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Abstract
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 + H(2)
--> HF + H and H + H(2) --> H(2) + 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 determined, confirming previous conclusions that Feshbach resonances cause
state-specific forward scattering of product molecules. Forward scattering has also
been observed in the H + D(2) --> HD + D reaction and attributed to a time-delayed
mechanism. But despite extensive experimental and theoretical investigations, the
details of the mechanism remain unclear. Here we present crossed-beam scattering experiments
and quantum calculations on the H + HD --> H(2) + 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 suggested, through
a well-defined 'quantized bottleneck state' different from the trapped Feshbach resonance
states observed before.