  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.

Atomic Physics