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Observation of a reactive resonance in the integral cross section of a six-atom

reaction: F + CHD 3

Jingang Zhou, Jim J. Lin, and Kopin Liu

Citation: The Journal of Chemical Physics 121, 813 (2004); doi: 10.1063/1.1761051

View online: http://dx.doi.org/10.1063/1.1761051

View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/121/2?ver=pdfcov

Published by the AIP Publishing

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Observation of a reactive resonance in the integral cross section

of a six-atom reaction: F

¿

CHD

3

Jingang Zhou, Jim J. Lin,a)and Kopin Liub)

The Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 106

共Received 30 March 2004; accepted 22 April 2004兲

The title reaction was investigated under crossed-beam conditions at collisional energies ranging from about 0.4 to 7.5 kcal/mol. Product velocity distributions were measured by a time-sliced, velocity-map imaging technique to explicitly account for the density-to-flux transformation factors. Both the state-resolved, pair-correlated excitation functions and vibrational branching ratios are presented for the two isotopic product channels. An intriguing resonance tunneling mechanism occurring near the reaction threshold for the HF⫹CD3 product channel is surmized, which echoes

the reactive resonances found previously for the F⫹HD→HF⫹D reaction and more recently for the F⫹CH4 reaction. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1761051兴

I. INTRODUCTION

One of the most fascinating subject in reaction dynamics has been the role of dynamical trapping in the vicinity of the transition state of the reaction.1– 4This trapping can be traced to the existence of reactive or Feshbach resonances,2–5or to the slowing down of motion along the reaction coordinate near the top of an adiabatic barrier—a phenomenon normally associated with the vibrational threshold with large angular momenta in the transition state region.1,6 – 8The underlying physics of these two trapping mechanisms are different共see for example Fig. 2 of Ref. 2兲,8though their distinctions are often blurred. There are a number of well-established theo-retical methods, such as L2-stabilization techniques,9,10 time-delay analysis,11–13 and the spectral quantization method,14 etc., which can be used to decipher the existence of a reac-tive resonance or not. The experimental observation of the resonance is, however, much more challenging. The main obstacle is not merely a technical experimental issue, but rather the lack of the knowledge of a unique and identifiable resonance fingerprint in experimental observables. To state it in simple terms, the dynamics of a full collision reaction will inevitably involve many partial waves corresponding classi-cally to a range of impact parameters. Owing to the short-lived nature of resonance states, any resonance, which al-ways occurs as a rotational progression of many partial waves, will generally be spaced by energies far less than the resonance widths—thus, sometimes referred to as ‘‘broad resonance.’’15 Consequently, even in an experiment with a well-defined collision energy, Ec, the resonance feature could be smeared out over a broad energy range making it difficult to unambiguously identify resonance fingerprints in the integral cross section共ICS兲 measurements.15,16Although this impact parameter averaging will be partly lifted when

the state- and angle-resolved quantity is examined, the afore-mentioned adiabatic threshold effect6 – 8 could also yield structures in this highly resolved quantity,17–22masking the resonance imprints in experimental observations.

At this point in time, the only unequivocal evidence for a reactive resonance in a full collision experiment is that for the F⫹HD→HF⫹D reaction.5,14,23–26In these reports, sev-eral identifiable resonance imprints in experimental observ-ables were suggested and elucidated. Concurrent theoretical simulations and analyses not only confirmed the experimen-tal conjectures, but also provided deeper insights into the nature of this resonance state. For the integral cross sections, a distinct step for Ecⱗ1 kcal/mol was observed in the reac-tive excitation function for the HF⫹D product channel, whereas it is entirely absent for the other DF⫹H channel.14 Anomalous collision energy dependence of the HF vibra-tional branching was also observed. In fact, the state-specific excitation function for HF(v

⫽2) features a step-like

struc-ture span from 0.2 to 1 kcal/mol, while HF(v

⫽3) is

char-acterized by a broad peak starting from its energetic thresh-old⬃1.16 to about 3 kcal/mol.24Two striking signatures for resonance were also identified in the total angular distribu-tion. In the three dimensional plot of␴(␪,Ec)-␪-Ec, a ridge structure is formed at low energy, and highly oscillatory forward-backward peaking appears at higher collision energy.23 In terms of state- and angle-resolved quantity, it was found that in the resonant tunneling regime the HF(v

⫽2, j

) products are characterized by a bimodal rotation dis-tribution which exhibits complicated, fast-evolving angular distributions with respect to both the final j

states and the initial collision energies.25,26All of these are unveiled from a single reaction system. Since every chemical reaction has its own characteristics, it is not clear how general those reso-nance signatures are—namely, will the similar features mani-fest in the other chemical reactions? Or are they just the resonance fingerprints for that particular reaction?

