DYNAMICS OF ION MOLECULE RECOMBINATION .5. A STUDY OF ENERGY-TRANSFER PATHWAYS

The Li+ + H2O --> Li+ (H2O) association reaction is used as a model system to study translation to rotation (T --> R) and translation to vibration (T --> V) energy transfer pathways which lead to the formation of ion-molecule complexes. Classical trajectories are used to study the association probability vs H2O bend frequency, impact parameter, and rotational temperature. For low and high bend frequencies association only occurs by T --> R energy transfer. For intermediate bend frequencies a T --> V energy transfer pathway becomes available. There is a well-defined peak in the association probability vs bend frequency, which apparently arises from a resonance between the Li+ + H2O relative translational motion and the H2O bend. Energy transfer and association are affected by the orientation of the H2O dipole during the collision. There is considerable structure in an association probability contour map plotted vs impact parameter and rotational temperature. In contrast to ion-molecule capture which is most probable for an impact parameter of zero and a rotational temperature of 0 K, the formation of an ion-molecule complex by energy transfer is most probable for nonzero values of impact parameter and rotational temperature. Both short-range and long-range Li+ (H2O) complexes are formed. The long-range complexes are important at large rotational temperatures and impact parameters. A discussion is given of the sensitivity of the trajectory results to the treatment of the zero point energy motion of H2O.