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 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.