Observation of a Reactive Rainbow in F+ CH3Dâ CH2D (v= 0)+ HF (v

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Observation of a Reactive Rainbow in F + CHD # CHD(v = 0) + HF(v = 3)? Huilin Pan, and Kopin Liu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07772 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 17, 2016

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The Journal of Physical Chemistry

J. Phys. Chem. A: Manuscript ID jp-2016-077723

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

Huilin Pan1 and Kopin Liu1,2* 1

Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica, P. O. Box 23-166, Taipei,

Taiwan 10617 2

Department of Physics, National Taiwan University, Taipei, Taiwan 10617

＊ (K. L.) Email: [email protected] ; Phone: 886-2-2366-8259

ACS Paragon Plus Environment

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ABSTRACT 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 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 transition state. In a sense the situation bears striking similarity to the more familiar elastic rainbow – thus, coined “reactive rainbow”.


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I.

INTRODUCTION Rainbow scattering is a well-known phenomenon in elastic collisions of particles − a

potential rainbow mediated by the van der Waals (vdW) well in the interaction potential.1 In essence, the interplay of the attractive and repulsive forces deflects the collision trajectories from a range of impact parameters (b) into the same scattering angle θ, resulting in a constructive interference between the scattering amplitudes from different paths and a buildup of the scattering intensity (a bulge) at rainbow angle. Rotationally inelastic rainbow has also been extensively investigated.2-4 In contrast to the elastic scattering where the experimental observable is in θ with the scattering variable in b, there are now two observables, in θ and ∆j (the change of the rotational states), accompanied by two variables, in b and γ (the impact angle of the rotor axis with respect to the initial relative velocity vector k). Thus the rotational rainbow can in principle occur from either the potential well as in the elastic case or the orientation of the rotor (the orientation or stereodynamic rainbow). As the rotational energy transfer necessarily invokes the anisotropic part of interactions, orientation rainbow usually dominates. Indeed, many of the key features of rotational rainbow can be captured by a simple hard-ellipsoid model,3,4 in which the attractive part of potential is neglected and the rotational rainbow arises from trajectories of different initial b andγ accumulating into the same scattering angle or ∆j . Here we report an intriguing observation of a distinct bulge in the scattering angular distribution for a particular product state-pair at specific collision energy (Ec) in the ground-state reaction of F + CH3D. An explanation is proposed for physical insight as to the origin of the structure. We coined this phenomenon the reactive rainbow of potential type. The reaction of Fatom with ground-state methane serves as a benchmark for understanding polyatomic reaction dynamics. Rich experimental information has been accumulated over the past decades. Remarkable advancements on the theoretical front have also been made in recent years. Highly accurate ab initio potential energy surfaces (PES) are now available.5-7 Both quasiclassical trajectory (QCT) and reduced dimensionality quantum dynamics (QD) calculations (mainly on the integral cross section and thermal rate constant) have been carried out8-17 with the results of the ground-state reaction compared reasonably well with the available experiments. The key dynamical attributes from previous investigations are: (1) the vibrational excitations of HF/DF are highly inverted,18 as anticipated by Polanyi’s rules19 for a highly


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exothermic reaction with an early barrier. The rotational excitations of HF/DF are relatively low,20 which can be understood from the kinematic constraint of angular momentum disposal for reactions with the H + LH' mass combination1,21, where H (H') and L denote the heavy and light particles, respectively. (2) The vibrational excitations of methyl products are moderate and primarily in the umbrella mode.22 The rotational excitations are also low with a propensity toward low K sublevels, i.e., in favor of tumbling rotations (K is the projection of the rotational angular momentum onto the molecular symmetry axis).22 (3) The reaction appears to be governed by two microscopic mechanisms: a direct abstraction and a resonance-mediated pathway.23 Experimental signatures for reactive resonances, particularly paramount at low Ec, in both the integral and differential cross sections have been proposed for reactions with CH4, CHD3 and CD4.24-27 Several recent reduced dimensionality QD calculations10-14 supported the experimental claim of resonance. And (4) the pair-correlated (the coincidently formed quantum states of the two departing products) angular distributions are rich in structures and highly sensitive to the product state-pairs and Ec.21,26-28 Such intricate patterns are, however, largely unexplained. This report focuses on one of such intriguing features.

