Fig. 1. Experimental setup. Electrons (red) and ions (blue) are created in the supersonic gas jet target. The thin copper rings create a homogeneous electric field, the large Helmoltz coils an additional magnetic field. These fields guide the charged particles onto fast time and position sensitive channel plate detectors (Roentdek www.roentdek.com). Time-of-flight and position of impact of each electron-ion pair is recorded in list mode. From this the three dimensional momentum vector of each particle can be calculated. [Media 1]

Fig. 2. Momentum distribution of He ¹⁺ ions. Left: For 85 eV single photon absorption. Right: 1.5 eV (800nm), 220 fsec, 1.4×10¹⁵ W/cm². The polarization vector of the light is horizontal. The photon momentum is vertical. In the left figure the momentum component in the third dimension out of the plane of the figure is restricted to ±0.4a.u., the right panel is integrated over the momenta in the direction out of the plane of the figure

Fig. 3. Momentum distribution of He ²⁺ ions. Left: For 99 eV single photon absorption. Right: 1.5 eV (800nm), 220 fsec, 6.6×10¹⁴ W/cm². The polarization vector is horizontal. The two bottom panels show projections of the distributions on the polarization axis.

Fig. 4. Momentum correlation between the two emitted electrons. Left and middle: Helium, doubly ionized by absorption of one 99eV photon. Left: The horizontal axis shows the momentum components of one electron along the polarization axis, the vertical axis the same momentum component of the corresponding second electron. Middle: Like left, but the momentum components along an axis perpendicular to the polarization is shown. The Quicktime movie (0.7MB) shows the transition between the left and middle distribution. I.e. the axis along which the momentum components are taken is rotating as indicated by the red double arrow. Right: Corresponding figure to the left one but for multi-photon double ionization of Argon by in the focus of a 220 fsec, 800nm laser pulse at peak intensities of 3.8.10¹⁴ W/cm². In all three figures the same sign of the momenta for both electrons means emission to the same half sphere, however the plane which divides the two half spheres is rotated in the middle panel. The data are integrated over the momentum components in the direction perpendicular to the polarization. The color coding shows the differential rate on linear scale in arbitrary units.

Fig. 5. Double ionization of helium by single photon absorption at 99 eV. The electrons have equal energies. One electron is emitted along the red arrow, the angular distribution of the second electron is shown by the data points in a polar plot. The distance from the origin shows the cross section. The blue line shows the location of the node according to selection rule 2 (see text). The dotted red line shows the node according to selection rule 1 (see text). The polarization axis is horizontal. The Quicktime Movie (1.7MB)shows the change of the distribution as the direction of the first electron changes its angle with respect to the polarization axis