In a recent report the first experimental evidence for de-tecting reactive resonances in a polyatomic reaction was a兲Also at: Department of Applied Chemistry, National Chiao Tung

Univer-sity, Hsinchu, Taiwan 300.

b兲Also at: Department of Chemistry, National Taiwan Normal University,

Taipei, Taiwan 106. Electronic mail: kpliu@gate.sinica.edu.tw

813

0021-9606/2004/121(2)/813/6/$22.00 © 2004 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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suggested.27 Near the reaction threshold of F⫹CH4→HF ⫹CH3, several remarkable observations were uncovered. A

significant reactivity, occurring only near the threshold, for the formation of the high-frequency symmetric stretch mode of the CH3(v1⫽1) product was discovered. Its coincident

co-product is highly state specific of HF(v

⫽2), and the

angular distribution is characterized by a distinct forward scattered peak. Moreover, at low collision energies the con-comitant HF(v

⫽2,j

) states to the vibrational ground state CH3 (v⫽0) product exhibit a bimodal rotation distribution

up to the energetic limit, in sharp contrast to the low rotation excitation observed for this reaction at higher collision ener-gies and for the isotopic reaction of F⫹CD4 →DF⫹CD3.28,29And the angular distributions of those high

j

states of the HF(v

⫽2)⫹CH3(v⫽0) product pairs also

display a clear preference for forward scattering. A resonant tunneling mechanism analogous to the F⫹HD→HF⫹D reaction,23,25in conjunction with a competing intramolecular energy redistribution 共IVR兲 within the transient resonant complex, was proposed to rationalize those anomalies.27

Reported here is the investigation of the F⫹CHD3

reac-tion. Only the ICS aspects will be presented, the detailed analysis of the differential cross section is currently in progress. As will be shown, similar observations in ICS as those found previously for the F⫹HD reaction14,22were dis-covered. We assert the existence of reactive resonances in the title reaction, which echoes our recent claim for the F ⫹CH4 reaction

27 although the resonance in the latter

reac-tion manifests itself in very different dynamical attributes— rather than from the appearance of the excitation function as in the present reaction.

II. EXPERIMENT

The experiment was carried out using a rotating-sources, crossed-beam apparatus. The experimental details and proce-dures have been described previously,30,31therefore, only the relevant features are presented here. A double-skimmed F-atom beam was generated by discharging a F2/rare-gas

mixture at the nozzle of a pulsed valve. Typically, a 5% F2

mixture in Ne or He was used in this work. The measured speeds of the F-atom beams are 1.13 and 1.62 km/s, respec-tively, which in turn can cover two ranges of collisional en-ergies, 0.43– 4.4 and 2.3– 8.2 kcal/mol, for the present reac-tion. Fine tuning of the collision energy was achieved by rotating the source chambers to change the intersection angle of the two molecular beam velocity vectors. A double-skimmed CHD3 beam was generated by expanding neat

CHD3 gas共Cambridge Isotope Inc., 99%兲 through a fast

so-lenoid pulsed valve共Evan-Lavie valve兲.32

The product of methyl radicals was detected by using a (2⫹1) resonance-enhanced multiphoton ionization scheme via the 3 pzintermediate state.28,29,33The Q branch of the 00 0

vibronic transition was used to select the ground vibrational state of CHD2 or CD3, for which the sampling of the

rota-tional states is approximately averaged by scanning the laser frequency back and forth over the Q branch spectral profile. This rotational averaging is weighted by the transition line strengths, and the results will be somewhat different from

those by the partial rotational selection 共i.e., laser frequency fixed兲 in previous studies.28 –30,34

