Quantum dynamical processes near the energy barrier that separates reactants from products influence the detailed mechanism by which elementary chemical reactions occur. In fact, these processes can change the product scattering behaviour from that expected from simple collision considerations, as seen in the two classical reactions F + H2 → HF + H and H + H2 → H2 + H and their isotopic variants. In the case of the F + HD reaction, the role of a quantized trapped Feshbach resonance state had been directly determined1, confirming previous conclusions2 that Feshbach resonances cause state-specific forward scattering of product molecules. Forward scattering has also been observed in the H + D2 → HD + D reaction3,4 and attributed to a time-delayed mechanism3,5,6,7. But despite extensive experimental8,9,10,11,12 and theoretical13,14,15,16,17,18 investigations, the details of the mechanism remain unclear. Here we present crossed-beam scattering experiments and quantum calculations on the H + HD → H2 + D reaction. We find that the motion of the system along the reaction coordinate slows down as it approaches the top of the reaction barrier, thereby allowing vibrations perpendicular to the reaction coordinate and forward scattering. The reaction thus proceeds, as previously suggested7, through a well-defined ‘quantized bottleneck state’ different from the trapped Feshbach resonance states observed before.
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We thank K. Liu and Y.T. Lee for many helpful discussions. The experimental work was supported mainly by the National Science Council and Academia Sinica of Taiwan, and the theoretical effort was supported by the National Science Foundation of USA, and by the Ministry of Education, Culture, Sports, Science and Technology of Japan. D.D. and X.Y. also acknowledge support for this work at DICP by the Ministry of Science & Technology of China.
The authors declare that they have no competing financial interests.
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Harich, S., Dai, D., Wang, C. et al. Forward scattering due to slow-down of the intermediate in the H + HD → D + H2 reaction. Nature 419, 281–284 (2002) doi:10.1038/nature01068
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