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Circular Dichorism in Helium photo double ionization experiment performed by the in collaboration with Y. Azuma at the Photon factory and COLTRIMS collaboration at ALS:


Berakdar and Klar have first predicted that the angular distribution of the electrons after photodouble ionisation of Helium will depend on the helicity of the light, even so the initial state is sperical symmetric. The helical geometry is in this case created by the detection of the two electron momentum vectors and the incoming photon direction.

Two electron continuum after double ionization of Helium 20 eV above threshold. The direction of one of the electrons is indicated by the arrow, the momentum distribution of the other electron is indicated in color. For linear light (left panel) the data are integrated over all direction of the polarization with respect to the plane shown, which is defined by the two electron momenta, for left and right cirular polarized light (middle and right panel) the plane of the figure is perpendicular to the light propagation. The inner circle indicates the locus of events of equal energy sharing. The node for back to back emission is seen on this circle. For more information compare achlerjpb

The experiments have been performed at Hasylab, ESRF, the photon factory and within the COLTRIMS-collaboration at the Advanced Light Source ALS of LBNL Berkeley.

Double ionization of Helium 20 eV above threshold. The photon direction is into the plane of the graph, the energy of the electron given by the arrow is indcated in the inset. Details can be found in Achlerjpb. Note that the up down asymmetry in the figure, i.e. the circular dichroism, vanishes if both electrons have 10eV. In this case the they are both indistinguishable and do not allow to define a handedness.

At present experiments at 100eV and 450eV above threshold are in preparation.