The velocity distribution of the ground vibrational state of the methyl products was measured by a time-sliced, ion-velocity imaging technique.30 The recorded image corre-sponds to the central slice of the velocity Newton sphere, which represents the product velocity distribution as

d3␴/dvxdvydvz. As detailed previously,

31

to normalize the relative signals at different collisional energies, a series of ion images without time slice were also independently re-corded using a sufficiently wide time gate of the detector. This mode of operation requires only several minutes of ac-cumulation for each image, thus minimizes the possible long-term drift problems. The apparent reaction cross sec-tions were then obtained by dividing the total ion counts of the wide-gated images by the relative velocity of the two reactants. A density-to-flux correction30 was then performed for the time-sliced image at each collisional energy to re-cover the true excitation function. Several efforts were taken to make the density-to-flux transformation more robust, in-cluding the use of an artificial laser sheet,30 proper selection of the time-slicing gatewidth,31etc. The errors of raw images from repeated measurements were typically within 5%. Con-sidering the possible uncertainties in the density-to-flux transformation and from the partition of the ring structures, we estimated the overall errors in the reported ICS are within ⫾10%, which is also evidenced from the scatterings of the data points to be presented later.

III. RESULTS AND DISCUSSION

A. Pair-correlated excitation functions— resonance signatures

Exemplified in Figs. 1 and 2 are a few raw ion velocity images of the two isotopic product channels. The images recorded with and without time slicing in a back-to-back manner are presented. By conservations of energy and mo-mentum, the ring-like features in the images of Figs. 1 or 2 can readily be assigned to the coincidently formed vibration states of the HF or DF products, respectively. Clear separa-tions of the vibrational rings allow us to partition the corre-lated vibrational states without ambiguity. At low collisional energies, even the rotational levels of the HF coproducts can be resolved from the velocity images of CD3⫹, as demon-strated in Fig. 1共c兲. The raw images display a slight asym-metry around the initial relative velocity vector that lies hori-zontally in the figures. It reflects the effect of the nonuniform sensitivity of detecting the reactive events. With the density-to-flux correction, the cylindrical symmetry about the rela-tive velocity is recovered. Nonetheless, as is evident from Figs. 1 and 2, even a casual inspection of the raw images can qualitatively reveal the angular distributions of the correlated product pairs.共The relative velocity lies horizontally and the forward direction is to the left.兲 The more detailed analysis of DCSs and possible interpretations are in progress. Figures 1共f兲 and 2共f兲 summarize the collisional energy dependences of the apparent cross sections of the two isotopic channels.

With the density-to-flux correction to each image, the ‘‘true’’ excitation functions of the two isotopic channels can 814 J. Chem. Phys., Vol. 121, No. 2, 8 July 2004 Zhou, Lin, and Liu

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then be obtained31and the results are shown in Fig. 3. Com-pared to the apparent excitation functions shown in Figs. 1共f兲 and 2共f兲, the effects of the density-to-flux correction are dis-cernible. For the HF⫹CD3 channel the effect is noticeable,

albeit relatively minor. On the other hand, the correction fac-tor becomes quite significant for the DF⫹CHD2 channel—

changing the excitation function into an entirely different shape关see Fig. 2共f兲 versus Fig. 3共b兲兴. The two product chan-nels display significantly different features. For the DF ⫹CHD2(v⫽0) product channel, the excitation function rises

almost linearly after the reaction threshold, followed by an intriguing change in slope. Its appearance is somewhat at variance with the previous report on the F⫹CD4→DF

⫹CD3(v⫽0) reaction. 30

As to the HF⫹CD3(v⫽0) channel, the excitation

func-tion is characterized by a distinct finite step at low collisional energies, which followed by an abrupt rise near 1.2 kcal/mol. It is interesting to note that the onset of the finite step in the HF⫹CD3 channel occurs around the same energy as where

the excitation function for the DF⫹CHD2channel displays a

kink in shape. The behavior of the HF⫹CD3 channel,

par-ticularly near the threshold region, is very different from the DF⫹CHD2 channel, as well as from the reactions of F

⫹CH4→HF⫹CH331 and F⫹CD4→DF⫹CD3.33 Instead,

the step-like feature near threshold is strongly reminiscent of

the F⫹HD→HF⫹D reaction,14for which the resonance tun-neling has been shown to be the sole contribution at low collision energies of 0.4 –1.2 kcal/mol.14,25In an early theo-retical investigation of the Cl⫹HCl→ClH⫹Cl reaction, FIG. 1. 共Color兲 Exemplified in 共a兲–共c兲 are the time-sliced raw images for

probing the CD3(v⫽0) state at Ec⫽6.3, 2.0, and 0.97 kcal/mol, respec-tively. A gatewidth of 30 ns was used. Shown in共d兲 and 共e兲 are the same as

共a兲 and 共b兲, respectively, but without time slicing 共230 ns gatewidth兲. The

apparent excitation function for the F⫹CHD3→HF⫹CD3(v⫽0) reaction is

summarized in 共f兲. The symbols are: 䊊 for Ne as the carrier gas of the F-atom beam source, and䊉 for He as the carrier gas. The line is to guide the eyes.