II.

EXPERIMENT The experiment was conducted in a rotatable-source, crossed-beam machine equipped with

a time-sliced velocity-map imaging detector, as described in detail previously.28,29 Only the key features are mentioned here. In short, the F-atom beam was generated by pulsed discharge of supersonically expanded mixture of 5% F2 in Ne. After double-skimming, it crossed with another doubly-skimmed CH3D beam at the center of the ion optics assembly. The rotational temperature of the CH3D beam was estimated to be about 9 K from the measured speed distributions.30 A number of product states CH3(v) and CH2D(v) were detected by a (2+1) resonance-enhanced multiphoton ionization (REMPI) process,22,31 and the state-tagged ion signals were recorded by the imaging detector. We will focus on CH2D(v = 0) in this report. As will be presented below, the chief observation of this work is a peculiar bulge in the angular distribution for the product pair (vHF, vCH2D) = (3, 00) at Ec = 4.3 kcal mol-1. Two issues are relevant to this observation. Firstly, it could be an experimental artifact, for example the spacecharge effects. The REMPI detection in our apparatus was made of a laser sheet (soft-focusing by


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a f = 500 mm cylindrical lens and spatially scanning of probe laser position in a direction (x) orthogonal to the laser propagation axis z with y as the ion time-of-flight axis).29 The resultant large ionization volume enables a higher count rates with minimal space charge complications. The typical signals in this work amounted to about < 50 ions/pulse, and the image features remained unchanged at lower signal count rates. Alternatively, the observed “local distortion” could come from the instrumental problems such as imperfect velocity map and/or distortion of MCP/image detector. Considerable efforts, e.g., by changing the electric fields of the ion optics (so that the ring size became larger or smaller on detector) or rotating both sources while keeping at the same Ec (so that the Newton sphere was rotated with respect to the probe laser and the ions would land at different regions of the detector system), were devoted to ensuring that is not case. Special attention was also paid to locating the center of the images in data analysis. Secondly, since the product rotational states are not resolved in the product images at higher Ec, the question becomes which product rotor, CH2D(00) and/or HF(v = 3), is responsible for the bulge formation. To this end, two modes of image acquisition were performed by either fixing the probe laser wavelength at the peak of the Q-head of the CH2D( 0 00 ) REMPI band near 333.7 nm (λ-peak) or scanning back and forth over the entire band (λ-scan), as illustrated in Figure 1. Two operations amount to detecting a subset of rotational states (presumably mainly the low- jCH

2D

states) and all rotational states of CH2D( 0 0 ), respectively. Comparisons of the results

from the two modes of operations allow us to address the rotational-probe effects.32 After the density-to-flux correction to each image,29,33 the correlated HF state and angular distributions were obtained. Tuning the Ec is achieved by changing the intersection angle of the two molecular beams, and the results at Ec = 0.88, 2.1 and 4.3 kcal mol-1, are reported here.

III.

RESULTS AND DISCUSSION Figure 2 (top) presents three raw images of the CH2D(00) products for the λ-peak probe.

On energetic grounds, the image features can readily be identified as the ro-vibrational states of the HF coproducts. The differential cross sections derived from the analyzed images are shown in the second row by a two-dimensional (uCH2D, θ) representation. In this representation, signals along a straight horizontal line (at a given recoiled speed uCH2D) will be anticipated if the corresponding ring feature in the image is perfectly round. As is seen, this largely holds for all, including those


