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Figure 1. a) The experimental three dimensional product flux
diagram for the F(2P3/2)+H2(j=0) reaction at the collision energy of
0.52 kcal/mol. b) The theoretically calculated three dimensional
product flux diagram for the F(2P3/2)+H2(j=0) reaction at the collision
energy of 0.52 kcal/mol.
Figure 2. The crossed beam experimental apparatus at
DICP used to study the F+H2(j=0,1) reaction.
Reaction resonance is a transiently trapped transition state along the reaction coordinate in chemical reactions. Reaction resonance has a great impact on the reaction mechanism, reaction rate, product branching ratios and product quantum state distribution etc. It is, however, highly challenging to capture experimentally. In the issue of March 10, 2006, the Science magazine published a combined experimental and theoretical study titled "Observation of Feshbach Resonances in the F+H2->HF+H Reaction" by scientists from CAS Dalian Institute of Chemical Physics and Nanjing University, in collaboration with scientists from University of Colorado at Boulder. The F+H2 ->HF+H reaction is a well known example for reaction resonance, which is also known to be the source for the powerful HF chemical laser. Theoretical predictions of a reaction resonance in the F+H2 reaction were first made in 1970s. In a landmark crossedbeams experiment on the F+H2 reaction by Lee and coworkers, a forward scattering peak for the HF(v'=3) product was clearly observed, and attributed to a reaction resonance. Subsequent theoretical studies using both the quasiclassical trajectory method and the quantum mechanical scattering method on the Stark-Werner PES (SW-PES), however, did not confirm this conjecture.
The research team at Dalian Institute of Chemical Physics conducted recently a crossed molecular beams scattering study with full quantum state resolution on the F+H2->HF+H reaction, using the highly sensitive H atom Rydberg tagging time-of-flight method. Pronounced forward scattered HF products in the v'=2 vibrational state were clearly observed at the collision energy of 0.52 kcal/mol (Fig. 1), which was attributed to both the ground and the first excited Feshbach resonances trapped in the peculiar HF(v'=3)'H'vibrationally adiabatic potential, with significant enhancement by constructive interference between the two resonances, by full quantum dynamical calculations, carried out using an accurate, full potential energy surface constructed for this study, using high level ab initio methods with small empirical corrections. Because of the importance of this work, Science magazine published a perspective in the same issue by Prof. Richard N. Zare, a worldly renowned expert in this field and Wolf Prize winner in chemistry in 2005.
Very recently, full quantum state resolved scattering of the F atom reaction with H2(j=0) and H2(j=1) was investigated by the experimental team. Dramatic difference between the dynamics for the F+H2(j=0,1) reactions at both collision energies have been observed. Forward scattering HF(v'=2) products have been observed unambiguously for the F+H2(j=1) reaction at low collision energies, which was attributed to the Feshbach resonances. This work was recently published in the Journal of Chemical Physics as a communication (J. Chem. Phys. 125, 151102 (2005)). This study provides a textbook example of reaction resonances involving a rotationally excited reagent.
(Quoted from 2007 Annual Report)
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