(HE, H-2(- A PHASE PORTRAIT ANALYSIS()) DYNAMICS )

By a detailed analysis of the phase portrait of collinear nonreactive (He, H-2(+)) collisions plotted in terms of Poincare surface of sectio n, we show that there exists an intermolecular bottleneck which can be identified with the zero-order separatrix. It is shown that the regul ar trajectories do not cross the intermolecular bottleneck, but the ir regular trajectories do. While a large number of irregular trajectorie s enter through the flux-in region and exit through the flux-out regio n of the bottleneck, in conformity with the earlier findings of Davis and Gray for (He, I-2) collisions, a significant number of trajectorie s enter directly, presumably due to the fact that these trajectories t raverse through the product channel of the configuration space althoug h the energy of the system is well below the reaction threshold. This is reinforced by the observation that, with a decrease in energy, a la rger number of irregular trajectories enter through the flux-in region . The fractal characteristics of the action-angle plots revealed in ou r earlier study (J. Chem. Phys. 1991, 95, 4160) are related to the exi stence of concentric layers of ''baseball bat'' like structures in pha se space. We also identify ''islands'' in the phase space correspondin g to quasiperiodic motion of bound trajectories in the neighborhood of chaotic scattering trajectories.