**1. Introduction**

The quasifree mechanism (QFM) is a special case of photo-double ionization (PDI) that was predicted by M. Amusia et al. in 1975 [1]. The name of the process originates from the idea that the photon interacts with a quasifree electron pair without involvement of the nucleus. The kinematic profile of QFM is characterized by electrons being emitted back-to-back with equal kinetic energy, which leave the nucleus with close-to-zero recoil momentum [2,3]. The interaction between photons and atoms is generally dominated by electric-dipole contributions, but the QFM profile is forbidden in a dipole transition due to angular momentum and parity conservation [4]. Hence, QFM facilitates double ionization by means of a pure electric-quadrupole transition. As QFM ejects two electrons only from the small part of the initial-state two-electron wave function where both electrons are spatially close together [5,6], its transition amplitude is extremely small and experimental investigations of QFM are challenging [7]. However, nearly four decades after the prediction, the existence of the quasifree mechanism was finally experimentally confirmed by Schöffler et al. in 2013 through the observation of doubly charged helium nuclei with close-to-zero momentum [8]. Note that this signature of QFM is similar to what is found for double ionization by Compton scattering which becomes the dominant double-ionization channel at high photon energies [9–11].

The recently renewed interest in nondipolar photoionization in the one-photon and strong-field ionization regimes (see, e.g., Refs. [12–17]) encouraged further experimental

**Citation:** Grundmann, S.; Trinter, F.; Fang, Y.-K.; Fehre, K.; Strenger, N.; Pier, A.; Kaiser, L.; Kircher, M.; Peng, L.-Y.; Jahnke, T.; Dörner, R.; Schöffler, M.S. Quasifree Photoionization under the Reaction Microscope. *Atoms* **2022**, *10*, 68. https://doi.org/ 10.3390/atoms10030068

Academic Editor: Yew Kam Ho

Received: 18 May 2022 Accepted: 22 June 2022 Published: 28 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and theoretical investigations of QFM. In 2018, electrons emitted back-to-back with equal energy were observed for helium PDI at 1100 eV photon energy [18]. This work displayed the angular emission pattern of electrons originating from a pure quadrupole transition. Two years later, QFM was confirmed for H2 PDI at 800 eV photon energy and it was shown how the QFM cross section relates to the initial spatial probability density at the two-electron cusp, which is the point where both electrons coalesce [19]. In the present work, we examine once again the experimental data used in Refs. [18,19] to continue the investigation of QFM. First, we will show that the angular distribution of QFM electrons originating from H2 PDI displays the same four-fold symmetry that was already observed for He PDI. Furthermore, second, we will provide evidence for the assumption that the photon momentum is not imparted onto the center of mass in quasifree photoionization. The latter finding is supported by numerical results from solving the full-dimensional time-dependent Schrödinger Equation (TDSE) beyond the dipole approximation.
