them-including partially and fully deuterated isotopologs-four out of twenty vibrational frequencies in the intermediate 3p states have so far eluded direct spectroscopic determination. Here, by exploiting the imaging spectroscopy approach to a few judiciously selected chemical reactions, the four long-sought REMPI bands-CHD2(611), CH2D(311), CH2D(511), and CH2D(611)-are discovered, which complete the REMPI identification for probing any vibrational mode of excitation of methyl radical and its isotopologs. These results, in conjunction with those previously reported yet scattered in the literature, are summarized here for ready reference, which should provide all necessary information for further spectral assignments and future studies of chemical dynamics using this versatile REMPI scheme.

Rotational-mode specific effects on the stereo-requirement in the reaction of prealigned-CHD3(v1 = 1; |JK = |10 or |1 ± 1) with the chlorine atom.

Several aspects of the stereo-specific requirement in the title reaction are systematically investigated in a crossed-beam experiment using a time-sliced, velocity-mapped imaging technique. Specifically, we explored (1) the differential steric effect from pre-aligning two different reagent rotational states and (2) the effect from probing different product rotational states. In the reaction with an aligned JK=10 reagent at Ec = 3.2 kcal mol-1, the head-on geometry yields a predominantly backward-scattered CD3(00) + HCl(v = 0) product pair, whereas the side-on approach results in a pronounced sideway-scattered distribution. The alternative CD3(00) + HCl(v = 1) channel exhibits a sharply forward-scattering feature for both the collisional geometries. The branching of the two product channels shows sensitive dependency on the collisional geometries. Probing different rotational states of CD3(00) reveals little variation in pair-correlated angular distributions, yet yields notable effect on the correlated vibrational branching of the HCl(v = 0, 1) coproducts. Similar steric propensities hold at lower collisional energy of 1.3 kcal mol-1. In stark contrast, diminishing steric effects were observed in the reaction with an aligned 1±1 reagent. Such huge differential, K-dependent stereo-requirements are largely attributed to the distinct "shapes" of the two rotational states of the aligned CHD3(v1 = 1) reagents.

Crossed beam polyatomic reaction dynamics: recent advances and new insights.

Over the past ten years or so, great advances in our understanding of the dynamics of elementary (bimolecular) polyatomic reactions in the gas-phase have occurred. This has been made possible by critical improvements (a) in crossed molecular beam (CMB) instruments with rotating mass spectrometric detection and time-of-flight analysis, especially following the implementation of soft ionization (by tunable low energy electrons or vacuum-ultraviolet synchrotron radiation) for product detection with increased sensitivity and universal detection power, and (b) in REMPI-slice velocity map ion imaging (VMI) detection techniques in pulsed CMB experiments for obtaining product pair-correlated information through high-resolution measurements directly in the center of mass system. The improved universal CMB method is permitting us to identify all primary reaction products, characterize their formation dynamics, and determine the branching ratios (BRs) for multichannel non-adiabatic reactions, such as those of ground state oxygen atoms, O(3P), with unsaturated hydrocarbons (alkynes, alkenes, dienes). The improved slice VMI CMB technique is permitting us to explore at an unprecedented level of detail, through pair-correlated measurements, the reaction dynamics of a prototype polyatomic molecule such as CH4 (and isotopologues) in its ground state with a variety of important X radicals such as F, Cl, O, and OH. In this review, we highlight this recent progress in the field of CMB reaction dynamics, with an emphasis on the experimental side, but with the related theoretical work, at the level of state-of-the-art calculations of both the underlying potential energy surfaces and the reaction dynamics, noted throughout. In particular, the focus is (a) on the effect of molecular complexity and structure on product distributions, branching ratios and role of intersystem crossing for the multichannel, addition-elimination reactions of unsaturated hydrocarbons with O atoms, and (b) on the very detailed dynamics of the abstraction reactions of ground-state methane (and isotopologues) with atoms (F, Cl, O) and diatoms (OH), with inclusion of also rotational mode specificity in the vibrationally excited methane reactions.

Vibrational Control of Bimolecular Reactions with Methane by Mode, Bond, and Stereo Selectivity.

