  We investigate single photon double ionization (PDI) of helium at photon
energies of 440 and 800 eV. We observe doubly charged ions with close to zero
momentum corresponding to electrons emitted back-to-back with equal energy.
These slow ions are the unique fingerprint of an elusive quasi-free PDI
mechanism predicted by Amusia et al. nearly four decades years ago [J. Phys. B
8, 1248, (1975)] . It results from the non-dipole part of the electromagnetic
interaction. Our experimental data are in excellent agreement with calculations
performed using the convergent close coupling and time dependent close coupling
methods.
