Potential anisotropy and the lifetime of triatomic collision complexes

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Abstract
In 1984, Schelling and Castleman discussed whether the lifetime of a complex formed in an ion–molecule reaction might depend on the anisotropy of the ion–molecule potential. Their model was that of a rigid rotor with spherical repulsion, but anisotropic polarizability. In this paper, we contend that models employing a rigid diatom are quite unrealistic to discuss complex lifetime, since the latter is predominantly determined by intermediate energy storage into vibration. We use, instead, a model system with three pair potentials of the Morse type and vary the equilibrium distances to generate different anisotropies of the potential. Trajectory calculations show a definite influence of this parameter both on the complex formation cross section and on lifetime. These lifetimes are one order of magnitude longer than those computed with the rigid diatom model, after the calculations have been scaled to the same masses and potential parameters. The lifetimes turn out to be roughly proportional to the period of the fastest normal mode of the intermediate molecule, and their behavior is generally in accord with RRKM theory.