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rotationally resolved rings, except for the feature near uCH2D ~ 1.0 km s-1 at Ec = 4.3 kcal mol-1, which displays a pattern tilt toward larger recoiled speed for sideway-scattered products. Since the product rotational states are not resolved, measurements were also performed by probing the entire CH2D( 0 00 ) REMPI band in order to pinpoint the roles of two product rotors. The corresponding results are presented in the third and bottom rows, which display again a tilt pattern for Ec = 4.3 kcal mol-1, suggesting the origin of such behavior being primarily associated with the coproducts HF. Figure 3 (top) compares the CH2D(v = 0) product speed distributions derived from the λpeak (for low-jCH2D, in blue) and λ-scan probed (for all-jCH2D, in red) images. (Fig. S1 compares the results from two additional λ-probes at half heights.) An anti-correlated behavior is obvious: The colder jCH

2D

is, the warmer ν HF will be. Such rotational-probe effects have been observed in

the F + CHD325,26 /CD434,35 and Cl + CHD3(v1 = 1)32 reactions. As discussed previously,34,35 this anti-correlated excitation between the rotational motions of methyl products and the vibrational distributions of the hydrogen halide coproducts is mainly kinematic in origin for the H + LH' mass-combination reactions. Interestingly, in terms of the correlation between the two departing rotors jCH2D and jHF of the (2, 00) pair, we noted from the shape of the speed distributions that while a negative one is seen at 0.88 kcal mol-1, a positive one appears for the two higher Ec (see also Fig. S1). This counter behavior suggests a change of reaction mechanisms of stereodynamic nature as Ec evolves. The second and third panels of Fig. 3 contrast the more detailed, angle-resolved speed distributions ─ forward (0o-20o), sideway (80o-100o), and backward (160o-180o) ─ for the λ-peak and λ-scan probes, respectively. Similar trend is noted: the correlated HF vibrational distributions are significantly colder in backwards than those in sideways and forwards, particularly at higher Ec. More striking is the distinct peak shift of the HF(v = 3) features near uCH2D = 1 km s-1 at Ec = 4.3 kcal mol-1 ─ a more vivid view of the tilt pattern displayed in Fig. 2, whereas all other peaks appear invariant to the scattering angles. In particular, a rotational resolved feature (j = 11) at Ec = 0.88 kcal mol-1, which also lies around uCH2D = 1 km s-1, does not exhibit any obvious shift. The pair-correlated HF(v = 3) angular distributions are summarized in Figure 4. Broadly speaking, the total distributions (the black lines) show a pronounced forward-glory feature that becomes sharper with increasing Ec. A bulge near θ = 50o is notable at Ec = 4.3 kcal mol-1. The


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occurrence of state-specific forward peak in a chemical reaction generally suggests either a direct stripping mechanism or a slow-down mechanism.21,23 In the latter case the resultant time-delay can have several causes. Classically, it may originate from the passage of large impact parameter trajectories over the centrifugal-shifted reaction barrier, as first demonstrated in a QCT study of the F + H2 → HF(v = 3) + H reaction.36 Quantum mechanically, tunneling through the centrifugal barrier and/or the presence of a resonance state will further enhance the forward intensity. Hence, mere sighting of a state-specific forward angular distribution may or may not implicate quantum resonance because other mechanisms are possible.23 Suggested by the peak shift of the (vHF, vCH2D) = (3, 00) pair at Ec = 4.3 kcal mol-1 (Fig. 3), each HF-vibrational peak was partitioned into two parts corresponding to the slow and faster speed components (Fig. 3, the dotted lines ─ albeit somewhat arbitrarily). The resultant speeddependent angular distributions are presented in Fig. 4 as the blue and red lines, respectively. At Ec = 0.88 and 2.1 kcal mol-1 the angular distributions of the two speed components are practical identical for HF(v = 3). At Ec = 4.3 kcal mol-1, only the faster component of HF(v = 3) exhibits the bulge near 50o. Similar results were obtained from additional λ-probes at half heights (Fig. S2), clearly demonstrating that the distinct bulge is attributable to the low-jHF products, irrespective of the probed jCH D . The speed partition for HF(v = 3) corresponds roughly to jHF ~ 4−7 for the slow 2

