Wave packet calculations for H-2(v(1)=10-14)+H-2(v(2)=0-2): Reaction and dissociation mechanisms

Collision induced dissociation and four center exchange reaction in H-2(v(1 )=high)+H-2(v(2)=low) are studied by means of time-dependent wave packet ca lculations and within a three-degree-of-freedom reduced dimensionality mode l. The role of both-vibrationally excited and vibrationally cold-collision partners is examined varying v(1) between 10 and 14 and v(2) between 0 and 2, respectively. From the analysis of the results, a clear picture of the m ain mechanisms of dissociation and reaction has been obtained, and the regi ons of the potential energy surface most sensitive to the dynamics have bee n identified. In this way, reaction bottlenecks are found to significantly depend on the initial v(1) state, owing to the anharmonicity of these v(1) states near dissociation and the different regions of the potential explore d by the associated wave packets. The topography of such bottlenecks provid e a basis for the existence of tunneling in (v(1)=10,12, v(2)=0-2) reaction s. Regarding the dissociation process, we find that there are two main mech anisms for dissociation; one where the unbroken diatom recoils with respect to dissociated fragments, and the second, where the diatom passes through the dissociated fragments. These mechanisms are responsible of a double pea k observed in some dissociation probabilities. For (v(1)=14, v(2)=0-2) reag ents, new processes appear with non-negligible probabilities: (i) inelastic collision by insertion of the cold diatom into the vibrationally excited o ne and (ii) dissociation of the initially cold diatom. These features, toge ther with the observation of structures in all channel probabilities, sugge st that four-atom complexes are formed during collision. (C) 2001 American Institute of Physics.