Classical dynamics of two-electron atoms at zero energy

We give a complete description of the classical dynamics of two electrons in the Coulomb potential of a positively charged nucleus for total energy E=0 and angular momentum L=0. The effectively four-dimensional phase space can be divided into partitions spanned by the stable and unstable manifold of the Wannier ridge space. We identify a further approximate symmetry by choosing an appropriate Poincare surface of section in this dynamical system. In addition, a dividing surface between the dynamics influenced by the two collinear spaces, the stable Zee space and the strongly chaotic eZe space can be identified. We discuss potential extensions of the binary symbolic dynamics found in collinear two-electron atoms to the noncollinear parts of the phase space for E <= 0.