FIG. 2. 共Color兲 Similar to Fig. 1, except for probing the CHD2(v⫽0) state

at Ec⫽6.4, 2.2, and 0.91 kcal/mol, respectively. Note that the apparent cross sections of this isotopic channel of F⫹CHD3→DF⫹CHD2(v⫽0) are

scaled to Fig. 1共f兲 according to their relative signal strengths under other-wise identical conditions.

FIG. 3. Depicted in 共a兲 and 共b兲 are the excitation functions of F⫹CHD3

→HF(v⬘)⫹CD3(v⫽0) and F⫹CHD3→DF(v⬘)⫹CHD2(v⫽0), respec-tively. The vibrational state-specific excitation functions for the HF or DF coproducts are also shown. The lines are to guide the eyes. The relative cross sections of the two isotopic channels have been normalized to each other, except the unknown detection sensitivity factors.

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Schatz et al. also observed the step-like behavior in the inte-gral cross sections and attributed it to the influence of tran-sition state resonances in that reaction.35 In retrospect, the connection from an isolated resonance to a step-like feature, rather than a peak or a dip, in the reactive excitation function is not difficult to comprehend. In the absence of the direct scattering component, the reactivity originates from many resonance partial waves—each is characterized by a resonant Lorentzian-like peak—that form a rotational progression. It is then the overlaps of those Lorentzian profiles giving rise to the step-like feature in ICS.14

Similar mechanism is proposed here. Energetically, the endothermic HF(v

⫽3)⫹CD3(v⫽0) channel becomes

ac-cessible for Ecⲏ1.14 kcal/mol. From the vibrational state resolved excitation functions shown in Fig. 3共a兲, it is clear that once the HF(v

⫽3)⫹CD3(v⫽0) channel opens up

en-ergetically, its formation rises very rapidly—indicating no exit channel barrier, while that of HF(v

⫽2)⫹CD3(v⫽0)

displays small variations. A closer inspection suggests that the energy dependence of thev

⫽2 channel appears to

con-sist of two parts: the resonant scattering component at lower

Ecand predominantly the direct over-the-barrier mechanism for Ecⲏ2 kcal/mol. By comparison, the reactivity to form HF(v

⫽3) in the present reaction is much more significant

than the F⫹HD reaction, for which the HF(v

⫽2) product has a higher yield than HF(v

⫽3) over the entire energy

range of the investigation共0.4–4.6 kcal/mol兲.24Apparently, the substitution of the D atom of the HD reactant by a CD3

moiety makes the reaction more heavy-light-heavy alike, and causes a dramatic change in the vibrational branchings of the newly formed HF products. The exact nature of this differ-ence is, however, unclear: Either from the relative contribu-tions of the resonant mechanism versus the direct scattering component or from the different partial widths of the intri-cate resonance decays into product vibration states, or both. Examining the images for the HF⫹CD3 channel at low

energies关for example, Fig. 1共c兲 for one Ec] revealed that the product velocity distributions are strikingly similar to those of F⫹CH4→HF(v

)⫹CH3(v⫽0) for Ecⱗ1 kcal/mol.27 In both cases, high rotation states, up to the energetic limit, of HF(v

⫽2) are formed with strong preference toward the

forward direction, which is distinctly different from those at higher Ec.28,29,31,34 By analogy to the F⫹CH4 case,27 the resonance state in the present reaction is tentatively assigned as three quanta in the H–F stretching mode and zero quanta for all other modes in the local mode representation.

One of the most surprising and remarkable findings in the studies of the F⫹CH4reaction is the observation of

sym-metric stretching excited CH3 products, the v1⫽1 state,27

near the reaction threshold. Searching for the stretching ex-citations of CD3 and CHD2 products from the present reac-tion failed. This negative result is not totally unexpected from the proposed resonance scenario. The formation of the CH3(v1⫽1)⫹HF(v

⫽2) product pairs from the F⫹CH4

reaction was previously interpreted as a result of the combi-nation of a resonance state formation (F¯H¯CH3, with 3

quanta in the F–H stretch and zeros for all other modes兲 and a restricted IVR process in competition to the resonance de-cay of the transient collision complex.27 Replacing the CH3

moiety by either CD3or CHD2pushes the vibrationally

adia-batic surface, which leads to thev1⫽1 methyl products,

sig-nificantly away from that correlated to the resonance state, thus, reducing the coupling strength between the two adia-batic surfaces and making the competing IVR unfavorable.