component and jHF ~ 0−3 for the faster. In view of this subtle and minute energetic difference of the two components, it is truly remarkable how sensitive the bulge feature is to the selected jHF states.37 Similarly, Figure 5 presents the Ec-evolution of the correlated HF(v = 2) distributions. The total distributions (the black lines) show richer variations. It starts as forward-backward asymmetric one at low Ec, and gradually turns into a broad and predominantly backward feature at higher Ec. Such Ec-dependent patterns are not unprecedented,21,26-28 yet mechanistic interpretations await future theoretical analysis. In terms of speed-partitioned angular distributions, the faster recoiled products show more backward intensity at 0.88 kcal mol-1. As Ec increases to 2.1 kcal mol-1, the total distribution of HF(v = 2) is dominated by the slow component except in the backward direction. At Ec = 4.3 kcal mol-1, the faster component of HF(v = 2) shifts the peak away from backward direction. Results from additional probes are presented in Fig. S3.


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We propose that the observed bulge for HF(v = 3) at Ec = 4.3 kcal mol-1 is a manifestation of rainbow phenomenon of the potential type. First, the reaction of F + CH3D has a low, early barrier, and the CH2D(00) + HF(v = 3) product pair is nearly thermoneutral (endothermic by 0.72 kcal mol-1). The reduced mass of the products and reactants are similar, 8.90 vs. 8.99 amu. At Ec = 4.3 kcal mol-1 the reaction is dominated by a direct pathway, for which the qualitative correlation between b and θ − larger b for forward and smaller b for backward − should hold, although not necessarily being one-to-one mapping. Moreover, if the reactant rotational angular momentum j is small (for a supersonically expanded beam at Trot ~ 9K) the total angular momentum J can be expressed as J = j' + l' = j + l ≈ l, where l and l' are the initial and final orbital angular momentum, respectively. For a light-atom transfer reaction it has been shown that the kinematic aspect of angular momentum disposal in an A + BC reaction dictates l → l' with little rotational excitation of the AB products.38 In the present polyatomic reaction the CH2D(v = 0) products are rotationally cold and the bulge is indeed sighted only for concomitantly, newly formed low-j state of HF(v = 3). All the above make the reaction of F + CH3D(v = 0) → CH2D(00) + HF(v = 3, low j) elasticlike. Yet, the trajectories must pass the transition state for reaction to occur. Contrary to the elastic collision that samples the (pre-reaction) vdW well, the present reaction also exhibits two additional potential wells: an exit channel vdW well of collinear F-H---CH2D geometry and a vibrationally adiabatic HF(v = 3)••CH2D(00) dynamic well in the vicinity of transition state, supported by the entrance barrier and the endothermicity to HF(v = 3) + CH2D(00). Vibrationally adiabatic analysis of ab initio calculated PES5-7 indicate that the former has a depth 3.0 kcal mol-1 with the FH---C (F---HC) distance of 2.14 Å (0.93 Å), whereas the latter is about 3.1 kcal mol-1 deep with a significantly shorter (longer) FH••C (F••HC) distance of 1.38 Å (1.17 Å).7 Numerous quasi-bound states supported by the dynamic well have been calculated theoretically12,39 and many of them are delocalized over potential wells. Conceivably, reactive trajectories would sample one or more of those wells and be influenced by the attractive forces. The resultant deflection function might display multiple extremes ─ more complicated than usual elastic case. Particularly significant are those trajectories sensing the vibrationally adiabatic dynamic well. Large impact-parameter collisions will be favored for that to happen because the centrifugalshifted barrier in the exit channel will yield a hump along the effective potential, causing the time