Vibrational motions of a polyatomic molecule are multifold and can be as simple as stretches or bends or as complex as concerted motions of many atoms. Different modes of excitation often possess different capacities in driving a bimolecular chemical reaction, with distinct dynamic outcomes. Reactions with vibrationally excited methane and its isotopologs serve as a benchmark for advancing our fundamental understanding of polyatomic reaction dynamics. Here, some recent progress in this area is briefly reviewed. Particular emphasis is placed on the key concepts developed from those studies. The interconnections among mode and bond selectivity, Polanyi's rules, and newly introduced vibrational-induced steric phenomena are highlighted.

Imaging characterization of the rapid adiabatic passage in a source-rotatable, crossed-beam scattering experiment.

In order to achieve a more efficient preparation of a specific ro-vibrationally excited reactant state for reactive scattering experiments, we implemented the rapid adiabatic passage (RAP) scheme to our pulsed crossed-beam machine, using a single-mode, continuous-wave mid-infrared laser. The challenge for this source-rotatable apparatus lies in the non-orthogonal geometry between the molecular beam and the laser propagation directions. As such, the velocity spread of the supersonic beam results in a significantly broader Doppler distribution that needs to be activated for RAP to occur than the conventional orthogonal configuration. In this report, we detail our approach to shifting, locking, and stabilizing the absolute mid-infrared frequency. We exploited the imaging detection technique to characterize the RAP process and to quantify the excitation efficiency. We showed that with appropriate focusing of the IR laser, a nearly complete population transfer can still be achieved in favorable cases. Compared to our previous setup-a pulsed optical parametric oscillator/amplifier in combination with a multipass ring reflector for saturated absorption, the present RAP scheme with a single-pass, continuous-wave laser yields noticeably higher population-transfer efficiency.

Observation of a Reactive Rainbow in F + CH3D → CH2D(v = 0) + HF(v = 3)?

Rainbow structures in the scattering angular distribution play an important role in deepening our understanding about the elastic and rotationally inelastic collisions of atoms/molecules. Reported here is the discovery of a rainbow in a chemical reaction. At Ec = 4.3 kcal mol(-1) one of the correlated product pairs in the F + CH3D reaction, (vHF, vCH2D) = (3, 00), displays a distinct bulge in angular distribution. We showed that the bulge originates predominantly from the low-j states of the HF(v = 3) products. Heuristic considerations led us to propose that such a bulge could be regarded as a signature for rainbow scattering. The underlying mechanism for its occurrence in this nearly thermoneutral product pair is ascribed to a delicate interplay of the attractive and repulsive parts of interactions in the vicinity of the transition state. In a sense, the situation bears striking similarity to the more familiar elastic rainbow, thus coined "reactive rainbow".

Rotational Mode Specificity in Cl + CH4(v3=1,|jNl⟩): Role of Reactant's Vibrational Angular Momentum.

The effect of initial rotational states in the reaction of antisymmetric-excited CH4(v3=1,|jNl⟩) with Cl atom was investigated in a crossed-beam, product-imaging experiment over the collisional energy (Ec) range of 2-5 kcal mol(-1). We found that while the initial rotational excitations exert a noticeable effect on total reactivity, they leave little imprint on the more detailed product-state and angular distributions. This finding echoes the previous conclusion in the analogous Cl + CHD3(v1=1,|NK⟩) reaction. However, the rotational enhancement factor is substantial at low Ec and then becomes insignificant at higher Ec, in contrast to the Cl + CHD3 case. A more intriguing finding is the role of the vibrational angular momentum (l) in promoting the reactivity. A heuristic picture is proposed to rationalize the observations.

Differential steric effects in Cl reactions with aligned CHD3(v1 = 1) by the R(0) and Q(1) transitions. I. Attacking the excited C-H bond.

