Recollisions and Correlated Double Ionization with Circularly Polarized Light

It is generally believed that the recollision mechanism of atomic nonsequential double ionization is suppressed in circularly polarized laser fields because the returning electron is unlikely to encounter the core. On the contrary, we find that recollision can and does significantly enhance double ionization, even to the extent of forming a “knee,” the signature of the nonsequential process. Using a classical model, we explain two apparently contradictory experiments, the absence of a knee for helium and its presence for magnesium.

Figure 1

Computed double ionization probabilities for Hamiltonian (1) as a function of the intensity of $I$ for $\omega =0.0584\mathrm{a}.\mathrm{u}.$ (780 nm) and circular polarization, $a=3$, and $b=1$ (magnesium case). The inset displays the same computation for helium ($a=b=1$).

Figure 2

NSDI in magnesium with a laser intensity $I=2\times {10}^{13}\mathrm{W}·{\mathrm{cm}}^{-2}$. The upper panel displays the energy (kinetic energy plus Coulomb interaction with the nucleus) of each electron (red and blue curves) as a function of time, and the green curve represents the energy of the (Coulomb) repulsion between the two electrons. The arrow indicates the moment of recollision. Lower left panel: Position of each electron in the rotating frame. Lower right panel: Position of each electron in the rotating frame during the time interval indicated by the dashed lines in the upper panel. The arrows indicate the positions of the electrons at the recollision and the direction of their motion. In the lower panels, we label the position of the saddle point with a cross.

Figure 3

Poincaré sections of the inner electron reduced model in the 3D space ($x,y,P_{\varphi }=x{p}_y-y{p}_x$). The admissible set is represented by the surface, and Poincaré sections are represented by dots. Lower panel: Sections for a Jacobi constant $\mathcal{K}=-0.659$ which corresponds to the constant for the inner electron in Fig. 2 when it is close to the nucleus and the outer electron is far away. For comparison, we display the Poincaré section associated with this part of the trajectory for the full model (crosses). Upper panel: Poincaré surface for a Jacobi constant $\mathcal{K}=-0.5$, when the ionization channel is open. The arrow represents the jump, from an invariant torus up to the chaotic unbounded area, experienced by the inner electron during NSDI.