Reaction resonances, or transiently stabilized transition-state structures, have proven highly challenging to capture experimentally. Here, we used the highly sensitive H atom Rydberg tagging time-of-flight method to conduct a crossed molecular beam scattering study of the F + H2 --> HF + H reaction with full quantum-state resolution. Pronounced forward-scattered HF products in the v' = 2 vibrational state were clearly observed at a collision energy of 0.52 kcal/mol; this was attributed to both the ground and the first excited Feshbach resonances trapped in the peculiar HF(v' = 3)-H' vibrationally adiabatic potential, with substantial enhancement by constructive interference between the two resonances.

Reaction resonances are transiently trapped quantum states along the reaction coordinate in the transition state region of a chemical reaction that could have profound effects on the dynamics of the reaction. Obtaining an accurate reaction potential that holds these reaction resonance states and eventually modeling quantitatively the reaction resonance dynamics is still a great challenge. Up to now, the only viable way to obtain a resonance potential is through high-level ab initio calculations. Through highly accurate crossed-beam reactive scattering studies on isotope-substituted reactions, the accuracy of the resonance potential could be rigorously tested. Here we report a combined experimental and theoretical study on the resonance-mediated F + HD --> HF + D reaction at the full quantum state resolved level, to probe the resonance potential in this benchmark system. The experimental result shows that isotope substitution has a dramatic effect on the resonance picture of this important system. Theoretical analyses suggest that the full-dimensional FH(2) ground potential surface, which was believed to be accurate in describing the resonance picture of the F + H(2) reaction, is found to be insufficiently accurate in predicting quantitatively the resonance picture for the F + HD --> HF + D reaction. We constructed a global potential energy surface by using the CCSD(T) method that could predict the correct resonance peak positions as well as the dynamics for both F + H(2) --> HF + H and F + HD --> HF + D, providing an accurate resonance potential for this benchmark system with spectroscopic accuracy.

Crossed molecular beam experiments and accurate quantum dynamics calculations have been carried out to address the long standing and intriguing issue of the forward scattering observed in the F ؉ H2 3 HF(v ‫؍‬ 3) ؉ H reaction. Our study reveals that forward scattering in the reaction channel is not caused by Feshbach or dynamical resonances as in the F ؉ H2 3 HF(v ‫؍‬ 2) ؉ H reaction. It is caused predominantly by the slow-down mechanism over the centrifugal barrier in the exit channel, with some small contribution from the shape resonance mechanism in a very small collision energy regime slightly above the HF(v ‫؍‬ 3) threshold. Our analysis also shows that forward scattering caused by dynamical resonances can very likely be accompanied by forward scattering in a different product vibrational state caused by a slow-down mechanism.chemical reaction dynamics ͉ crossed molecular beam experiment ͉ potential energy surface C hemical reactions occur when one reactant collides with another and some rearrangements among reactants take place along a path connecting reactants to products. The path is called the reaction coordinate for a chemical reaction, along which the reactants will go through an intimate region to reach the product side. In a typical chemical reaction with an energetic barrier, no discrete quantum structure could exist along the reaction coordinate. However, quantized states do exist along coordinates perpendicular to the reaction coordinate. For each quantized state, there is an effective, vibrationally adiabatic potential. In certain cases, transiently trapped quantum states could exist on these vibrational adiabatic potentials along the reaction coordinate. Such quasi-bound quantized states along the reaction coordinate in the intimate region of a chemical reaction are normally called dynamical resonances, or reaction resonances. Because reaction resonances are very sensitive to the potential energy surface governing a chemical reaction, they provide possibilities for probing the critical region of the potential energy surface more directly. As a result, reaction dynamics has been a central topic in the study of chemical reaction dynamics in the last few decades (1-4).Probing of dynamical resonances experimentally is essential to the study of the resonances in chemical reactions. A key signature of reaction resonance is the product forward scattering caused by the time delay of the reaction system trapped in quasi-bound resonance states. However, forward scattering in a scattering experiment does not necessarily come from reaction resonances. Recently, Zare and coworkers (5) have attributed the forward scattering in the H ϩ D 2 reaction to a time delay mechanism. In the study of the H ϩ HD system by Harich et al., the forward scattering was attributed to the time delay when the reaction system passes over a specific reaction barrier with little translational speed (6, 7). Therefore, distinguishing which mechanism is causing the time delay and the forward scattering product in a specific reaction h...

Full quantum state resolved scattering of the F atom reaction with H(2)(j=0) and H(2)(j=1) was investigated at the collision energies of 0.19 and 0.56 kcalmol. Dramatic difference between the dynamics for the F+H(2)(j=0,1) reactions at both collision energies have been observed. Forward scattering HF(v(')=2) products have been observed unambiguously for the F+H(2)(j=1) reaction at low collision energies, which was attributed to the Feshbach resonances. This study provides a unique case of reaction resonances involving a rotationally excited reagent.

ABSTRACT:The multiphoton association reaction He þ H þ ! HeH þ in the electronic ground state is investigated using the time-dependent quantum wave packet method. It is shown that the collision pairs He þ H þ in continuum state transfer into m ¼ 0 state and then produce stable molecules HeH þ through emission of two or three photons. The multiphoton transition takes place via intermediate states, and the transfer probability is determined by the collision energy and the intermediate states. The populations of the intermediate states and m ¼ 0 state can be controlled by the laser duration. The three-photon transition is more efficient than the two-photon transition for producing the molecule HeH þ in m ¼ 0 state.

A crossed molecular beams, state-to-state scattering study was carried out on the F+H2HF+H reaction at the collision energy of 5.02 kJ/mol, using the highly sensitive H atom Rydberg tagging time-of-flight method. All the peaks in the TOF spectra can be clearly assigned to the ro-vibrational structures of the HF product. The forward scattering of the HF product at v=3 has been observed. The small forward scattering of the HF product at v=2 has also been detected. Detailed theoretical analysis is required in order to fully understand the dynamical origin of these forward scattering products at this high collision energy.

Articles you may be interested inA new crossed molecular beam apparatus using time-sliced ion velocity imaging technique Rev. Sci. Instrum. 79, 094104 (2008); Molecular-beam intensity and speed ratio are two major limiting factors in many molecular-beam experiments. This article reports a high-intensity, high-speed-ratio, pulsed supersonic fluorine atom beam source using a double-stage discharge beam source. Its performance is indicated by the high-resolution time-of-flight spectrum in the crossed beam experiment of F͑ 2 P͒ + para-H 2 .

In this article, we describe an apparatus in our laboratory for investigating elementary chemical reactions using the high resolution time-of-flight Rydberg tagging method. In this apparatus, we have adopted a rotating source design so that collision energy can be changed for crossed beam studies of chemical reactions. Preliminary results on the HI photodissociation and the F atom reaction with H 2 are reported here. These results suggest that the experimental apparatus is potentially a powerful tool for investigating state-to-state dynamics of elementary chemical reactions.