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We investigate single photon double ionization (PDI) of helium at photonenergies of 440 and 800 eV. We observe doubly charged ions with close to zeromomentum corresponding to electrons emitted back-to-back with equal energy.These slow ions are the unique fingerprint of an elusive quasi-free PDImechanism predicted by Amusia et al. nearly four decades years ago [J. Phys. B8, 1248, (1975)] . It results from the non-dipole part of the electromagneticinteraction. Our experimental data are in excellent agreement with calculationsperformed using the convergent close coupling and time dependent close couplingmethods.

Ejection of quasi-free electron pairs from the helium atom ground state by single photon absorption