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delay and the phase shift of scattering amplitudes quantum mechanically, thus enhancing the influence of the dynamic well. This raises an intriguing prospect that the observed bulge structure, if indeed proven to be rainbow, may open a new window to probe the adiabatically dynamic well that supports reactive resonances. Up to now, all scattering experiments aimed at detecting reactive resonance are performed at low Ec, with the undulatory structures in the integral and/or differential cross sections as the signatures for quasi-bounded metastable states.23,40,41 In contrast, the proposed reactive rainbow signifies at much higher Ec. It may occur, in principle, even if the quasi-bounded resonance states lie below the reactant asymptote, and would be particularly appealing for systems with numerous, unresolved resonance states as the present case. Since large-impact-parameter collisions are the prerequisite for the rainbow occurrence, this might explain the absence of rainbow feature at the two lower Ec’s. Experimental results at more Ec and how the bulge feature evolves with Ec will be reported in the near future. Theoretical prediction of a reactive rainbow in the state-to-state reaction of F + H2(v = 0) → HF(v = 3) + H has recently been made (yet to be confirmed experimentally) using a semiclassical complex angular momentum approach.42,43 Notably this is a nearly thermoneutral channel and the general topography of the PES along the minimum energy path, including the adiabatically dynamic well, is similar to the present one. Experimentally, the term of “rainbowlike” features in pair-correlated angular distributions have previously been noted in several product-pair channels of the F + CHD326 and CD421,28,34,35 reactions at certain Ec’s. But, the kinematic aspect of those pair-channels behaving as elastic-like reactive events remains unrecognized till now. Moreover, the present reaction also yields a number of vibrationally excited methyl products, CH3(v) and CH2D(v). Preliminary experiments indicated that for all thermoneutral (within 1 kcal mol-1) product pairs, the angular bulges were consistently detected at certain Ec’s; otherwise, no bulge was observed. Taken together, it appears that the bulge feature is not unique to the reaction reported here. The above considerations led us to conjecture that the reactive rainbow of potential type might be a general phenomenon when three conditions are met: (1) the reaction exhibits a vibrationally adiabatic well, (2) the reactive scattering behaves elastically for some specific product channels, and (3) at sufficiently high Ec with contributions from large b collisions. This


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type of reactive rainbow is expected to be exquisitely sensitive to the interplay of attractive and repulsive forces in the vicinity of transition state region, analogous to the elastic rainbow to the vdW well.1 The fact that only the correlated low-j states of newly formed HF(v = 3) products exhibit the rainbow signature further suggests that the rainbow structures arise from glancing-type encounters with gentle torque. Thus, it might provide, with proper theoretical framework, an invaluable tool to decode the bond-switching in the wake of a reactive encounter. In that regard, new and innovative way of measuring the (jHF-jCH2D-k'-k) vector correlation near the rainbow angle (admittedly, a daunting task) will be particularly rewarding.37 At present, theoretical investigations into the proposed rainbow structure appear extremely challenging. Not only a highly accurate PES is needed ─ which exists, but, a QD calculation at the level of rovibrationally resolved angular distributions will be necessary to model this quantum phenomenon. Practically, a judiciously chosen reduced-dimensionality approach could be the first step. Future theoretical advances will certainly shed more light on the phenomenon reported here and possibly uncover other types of reactive rainbow.

IV.

SUMMARY A distinct bulge structure was observed in the product angular distribution of the title

reaction. We showed experimentally its occurrence is associated with low j states (j < 3) of the correlated HF(v = 3), irrespective of the probed rotational states of CH2D(v = 0) coproducts. Heuristic considerations, based on the similar observations in previous studies of other isotopically analogous reactions and the theoretical predictions of a simpler F + HD system, led us to propose the observed bulge as a manifestation of rainbow phenomenon. The conditions for its occurrence are conjectured and delineated. It is our hope that the concept and example presented in this work will not only inspire theoreticians to develop necessary frameworks for deeper insights, but also provide experimentalists a template in their future studies of other reactions.

SUPPORTING INFORMATION Figures S1-S3 for additional λ-fixed probes at the half-height of the REMPI peak, references.


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ACKNOWLEDGMENTS We thank C.-C. Lin for assisting with image acquisitions. This work was supported by the Academia Sinica Investigator Award and the Minister of Science and Technology of Taiwan (MOST-105-2113-M-001-019-MY3).

