Quantum dynamical theories have progressed to the stage in which state-to-state differential cross sections can now be routinely computed with high accuracy for three-atom systems since the first such calculation was carried out more than 30 years ago for the H + H(2) system. For reactions beyond three atoms, however, highly accurate quantum dynamical calculations of differential cross sections have not been feasible. We have recently developed a quantum wave packet method to compute full-dimensional differential cross sections for four-atom reactions. Here, we report benchmark calculations carried out for the prototypical HD + OH → H(2)O + D reaction on an accurate potential energy surface that yield differential cross sections in excellent agreement with those from a high-resolution, crossed-molecular beam experiment.

Partial wave resonances, quasi-bound resonance states with well-defined rotation in the transition state region of a chemical reaction, play a governing role in reaction dynamics but have eluded direct experimental characterization. Here, we report the observation of individual partial wave resolved resonances in the F + HD --> HF + D reaction by measuring the collision energy-dependent, angle- and state-resolved differential cross section with extremely high resolution, providing a spectroscopic probe to the transition state of F + HD --> HF + D. The agreement of the data with the high-level theoretical calculations confirms the sensitivity of this probe to the subtle quantum mechanical factors guiding this benchmark reaction.

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.