Another quantity of considerable interests is the isotopic branching ratio of the two product channels. Unfortunately, the spectroscopic detection efficiencies of CD3(v⫽0) and

CHD2(v⫽0), which depend on the unknown Franck–

Condon factors and the isotope-dependent predissociation of the intermediate states, are difficult to be quantitied. Aside from the uncalibrated detection efficiency factors, the exci-tation functions shown in Fig. 3 have, nonetheless, been scaled according to their relative signal strengths for future comparisons.

B. Correlated vibrational branching ratios

Figure 4 depicts the collisional energy dependences of the correlated vibrational branching ratios of the two isotopic product channels. Both channels exhibit strong preference for high vibrational states of HF and DF, which is expected from either the proposed resonance mechanism24,27 or the direct scattering mechanism of a highly exoergic reaction with an early barrier.36Because of the dominance of the two highest vibrational states for both isotopic channels over the entire energy range of this study, the product vibrational branching ratios are overwhelmed by the behaviors of these two states, forming a mirror image as seen in Fig. 4. The lower vibrational states, i.e., the minor channels of HF(v

⫽1) and DF(v

⫽1), can only be observed at higher colli-sional energies, and their contributions increase slightly as the collisional energy increases. As seen from Figs. 1 and 2, these lower vibrational states are formed preferentially in the backward hemisphere, for which collisions with small im-pact parameters should dominate in a direct reaction. The combination of the collisional energy dependence and the angular distribution will then suggest that the lower HF/DF vibrational states are produced mainly through a direct re-FIG. 4. The correlated vibrational branching ratios as a function of colli-sional energy for the HF(v⬘)⫹CD3(v⫽0) and DF(v⬘)⫹CHD2(v⫽0) product channels in the upper and lower panels, respectively.

816 J. Chem. Phys., Vol. 121, No. 2, 8 July 2004 Zhou, Lin, and Liu

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bound mechanism, for which the line-of-center forces are stronger. This is a general trend observed in the F-atom re-actions with methane and its isotopic variants.28 –31,34

As to the CHD2(v⫽0)⫹DF(v

⫽2) product pair, its

branching ratio for Ec⭓1.5 kcal/mol displays a mild increase as Ecincreases—similar to the behaviors for DF(v

⫽1) and

CD3(v⫽0)⫹HF(v

⫽1). The amount of this increase is at

the expense of the adjacent higher vibrational state, DF(v

⫽3). For Ecⱗ1.2 kcal/mol, however, its branching ratio ex-hibits an opposite energy dependency. And the onset of this low-energy behavior coincides with the abrupt changes in branching ratios for DF(v

⫽3) and DF(v

⫽4).

Interest-ingly, this onset energy is also where the total excitation function shows a change in slopes关see Fig. 3共b兲兴. Energeti-cally, there is nothing special around Ec⬃1.2 kcal/mol for the CHD2⫹DF channel. On the other hand, it is near the

energetic threshold for CD3(v⫽0)⫹HF(v

⫽3) that is,

however, a chemically different channel. The exact origin of those observations, fortuitous or not, is yet unclear at the present time.

C. Correlated energy disposals

Figure 5 shows the collisional energy dependences of the fractional energy disposals of the correlated HF/DF vibra-tional energy, the product translavibra-tional energy, and the rota-tional energy. Both fV and fT are obtained directly from the image analysis, whereas fR is deduced from energy conser-vation, fR⫽1- fV- fT. Since the CD3 or CHD2rotational

dis-tributions are approximately sampled in this study, the quan-tity of fR is best regarded as the sum of the two product rotors. For these averaged quantities, similarity between the two channels is noted. In particular, except near thresholds,

fR always remains small and nearly invariant to collisional energies. As a consequence, the two remaining degrees of freedom, fV and fT, display an anticorrelated behavior.

