Features of chemical reactions at vanishing kineticenergies: The presence of internally “hot” reagents

The reactions between vibrationally and rotationally excited H-2 and D-2 molecules and the F atom are analyzed at ultra-low collision energies usin g the Coupled Channels quantum approach. The aim of this work is to compare the relative efficiency of the reactive scattering event with that of the vibra tional or rotational quenching process in the ultra-cold temperature regime in order to establish general trends, possibly amenable to experiments on this or on more complex systems. We have already compared the rotational de-excitation efficiency with the reactive one in the F+D-2 (nu=0, j=2)-->DF+D reaction [1] and we have seen in that case that rotational de-excitation is more efficient than reaction when going down to ultra-low energies. We are investigatin g here the vibrational excitation case when the internal energy of the molecule becomes large enough to be above the classical barrier, and we are also pre senting new results for the rotationally hot H-2 partner. We find that, with vibrationally "hot" molecules, the reaction becomes more efficient than the re laxation process; while the relative efficiency of such processes when having rotationally hot molecular partners is much more system-dependent.