10h00   Roberto Artuso   (Università dell'Insubria)
Weak chaos: dynamical effects
11h20   Cristel Chandre   (Centre de Physique Théorique)
Simple et double ionisation de l'atome d'Hélium

Résumé du séminaire de R. Artuso

Dynamical systems with mixed phase space generally exhibit peculiar dynamical features due to sticking near integrable structures of typical trajectories. We discuss simple models where the "regular region" collapses to a single (marginal) fixed point. In particular we consider 1d maps of Pomeau-Manneville type, and a class of two-dimensional area preserving maps with a parabolic fixed point.

Résumé du séminaire de C. Chandre

The Helium atom driven by an external pulse exhibits single and double ionizations. Some of the double ionized electrons result from sequential double ionization, others correspond to non-sequential double ionization. The double ionization probability takes the form of a ``knee'' as a function of the intensity of the pulse. Here, we investigate the classical dynamics of this system. Using Lyapunov exponents, periodic orbits, as well as reduced Hamiltonian models, we investigate the mechanisms occurring in phase space which shed light on the single and double ionization mechanisms. We find that although the Helium in the absence of the laser field is extremely chaotic, the mechanisms behind all three kinds of ionization are based on integrable models and explain the ``knee'' observed in experiments and numerical simulations.