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Two multi-institutional collaborations in Germany have independently confirmed that electrons act cooperatively when intense light liberates two of them from helium and other rare-gas atoms. One experiment, performed at the Max Born Institute in Berlin, studied the double photoionization of neon; the other, performed at the University of Marburg, studied photoionization of helium.
(above) Experimental setup for the photoionization experiment of helium. The helium atoms pass the focus of an ultra-intense laser beam. The pulse duration is around 200 femtoseconds, and the wavelength is 800 nanometers.
(above) Schematic drawing of correlated electron emission in a helium atom. Electrons act cooperatively when a laser light pulse ejects two of them from an atom.
(above) Schematic drawing of the a cooperative-electron behavior known as rescattering: One electron is liberated and accelerated back towards the helium atom by the light field. Both electrons are emitted in a correlated fashion after the scattering event takes place. (Above figures courtesy Harald Giessen, University of Marburg and colleagues.)
(above) Another schematic of the cooperative-behavior scheme known as rescattering. This figure was prepared by the authors of the photoionization experiment on neon, performed at the Max Born Institute in Berlin.The authors of the experiment suggest that their data support this rescattering mechanism.
Like pebbles in a bowl, electrons in an atom are normally at the bottom of an "energy well" which keeps them inside an atom. But an ultrashort laser pulse that passes through the atom can provide the energy to eject these electrons, in effect, providing enough energy for them to roll out of the bowl. The yellow wave represents the electric field associated with an ultrashort, intense laser pulse. The blue surfaces represent the energy wells for the two electrons (red balls) at various stages during which the electric field passes through.
When the laser pulse first hits the atom, the electrons first experience the electric field at a maximum value (leftmost energy well). This removes one of the electrons from the atom. As the pulse passes through the atom, the electric field briefly reaches a zero value (second energy well from left), and then the electric has a maximum value in the reverse direction (third energy well from left). According to the rescattering scenario, this reversed electric field pushes the ejected electron back to the atom. Finally, in the last energy well on the right, this ejected electron knocks out another one of its comrades, and both make their escape from the atomic prison. (Figure courtesy Robert Moshammer, University of Freiburg.)
These experiments were performed by two independent collaborations in Germany at the University of Marburg and at the Max-Born-Institute Berlin. For more information, contact Robert Moshammer, University of Freiburg, moshammer@physik.uni-freiburg.de, Horst Rottke, Max Born Intitut, Berlin, rottke@mbi-berlin.de), and Reinhard Doerner, University of Frankfurt, doerner@hsb.uni-frankfurt.de; Harald Giessen, University of Marburg, giessen@mailer.uni-marburg.de
reported by: Weber et al.and Moshammer et al., in the 17 January Physical Review Letters