Separating Dipole and Quadrupole Contributions to Single-Photon Double Ionization
Bray, A. W.
Schmidt, L. Ph H.
Williams, J. B.
Schoeffler, M. S.
Kheifets, A. S.
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We report on a kinematically complete measurement of double ionization of helium by a single 1100 eV circularly polarized photon. By exploiting dipole selection rules in the two-electron continuum state, we observed the angular emission pattern of electrons originating from a pure quadrupole transition. Our fully differential experimental data and companion ab initio nonperturbative theory show the separation of dipole and quadrupole contributions to photo-double-ionization and provide new insight into the nature of the quasifree mechanism.