**4. Conclusions**

We have investigated the quasifree mechanism of one-photon double ionization of He and H2 at 800 eV photon energy and addressed two open questions concerning this intriguing process. We found the four-fold symmetry in the angular emission pattern of QFM electrons from H2 double ionization that underlines the quadrupole nature of the process. Furthermore, we showed that the photon momentum is not transferred onto the photo-ion in quasifree photoionization, which is in contrast to single ionization in general and double ionization by means of electron–electron correlation.

**Author Contributions:** Conceptualization, M.S.S.; Experimental preapration, S.G., F.T., T.J. and M.S.S.; Experiment performed, S.G., F.T., K.F., N.S., A.P., L.K., M.K., T.J. and M.S.S.; Data analysis, S.G.; Validation, F.T., T.J., R.D. and M.S.S.; Supervision, T.J., R.D. and M.S.S.; Theory calculations, Y.-K.F. and L.-Y.P.; Writing—original draft preparation, S.G.; Writing—review and editing, S.G., F.T., Y.-K.F., K.F., N.S., A.P., L.K., M.K., L.-Y.P., T.J., R.D. and M.S.S.; Visualization, S.G.; Funding acquisition, T.J., R.D. and M.S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was funded by DFG and BMBF. **Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The experimental team thank Miron Amusia for their continued encouragemen<sup>t</sup> of the experiments that are hunting for signals of QFM. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III and we would like to thank Jörn Seltmann and Kai Bagschik for excellent support during the beam time. We acknowledge support by DFG and BMBF.

**Conflicts of Interest:** The authors declare no conflicts of interest.
