Schöffler, M. S.Stuck, C.Waitz, M.Trinter, F.Jahnke, T.Lenz, U.Jones, M.Belkacem, A.Landers, A. L.Pindzola, M. S.Cocke, C. L.Colgan, J.Kheifets, A.Bray, I.Schmidt-Böcking, H.Dörner, R.Weber, Th.2023-12-072023-12-072013-07-02https://hdl.handle.net/2097/43896We investigate the single-photon double ionization 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 quasifree photon double ionization mechanism predicted by Amusia et al. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods.© American Physical Society (APS). This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).http://rightsstatements.org/vocab/InC/1.0/https://web.archive.org/web/20181120135245/https://journals.aps.org/copyrightFAQ.htmlText