*Article* **Quasifree Photoionization under the Reaction Microscope**

**Sven Grundmann 1, Florian Trinter 1,2, Yong-Kang Fang 3, Kilian Fehre 1, Nico Strenger 1, Andreas Pier 1, Leon Kaiser 1, Max Kircher 1, Liang-You Peng 3, Till Jahnke 4, Reinhard Dörner 1 and Markus S. Schöffler 1,\***



**Abstract:** We experimentally investigated the quasifree mechanism (QFM) in one-photon double ionization of He and H2 at 800 eV photon energy and circular polarization with a COLTRIMS reaction microscope. Our work provides new insight into this elusive photoionization mechanism that was predicted by Miron Amusia more than four decades ago. We found the distinct four-fold symmetry in the angular emission pattern of QFM electrons from H2 double ionization that has previously only been observed for He. Furthermore, we provide experimental evidence that the photon momentum is not imparted onto the center of mass in quasifree photoionization, which is in contrast to the situation in single ionization and in double ionization mediated by the shake-off and knock-out mechanisms. This finding is substantiated by numerical results obtained by solving the system's full-dimensional time-dependent Schrödinger equation beyond the dipole approximation.

**Keywords:** two-electron systems; one-photon double ionization; many-electron correlation
