The influence of vibrational excitation on chemical reaction dynamics is well understood in triatomic reactions, but the multiple modes in larger systems complicate efforts toward the validation of a predictive framework. Although recent experiments support selective vibrational enhancements of reactivities, such studies generally do not properly account for the differing amounts of total energy deposited by the excitation of different modes. By precise tuning of translational energies, we measured the relative efficiencies of vibration and translation in promoting the gas-phase reaction of CHD3 with the Cl atom to form HCl and CD3. Unexpectedly, we observed that C-H stretch excitation is no more effective than an equivalent amount of translational energy in raising the overall reaction efficiency; CD3 bend excitation is only slightly more effective. However, vibrational excitation does have a strong impact on product state and angular distributions, with C-H stretch-excited reactants leading to predominantly forward-scattered, vibrationally excited HCl.

We report a comprehensive study of the quantum-state correlation property of product pairs from reactions of chlorine atoms with both the ground-state and the CH stretch-excited CHD 3. In light of available ab initio theoretical results, this set of experimental data provides a conceptual framework to visualize the energy-flow pattern along the reaction path, to classify the activity of different vibrational modes in a reactive encounter, to gain deeper insight into the concept of vibrational adiabaticity, and to elucidate the intermode coupling in the transition-state region. This exploratory approach not only opens up an avenue to understand polyatomic reaction dynamics, even for motions at the molecular level in the fleeting transition-state region, but it also leads to a generalization of Polanyi's rules to reactions involving a polyatomic molecule. Over the past decades, there has been tremendous progress in experimental characterization of the structure of the transition state, notably by using the spectroscopic probes (2-4). Transition-state spectroscopy experiments performed to date are essentially the half-collision type in which the transition state is directly accessed either through photodetachment of negative ion precursor in a frequency-resolved experiment (3) or by the femtosecond pump-probe, time-resolved approach (4). As elegant and informative as those experiments are, half-collision results, in general, do not depict a full picture of how the reactants transform into the products. One way to think of this is as follows. The basic idea of a typical half-collision experiment is to initiate the reaction at transition state by a photoexcitation process. By virtue of photoabsorption, the total angular momentum, that is, the partial wave or the impact parameter, of the reactive system is then well specified and often limited to the lowest few quantum numbers in a restricted geometry of the Franck-Condon region. Consequently, the half-collision results are greatly simplified and more amenable to theoretical tests. In contrast, a chemical reaction inevitably constitutes the contribution from collisions with a full range of impact parameters and orientations. The resultant wave-interference patterns, arising from the coherent sum of scattering amplitudes of many partial waves, are manifested in the full-collision attribute such as product angular distribution (5, 6), which cannot be readily accounted for by the few-partial-wave, half-collision approach. On the horns of a dilemma, a full-collision experiment usually deals with asymptotic properties of the reaction, thereby rendering direct probes of the fleeting transition state difficult.Here, we propose an approach to delineate the dynamical aspects of the transition state in a full-collision experiment by tracking the energy flow along the reaction path. We previously introduced an experimental method to unfold the state-specific correlation of coincident product pairs in polyatomic reactions (7-9). More recently, we exploited the product pair-correl...

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