Observation of a transition state resonance in the integral cross section of the F+HD reaction

We have studied the reaction F + HD at low collision energies using a combi nation of experimental and theoretical methods. Clear evidence for a reacti ve resonance is found in the integral cross section for the reactive channe l F + HD --> HF + D. Using a crossed molecular beam apparatus, the total re active cross sections for the HF + D and DF + H channels were obtained in t he collision energy range of 0.2-5 kcal/mol. In addition, Doppler profiles were obtained over this range of energies, which provide information about the angularly resolved distribution of final vibrational states. The cross section shows a distinctive steplike feature near 0.5 kcal/mol. Furthermore , the Doppler profiles reveal a dramatic change in the angular distribution of products over a narrow energy range centered at 0.5 kcal/mol. This feat ure is shown to arise from a reactive resonance localized near the transiti on state. Theoretical scattering calculations have been carried out using t he Stark-Werner potential energy surface, which accurately reproduce the sh ape of the resonance feature. A detailed analysis of quantum dynamics using the spectral quantization method reveals that a quantum resonance exists n ear 0.52 kcal/mol, which is localized about the collinear FHD geometry. At collision energies below 1 kcal/mol, the reaction was found to proceed almo st exclusively through resonant tunneling with very little contribution fro m direct, over the barrier, reaction. The properties of the quantum resonan ce, such as the position, lifetime, and partial widths were found to correl ate well with the experimental results and the quantum scattering calculati ons. (C) 2000 American Institute of Physics. [S0021-9606(00)00709-1].