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(23) Liu, K. Quantum Dynamical Resonances in Chemical Reactions: from A + BC to Polyatomic Systems. Adv. Chem. Phys. 2012, 149, 1-46; and references therein. (24) Shiu, W.; Lin, J. J.; Liu, K. Reactive Resonance in a Polyatomic Reaction. Phys. Rev. Lett. 2004, 92, 103201. (25) Zhou, J.; Lin, J. J.; Liu, K. Observation of a Reactive Resonance in the Integral Cross Section of a Six-Atom Reaction: F + CHD3. J. Chem. Phys. 2004, 121, 813-818. (26) Zhou, J.; Lin, J. J.; Liu, K. Deciphering the Nature of the Reactive Resonance in F + CHD3: Correlated Differential Cross-Sections of the Two Isotopic Channels. Mol. Phys. 2010, 108, 957-968. (27) Wang, F.; Liu, K. Experimental Signatures for a Resonance-Mediated Reaction of BendExcited CD4(vb = 1) with Fluorine Atoms. J. Phys. Chem. Lett. 2011, 2, 1421-1425. (28) Lin, J. J.; Zhou, J.; Shiu, W.; Liu, K. State-Specific Correlation of Coincident Product Pairs in the F + CD4 Reaction. Science 2003, 300, 966-969. (29) Lin, J. J.; Zhou, J.; Shiu, W.; Liu, K. Application of Time-Sliced Ion Velocity Imaging to Crossed Molecular Beam Experiments. Rev. Sci. Instrum. 2003, 74, 2495-2500. (30) Wang, F.; Liu, K. Imaging the Effects of Bend-Excitation in the F + CD4(vb = 0, 1) → DF(v) + CD3(v2 = 1, 2) Reactions. J. Phys. Chem. A 2013, 117, 8536-8544. (31) Brum, J. L.; Johnson III, R. D.; Hudgens, J. W. Electronic Spectra of the Heteroisotopic CH2D and CHD2 Radicals by Resonance Enhanced Multiphoton Ionization. J. Chem. Phys. 1993, 98, 3732-3736. (32) Wang, F.; Lin, J.-S.; Cheng, Y.; Liu, K. Vibrational Enhancement Factor of the Cl + CHD3(v1 = 1) Reaction: Rotational-Probe Effects. J. Phys. Chem. Lett. 2013, 4, 323-327. (33) Sonnenfroh, D. M.; Liu, K. Number Density-to-Flux Transformation Revisited: Kinematic Effects in the Use of Laser-Induced Fluorescence for Scattering Experiments. Chem. Phys. Lett. 1991, 176, 183-190. (34) Zhou, J.; Shiu, W.; Lin, J. J.; Liu, K. Rotationally Selected Product Pair Correlation in F + CD4 → DF(v’) + CD3(v = 0, N). J. Chem. Phys. 2004, 120, 5863-5866. (35) Zhou, J.; Shiu, W.; Lin, J. J.; Liu, K. Rotationally Selected Product Pair Correlation: F + CD4 → DF(v’) + CD3(v2 = 0 and 2, N). J. Chem. Phys. 2006, 124, 104309.


