By a detailed analysis of the phase portrait of collinear nonreactive (He, H-2(+)) collisions plotted in terms of Poincare surface of section, we show that there exists an intermolecular bottleneck which can be identified with the zero-order separatrix. It is shown that the regular trajectories do not cross the intermolecular bottleneck, but the irregular trajectories do. While a large number of irregular trajectories enter through the flux-in region and exit through the flux-out region of the bottleneck, in conformity with the earlier findings of Davis and Gray for (He, I-2) collisions, a significant number of trajectories enter directly, presumably due to the fact that these trajectories traverse through the product channel of the configuration space although the energy of the system is well below the reaction threshold. This is reinforced by the observation that, with a decrease in energy, a larger number of irregular trajectories enter through the flux-in region. The fractal characteristics of the action-angle plots revealed in our earlier study (J. Chem. Phys. 1991, 95, 4160) are related to the existence of concentric layers of "baseball bat" like structures in phase space. We also identify "islands" in the phase space corresponding to quasiperiodic motion of bound trajectories in the neighborhood of chaotic scattering trajectories.