Vibrational relaxation in atom-exchange reactions: A classical, Monte Carlo, trajectory study

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Abstract
A classical trajectory study has been carried out in order to determine whether vibrationally excited molecules can be relaxed efficiently in collisions when an atom-exchange reaction is possible. Using simple London-Eyring-Polanyi-Sato potentials, calculations have been performed on systems modelled on the collision partners H + H2 (v = 1 and v = 2), H + ClH (v = 1 and 2), and Cl + HCl (v = 1 and 2). The vibrational excitation lowers the threshold and activation energies for reaction in all cases, although the extent of this lowering depends on the particular mass combination. The collisions where the activation energy barrier is not crossed during the reaction are highly adiabatic. In contrast, the diatomic products of collisions in which the energy barrier is crossed possess a wide distribution of vibrational energies and it is clear that these collisions play the major role in vibrational relaxation. The results of the calculations are compared to the rates which have been determined experimentally for de-excitation of HCl (v = 1, 2 and 3) by Cl atoms and of H2 (v = 1) by H atoms.