When a CHD3 molecule is pumped to the C-H stretching-excited state by absorbing a linearly polarized infrared (IR) photon via the R(0) branch of the v1 = 1←0 transition, the rotational angular momentum j of the prepared state jK=10 preferentially lies in a plane perpendicular to the IR polarization axis εIR. By way of contrast, when the Q(1) branch is used, the state of jK=1±1 is prepared with j aligned along the direction of εIR. Reported here is a detailed study of the title reaction by actively controlling the collision geometries under these two IR-excitation schemes at collision energy Ec = 8.6 kcal mol-1, using a crossed molecular beam, product imaging approach. We found that under the R(0) excitation, the polarization-dependent differential cross sections for the HCl(v = 0) + CD3(00) channel can largely be understood by invoking dual reaction mechanisms. The forward-scattered products are most likely mediated by a time-delayed resonance mechanism-as the formation of the HCl(v = 1) + CD3(00) channel, whereas the backward/sideways scattered products are governed by a direct abstraction mechanism. Compared to the previous results at lower Ec of 3.8 kcal mol-1, the sighting of opening-up the attack angle at the transition state of the direct pathway is proposed. Results under the Q(1) excitation are, however, perplexing and bear no obvious correlation to the corresponding ones for the R(0) excitation, defying simple intuitive interpretation. Possible reasons are put forward, which call for theoretical investigations for deeper insights. The results on the alternative isotope channel, DCl + CHD2, will be reported in the following paper.

Differential steric effects in Cl reactions with aligned CHD3(v1 = 1) by the R(0) and Q(1) transitions. II. Abstracting the unexcited D-atoms.

A complete set of four polarization-dependent differential cross sections in the reactions of Cl + aligned-CHD3(v1=1,jK)→DCl(v=0)+CHD2(v1=1) is reported here for two different, rotationally polarized states with j = 1: specifically the jK=10 state prepared via the R(0) excitation and the 1±1 state via Q(1). In stark contrast to the complicated situation of the HCl(v) + CD3(v = 0) channel reported in Paper-I, the stereo-requirement of this isotopic channel for both polarized reactants appears quite straightforward and consistent with a direct rebound mechanism. The extent of steric effects is moderate and relatively smaller than the alternative H-atom abstraction channel. All major findings reported here can qualitatively be understood by first noting that the present reaction invokes abstracting a D-atom, which is the spectator in the IR-excitation process. Next, it is recognized that the directional properties of two polarized states of CHD3(v1=1, jK) should manifest primarily in the IR-excited C-H bond, leaving secondary imprints in the unexcited CD3-moiety. The stereo-specificity of the DCl + CHD2 product channel is further reduced by the fact that the abstraction can occur with any one of the three spatially distinct D-atoms.

Correlated Dynamics of the O((3)P) + CHD3(v=0) Reaction: A Joint Crossed-Beam and Quasiclassical Trajectory Study.

Crossed beam experiments and quasiclassical trajectory computations on an ab initio potential energy surface are performed for the O((3)P) + CHD3(v=0) → OH(v'=0) + CD3(v2=0,2) and OD(v'=0,1) + CHD2(v=0) reactions. Both experiment and theory show that the excitation functions display a concave-up behavior and the angular distributions are backward scattered, indicating a direct rebound mechanism and a tight-bend transition state. The reaction produces mainly ground-state products showing the dominance of a vibrationally adiabatic reaction pathway. The standard histogram binning cannot reproduce the observed vibrational adiabaticity, whereas Gaussian binning gives good agreement with experiment.

Imaging the Effect of Reactant Rotations on the Dynamics of the Cl + CHD3(v1 = 1, |J,K⟩) Reaction.

The effect of the rotational excitations of CHD3(v1 = 1, |J,K⟩) in reaction with the Cl atom was investigated in a crossed-beam, product-imaging experiment over the collisional energy (Ec) range 1-6 kcal mol(-1). It was found that the initial J- and K-selections affect only the total reactivity, not the more detailed product-state and angular distributions-a surprising result that defies conventional wisdom. Although the higher reactivity with increasing J states could be understood as a result of a wider range of attack angles near the barrier, the observed K-propensity of σ|J0⟩ < σ|JJ⟩ for J = 1 at Ec < 2 kcal mol(-1), opposite to that reported previously at higher Ec's, is perplexing and yet to be explained.