Amplitude and phase reconstruction of electron wave packets for probing ultrafast photoionization dynamics.

Ultrafast atomic processes, such as excitation and ionization occurring on the femtosecond or shorter time scale, were explored by employing attosecond high-harmonic pulses. With the absorption of a suitable high-harmonic photon a He atom was ionized, or resonantly excited with further ionization by absorbing a number of infrared photons. The electron wave packets liberated by the two processes generated an interference containing the information on ultrafast atomic dynamics. The attosecond electron wave packet, including the phase, from the ground state was reconstructed first and, subsequently, that from the 1s3p state was retrieved by applying the holographic technique to the photoelectron spectra comprising the interference between the two ionization paths. The reconstructed electron wave packet revealed details of the ultrafast photoionization dynamics, such as the instantaneous two-photon ionization rate.

Scaling laws for the photoionization cross section of two-electron atoms.

The cross sections for single-electron photoionization in two-electron atoms show fluctuations which decrease in amplitude when approaching the double-ionization threshold. Based on semiclassical closed orbit theory, we show that the algebraic decay of the fluctuations can be characterized in terms of a threshold law sigma proportional to |E|(mu) as E --> 0(-) with exponent mu obtained as a combination of stability exponents of the triple-collision singularity. It differs from Wannier's exponent dominating double-ionization processes. The details of the fluctuations are linked to a set of infinitely unstable classical orbits starting and ending in the nonregularizable triple collision. The findings are compared with quantum calculations for a model system, namely, collinear helium.

Classical dynamics of two-electron atoms near the triple collision.

The classical dynamics of two electrons in the Coulomb potential of an attractive nucleus is chaotic in large parts of the high-dimensional phase space. Quantum spectra of two-electron atoms, however, exhibit structures which clearly hint at the existence of approximate symmetries in this system. In a recent paper [Phys. Rev. Lett. 93, 054302 (2004)], we presented a study of the dynamics near the triple collision as a first step towards uncovering the hidden regularity in the classical dynamics of two electron atoms. The nonregularizable triple collision singularity is a main source of chaos in three body Coulomb problems. Here, we will give a more detailed account of our findings based on a study of the global structure of the stable and unstable manifolds of the triple collision.

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 Poincaré 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< or =0.

Classical dynamics near the triple collision in a three-body Coulomb problem.

We investigate the classical motion of three charged particles with both attractive and repulsive interactions. The triple collision is a main source of chaos in such three-body Coulomb problems. By employing the McGehee scaling technique, we analyze here for the first time in detail the three-body dynamics near the triple collision in 3 degrees of freedom. We reveal surprisingly simple dynamical patterns in large parts of the chaotic phase space. The underlying degree of order in the form of approximate Markov partitions may help in understanding the global structures observed in quantum spectra of two-electron atoms.