Li Che

Observation of Feshbach resonances in the F + H2 --> HF + H reaction.

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.

Breakdown of the Born-Oppenheimer approximation in the F+ o-D2 -> DF + D reaction.

The reaction of F with H2 and its isotopomers is the paradigm for an exothermic triatomic abstraction reaction. In a crossed-beam scattering experiment, we determined relative integral and differential cross sections for reaction of the ground F(2P3/2) and excited F*(2P1/2) spin-orbit states with D2 for collision energies of 0.25 to 1.2 kilocalorie/mole. At the lowest collision energy, F* is ∼1.6 times more reactive than F, although reaction of F* is forbidden within the Born-Oppenheimer (BO) approximation. As the collision energy increases, the BO-allowed reaction rapidly dominates. We found excellent agreement between multistate, quantum reactive scattering calculations and both the measured energy dependence of the F*/F reactivity ratio and the differential cross sections. This agreement confirms the fundamental understanding of the factors controlling electronic nonadiabaticity in abstraction reactions.

The Extent of Non–Born-Oppenheimer Coupling in the Reaction of Cl(2P) with para-H2

Elementary triatomic reactions offer a compelling test of our understanding of the extent of electron-nuclear coupling in chemical reactions, which is neglected in the widely applied Born-Oppenheimer (BO) approximation. The BO approximation predicts that in reactions between chlorine (Cl) atoms and molecular hydrogen, the excited spin-orbit state (Cl*) should not participate to a notable extent. We report molecular beam experiments, based on hydrogen-atom Rydberg tagging detection, that reveal only a minor role of Cl*. These results are in excellent agreement with fully quantum-reactive scattering calculations based on two sets of ab initio potential energy surfaces. This study resolves a previous disagreement between theory and experiment and confirms our ability to simulate accurately chemical reactions on multiple potential energy surfaces.

Probing the resonance potential in the F atom reaction with hydrogen deuteride with spectroscopic accuracy

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 FH2 ground potential surface, which was believed to be accurate in describing the resonance picture of the F + H2 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 + H2 → HF + H and F + HD → HF + D, providing an accurate resonance potential for this benchmark system with spectroscopic accuracy.

HF(v′ = 3) forward scattering in the F + H2 reaction: Shape resonance and slow-down mechanism

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 → 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 → 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.

A double-stage pulsed discharge fluorine atom beam source

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 F(P2)+para-H2.

High resolution time-of-flight spectrometer for crossed molecular beam study of elementary chemical reactions

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 H2 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.

High Resolution Crossed Beams Scattering Study of the F+HD?DF+H Reaction

Crossed beams scattering study was carried out on the F+HDDF+H reaction using high-resolution H-atom Rydberg tagging time-of-flight technique. Vibrational state-resolved differential cross sections were measured, with partial rotational state resolution, at eight collision energies in the range of 2.515.60 kJ/mol. Experimental results indicated that the product angular distributions are predominantly backward scattered. As the collision energy increases, the backward scattered peak becomes broader gradually. Dependence of product vibration branching ratios on the collision energy was also determined. The experimental results show that the DF products are highly inverted in the vibrational state distribution and the DF (v = 3) product is the most populated state. Furthermore, the DF (v = 1) product has also been observed at collision energy above 3.97 kJ/mol.