Grundmann, STrinter, FBray, AlexanderEckart, SebastianRist, JonasKastirke, GMetz, DanielKlumpp, SViefhaus, JSchmidt, Lothar Ph HWilliams, J BKheifets, Anatoli2020-06-152020-06-150031-9007http://hdl.handle.net/1885/205048We 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.A. K. acknowledges support by the Wilhelm and Else Heraeus Foundation. S. K. acknowledges the funding of the EUCALL project within the European Unions Horizon 2020 Research and Innovation Programme under the Grant Agreement No. 654220. This work was supported by BMBF and DFG. We are grateful to the staff of PETRA III for excellent support during the beam time.5 pagesapplication/pdfen-AU© 2018 American Physical Societysingle-and few-photon ionization and excitation, atomic, molecular and optical2018-10-2310.1103/PhysRevLett.121.1730032019-12-22