As shown previously for the F⫹CD4 reaction, the

pro-pensity of ⌬Ec⬃⌬ETholds.37 Similar behavior is seen here

for both product channels at higher collisional energies, which results in a roughly linear rise of fT for Ec

ⲏ2 kcal/mol. Higher vibrational energy disposals are ob-served at lower collisional energies unless the highest vibra-tional levels become energetically closed. For example, as shown in Fig. 3共a兲, ␴HF(v⬘⫽3) drops sharply for Ec

ⱗ2 kcal/mol, while ␴HF(v⬘⫽2) remains about flat. The step

shape of fV(HF) simply reflects the particular shapes of the vibrational state resolved excitation functions. Upon the drop of fV, both fT and fR rise, at different rates, to balance the energy conservation. The rotational distribution of the HF(v

⫽2) products at low collision energies is always quite

broad and bimodal, as can be seen from the raw image shown in Fig. 1共c兲, which then boosts fR to a higher plateau for Ecⱗ1.2 kcal/mol.

As to the DF product channel, fR also rises for Ec

ⱗ1.0 kcal/mol, though to less extent. We note again that it is over the same energy range as where the branching ratio of DF(v

⫽2) displays an unusual energy dependence, as

al-luded to early. Whether this rise in fR comes from a warmer rotational excitation of DF products and if it can also be regarded as a resonance imprint or just a threshold anomaly require deeper scrutiny because of the contributions from both product rotors in this study. Further experiments with either N-state tagged or partially rotational selection of the CHD2 coproduct will be needed to clarify it.

IV. SUMMARY

In this work, several dynamical aspects of the integral cross sections of the title reactions are presented. As eluci-dated here, examining the collisional energy evolutions of those attributes not only provides a global view of dynamics of this particular reaction, but contains rich information that, in conjunction with future theoretical investigations, can deepen our general understanding about chemical reactivity of more complex systems. Perhaps, the most significant con-tribution of the present study is the observation of a step-like structure in the excitation function of the HF⫹CD3 product

channel. In many respects this unusual feature is entirely in analog to that discovered previously for the F⫹HD→HF ⫹D reaction,14 and is suggestive of the existence of a

reac-tive resonance in this six-atom reaction. From the behaviors of the vibrational state-specific excitation functions, it ap-pears that the resonant contribution to the present reaction might be more significant than in the previous F⫹HD reac-tion. Future theoretical confirmation is warranted.

The step-like behavior in reactive excitation function has now been reported in the literature for three chemical reac-tions: F⫹HD,14 the theoretical study of Cl⫹HCl,35 and the present F⫹CHD3 reaction. It appears that the resonance

fin-gerprints can after all survive the impact parameter averag-ings in ICS in favorable cases. And we may very well en-counter more examples in the future than what we have previously conceived.14 –16

ACKNOWLEDGMENTS

The authors thank Li-Chuan Zhou 共Dalian Institute of Chemical Physics兲 for the help in some experiments. This FIG. 5. Summary of the collisional energy dependencies of the fractional

energy disposals of correlated product pairs for 共a兲 HF(v⬘)⫹CD3(v⫽0) and共b兲 DF(v⬘)⫹CHD2(v⫽0) channels. Note that fRis deduced from the

energy conservation relationship, fR⫽1- fV- fT, and contains the rotational

energies of both product rotors in this report.

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work was supported by National Science Council of Taiwan 关NSC 040 共K.L.兲 and NSC 92-2113-M-001-044 共J.J.L.兲兴.

1

D. C. Chatfield, R. S. Friedman, D. W. Schwenke, and D. G. Truhlar, J. Phys. Chem. 96, 2414共1992兲.

2K. Liu, Annu. Rev. Phys. Chem. 52, 139共2001兲.

3F. Fernandez-Alonso and R. N. Zare, Annu. Rev. Phys. Chem. 53, 67

共2002兲.

4S. D. Chao and R. T. Skodje, Theor. Chem. Acc. 108, 273共2002兲. 5K. Liu, R. T. Skodje, and D. E. Manolopoulos, Phys. Chem. Commun. 5,

27共2002兲.

6

S. C. Althorpe, F. Fernandez-Alonso, B. D. Bean, J. D. Ayers, A. E. Pomerantz, R. N. Zare, and E. Wrede, Nature共London兲 416, 67 共2002兲.

7S. A. Harich, D. Dai, C. C. Wang, X. Yang, S. D. Chao, and R. T. Skodje,

Nature共London兲 419, 281 共2002兲.

