Research Papers in Physics and Astronomy


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Published by American Institute of Physics; CP811 Ionization, Correlation, and Polarization in Atomic Collisions, edited by A. Lahmam-Bennani and B. Lohmann 0 2006 American Institute of Physics 0-7354-0303-11061$23.00.


Lowest-order nondipole effects are studied in double photoionization (DPI) of the He atom. Ah initio parametrizations of the quadrupole transition amplitude for DPI from the 1S0state are presented in terms of the exact two-electron reduced matrix elements. Parametrizations for the dipole-quadrupole triply differential cross section (TDCS) and doubly differential cross section (DDCS) are presented in terms of polarization-independent amplitudes for the case of an elliptically polarized photon. Expressions for the DDCS in terms of the reduced two-electron matrix elements are also given. A general analysis of retardation-induced asymmetries of the TDCS including the circular dichroism effect at equal energy sharing is presented. Our numerical results exhibit a nondipole forward-backward asymmetry in the TDCS for DPI of He at an excess energy of 450 eV that is in qualitative agreement with existing experimental data.

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