Energy sharing and asymmetry parameters for photo double ionization of helium 100 eV above threshold in single-particle and Jacobi coordinates

Abstract

We present a joint experimental and theoretical study of the double differential cross section for the double ionization of helium by absorption of one 179 eV linear polarized photon (100 eV above threshold). We show the energy sharing and the asymmetry parameters in both single particle coordinates and in Jacobi coordinates. An asymmetric sharing of the electron excess energy and a breakup of the electron pair preferentially parallel to the polarization axis is found. This marks a deviation from the propensity rule favoring perpendicular relative electron motion which has been observed to hold at energies closer to the double ionization threshold. * Electronic address: doerner@hsb.uni-frankfurt.de 1 Double ionization of helium by single photon absorption is a benchmark process for the investigation of electron-electron correlation in few-body systems. The three particles final state is completely kinematically determined by a set of 5 linear independent degrees of freedom (the remaining 4 are fixed by momentum and energy conservation). This fivefold differential cross section can be reduced by one degree of freedom by taking the dipole approximation into account. For examination within the dipole approximation it is helpful to integrate over 2 or 3 of the degrees of freedom to obtain doubly or singly differential cross sections. These cross sections give an overview of which regions of this 4 dimensional phase space are most important. The cross section differential in an arbitrary momentum vector k can be described by only two parameters, the cross section differential in energy dσ/dE and the asymmetry parameter β. It can always be expressed in the simple form. Hereǫ is the linear light polarization axis and P 2 the second legendre polynomial. While such cross sections are extremely valuable for the characterization of the process, there has been only three experiments reported [1][2][3].Two coordinate sets are widely used in the literature to describe the momentum configurations: The first set is the electron momenta k 1,2 relative to the center of mass of all particles with the excess energy given by E excess = E 1 + E 2 and E 1,2 = k 2 1,2 /2. The second set is the Jacobi momenta -the center of mass momentum of the electron pair k + = k 1 + k 2 , where − k + is the momentum of the recoiling nucleus, and k − = ( k 1 − k 2 )/2 is the relative momentum of the electrons. The excess energy shared in the corresponding energies readsAtomic units are used throughout. The two coordinate systems suggest two different perspectives on the double ionization process: single electron coordinates describe the escape of each electron from the nuclear potential. Therefore they would be most appropriate if the coupling between the electrons could be treated as perturbation while their motion is mainly governed by the nuclear field and the photon. This can be expected for example at very high photon energies, where one expects [4,5] that one electron absorbs the majority of the photon energy and angular momentum while ...

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