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(36) Aoiz, F. J.; Banares, L.; Herrero, V. J.; Rabanos, V. S.; Stark, K.; Werner, H.-J. Classical Dynamics for the F + H2 → HF + H Reaction on a New ab initio Potential Energy Surface. A Direct Comparison with Experiment. Chem. Phys. Lett. 1994, 223, 215-226. (37) In addition to the extent of rotational excitations, we speculated that the reactive rainbow reported here may also be sensitive to the polarization or the sense of rotation (orientation) of the two rotors, jHF and jCH2D, which carries the imprints from a geared (the recoiled HF and CH2D products counter rotating in opposite directions from repulsion) or anti-geared (the two rotating in the same direction from attraction) force acting on the scattered products. (38) Elsum, I. R.; Gordon, R. G.; A Kinematic, Classical Mechanical Theory of Reactive Collisions. J. Chem. Phys. 1982, 76, 3009-3018. (39) Schapers, D.; Manthe, U. Quasi-Bound States of the F·CH4 Complex. J. Phys. Chem. A 2016, 120, 3186-3195. (40) Yang, X.; Zhang, D. H. Dynamical Resonance in the Fluorine Atom Reaction with the Hydrogen Molecule. Acc. Chem. Res. 2008, 41, 981-989. (41) Costes, M.; Naulin, C. Observation of Quantum Dynamical Resonances in Near Cold Inelastic Collisions of Astrophysical Molecules. Chem. Sci. 2016, 7, 2462-2469. (42) Xiahou, C.; Connor, J. N. L. A New Rainbow: Angular Scattering of the F + H2(vi = 0, ji = 0) → FH(vf = 3, jf = 3) + H Reaction. J. Phys. Chem. A 2009, 113, 15298-15306. (43) Xiahou, C.; Connor, J. N. L.; Zhang, D. H. Rainbows and Glories in the Angular Scattering of the State-to-State F + H2 Reaction at Etrans = 0.04088 eV. Phys. Chem. Chem. Phys. 2011, 13, 12981-12997.


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Figure 1. REMPI spectrum of the ground state products CH2D(00) from the reaction of F + CH3D at Ec = 4.61 kcal mol-1. The vertical dashed lines indicate the two-photon frequency range for the λ-scan probe, and the arrows for the other three fixed-λ probes when acquiring the product images. The results from blue- and red-shifted λ1/2 are presented in the Supporting Online Materials.


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Figure 2. The top row displays three raw images of the CH2D(00) products probed at the peak of the REMPI origin band. On energetic grounds, the image feature can be identified as the state-pair of the two products as labeled. At Ec = 0.88 kcal mol-1 the correlated rotation states of HF(v = 2) are partially resolved, as exemplified by the j quantum numbers. The second row displays the image results in a two-dimensional (uCH2D, θ)-representation, which are density-to-flux corrected and weighted with the Jacobian factor (uCH2D)2 by convention. The black horizontal dashed lines are to guide eyes. The lower two rows are as the upper two, except for λ-scan that probes all rotational state of CHD2(00).


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Figure 3. (Top) Comparisons of the product speed distribution for the two different λ-probes at three Ec. For ready comparison the red (λ-scan) and blue (λ-peak) distributions are normalized by the same total areas. The comb in the top-left panel indicates the rotational state of the correlated HF(v = 2, j) products. The very weak (1, 00) features at Ec = 4.3 kcal mol-1 are amplified by tenfold in the inset. Comparisons of the angle-resolved speed distributions for the λ-peak (Middle) and λ-scan (Bottom) probes: “f” denotes forward (0o-20o), “s” for sideway (80o-100o), and “b” for backward (160o-180o). The solid vertical lines encompass the speed range over which the respective HF state-resolved angular distribution is derived. The vertical dotted line separates each vibrational feature into a slow and a faster component for the speed-dependent angular distribution to be presented in Figs. 3 and 4. Note the significant shift of the angle-dependent peaks for the (3, 00) pair at Ec = 4.3 kcal mol-1.


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Figure 4. Correlated angular distributions for HF(v = 3) by two different probes, λ-peak (Upper) and λ-scan (Lower). The black curve is for total and red (blue) curve is for the faster (slow) speed component. The speed partition is indicated by the vertical dotted line in Fig. 2. Note the distinct bulges (the red arrow, in both probes) near the scattering angle of 50o for the faster speed component of HF(v = 3) at Ec = 4.3 kcal mol-1.


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Figure 5. As Fig. 4, except for the HF(v = 2) coproducts.


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TOC


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Fig 1 254x250mm (96 x 96 DPI)



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Fig 3 254x250mm (96 x 96 DPI)



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Fig 5 254x250mm (96 x 96 DPI)



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Observation of a Reactive Rainbow in F+ CH3Dâ CH2D (v= 0)+ HF (v

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