  Quantum scattering calculations are reported for the H+HCl(v,j=0) and
H+DCl(v,j=0) collisions for vibrational levels v=0-2 of the diatoms.
Calculations were performed for incident kinetic energies in the range 10-7 to
10-1 eV, for total angular momentum J=0 and s-wave scattering in the entrance
channel of the collisions. Cross sections and rate coefficients are
characterized by resonance structures due to quasibound states associated with
the formation of the H...HCl and H...DCl van der Waals complexes in the
incident channel. For the H+HCl(v,j=0) collision for v=1,2, reactive scattering
leading to H_2 formation is found to dominate over non-reactive vibrational
quenching in the ultracold regime. Vibrational excitation of HCl from v=0 to
v=2 increases the zero-temperature limiting rate coefficient by about 8 orders
of magnitude.
