title: Chaotic dynamics in classical s-wave helium.
abstract: Classical dynamics of s-wave helium in the case of E<0 is investigated by the geometric method. The ambiguousness of the orbits after two-electron critical collision (TECC) is eliminated by confining the motion to the fundamental domain defined by r1>r2. Scattering orbits are classified into undelayed and delayed (resonant) according to whether they can avoid the recurrence of reaction. A global Smale horseshoe on the surface of section is constructed for the case of Z=2 which implies that the bounded motion is purely chaotic and explains the onset of the chaotic (resonant) scattering when the incident energy is below a threshold. Immediately below the threshold, the probability of resonant scattering increases linearly with the energy difference. Moreover, we found that the permitted code sequences for the scattering orbits at a given incident energy are determined by the principal TECC orbit. Compared with the e-Ze- collinear helium, the s-wave helium is less chaotic and exhibits a more intricate threshold behavior.
authors.affiliation: Department of Astronomy and Applied Physics, Center of Nonlinear Science, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Z Q
authors.initials: ZQ
authors.lastname: Bai
authors.forename: Y
authors.initials: Y
authors.lastname: Gu