  Quantum reactive scattering calculations on the vibrational quenching of HD
due to collisions with H were carried out employing an accurate potential
energy surface. The state-to-state cross sections for the chemical reaction HD
($v=1, \ j=0$) + H $\rightarrow$ D + H$_2$ ($v'=0, \ j'$) at collision energies
between 1 and 10,000 cm$^{-1}$ are presented, and a Feshbach resonance in the
low-energy regime, below the reaction barrier, is observed for the first time.
The resonance is attributed to coupling with the vibrationally adiabatic
potential correlating to the $v=1, \ j=1$ level of the HD molecule, and it is
dominated by the contribution from a single partial wave. The properties of the
resonance, such as its dynamic behavior, phase behavior, and lifetime, are
discussed.