8D. E. Manolopoulos, Nature共London兲 419, 266 共2002兲. 9

D. E. Manolopoulos, Faraday Discuss. 110, 213共1998兲.

10T. Takayanagi and A. Wada, Chem. Phys. Lett. 348, 514共2001兲. 11J. F. Castillo, D. E. Manolopoulos, K. Stark, and H.-J. Werner, J. Chem.

Phys. 104, 6531共1996兲.

12S. D. Chao and R. T. Skodje, J. Chem. Phys. 119, 1462共2003兲. 13

S. C. Althorpe, J. Phys. Chem. A 107, 7152共2003兲.

14R. T. Skodje, D. Skouteris, D. E. Manolopoulos, S.-H. Lee, F. Dong, and

K. Liu, J. Chem. Phys. 112, 4536共2000兲.

15J. Z. H. Zhang and W. H. Miller, J. Phys. Chem. 95, 12共1991兲. 16

W. H. Miller, Annu. Rev. Phys. Chem. 41, 245共1990兲.

17F. J. Aoiz, V. J. Herrero, and V. Saez Rabanos, J. Chem. Phys. 97, 7423

共1992兲.

18S. D. Chao and R. T. Skodje, Chem. Phys. Lett. 336, 364共2001兲. 19S. C. Althorpe, J. Chem. Phys. 117, 4623共2002兲.

20

F. J. Aoiz, L. Banares, J. F. Castillo, and D. Sokolovski, J. Chem. Phys. 117, 2546共2002兲.

21

D. Sokolovski, Chem. Phys. Lett. 370, 805共2003兲.

22D. Dai, C. C. Wang, S. A. Harich, X. Wang, X. Yang, S. D. Chao, and

R. T. Skodje, Science 300, 1730共2003兲.

23

R. T. Skodje, D. Skouteris, D. E. Manolopoulos, S.-H. Lee, F. Dong, and K. Liu, Phys. Rev. Lett. 85, 1206共2000兲.

24F. Dong, S.-H. Lee, and K. Liu, J. Chem. Phys. 113, 3633共2000兲. 25

S.-H. Lee, F. Dong, and K. Liu, J. Chem. Phys. 116, 7839共2002兲.

26S.-H. Lee, F. Dong, and K. Liu, Faraday Discuss.共in press兲. 27W. Shiu, J. J. Lin, and K. Liu, Phys. Rev. Lett. 92, 103201共2004兲. 28J. J. Lin, J. Zhou, W. Shiu, and K. Liu, Science 300, 966共2003兲. 29

J. Zhou, J. J. Lin, and K. Liu, J. Chem. Phys. 119, 8289共2003兲.

30J. J. Lin, J. Zhou, W. Shiu, and K. Liu, Rev. Sci. Instrum. 74, 2495共2003兲. 31W. Shiu, J. J. Lin, K. Liu, M. Wu, and D. H. Parker, J. Chem. Phys. 120,

117共2004兲.

32

U. Even, J. Jortner, D. Noy, N. Lavie, and C. Cossart-Magos, J. Chem. Phys. 112, 8068共2000兲.

33J. Zhou, J. J. Lin, W. Shiu, S.-C. Pu, and K. Liu, J. Chem. Phys. 119, 2538

共2003兲.

34

J. Zhou, W. Shiu, J. J. Lin, and K. Liu, J. Chem. Phys. 120, 5863共2004兲.

35G. C. Schatz, D. Sokolovski, and J. N. L. Connor, J. Chem. Phys. 94, 4311

共1991兲.

36R. D. Levine and R. B. Bernstein, Molecular Reaction Dynamics and

Chemical Reactivity共Oxford University Press, Oxford 1987兲.

37J. Zhou, J. J. Lin, W. Shiu, and K. Liu, J. Chem. Phys. 119, 4997共2003兲.

818 J. Chem. Phys., Vol. 121, No. 2, 8 July 2004 Zhou, Lin, and Liu

數據

FIG. 3. Depicted in 共a兲 and 共b兲 are the excitation functions of F⫹CHD 3
Figure 4 depicts the collisional energy dependences of the correlated vibrational branching ratios of the two isotopic product channels
Figure 5 shows the collisional energy dependences of the fractional energy disposals of the correlated HF/DF  vibra-tional energy, the product translavibra-tional energy, and the  rota-tional energy

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