The simplest molecule in the universe, H₂, is perfectly symmetric. Its electrons are delocalized all over the molecule. This is commonly assumed to hold true while a molecule is dissociating. The well-defined symmetry of the electron wave function will then create a hole with equal probability at each of the two fragments.

External fields which are present during the dissociation of a molecule can break this symmetry. Strong laser fields have been utilized to mix different electronic states, leading to a localization of the bound electron. Such a mixture of states can also occur already in the ionization step if the ionization energy is in the range of doubly excited resonances.

Lately, we showed experimental evidence for a third way to break the symmetry of H₂: The electric field of the ejected photoelectron is sufficient to preferentially localize the bound electron at one side of the remaining H₂⁺ ion. This retroaction of the photoelectron onto its parent molecule was recently suggested in pioneering theoretical work by Serov and Kheifets (V. Serov and A. S. Kheifets, Phys. Rev. A 89, 031402 (2014) but has never been recognized in an experiment.

In Fig. 1, we show the photoelectron angular distribution of the p-H breakup in a coordinate frame where the x axis is given by the molecular axis. Our data show a significant asymmetry of the p-H breakup for very slow photoelectrons, which decreases with increasing electron energy. During the dissociation, the bound electron clearly prefers to localize at the proton opposite to the direction of the photoelectron

Fig. 1: Angular distribution of the ejected photoelectron. Shown is the angle between the electron momentum vector and the molecular axis for photon energies of 19.1, 20.1, and 21.1 eV. The KER is restricted to intervals from 0 to 0.1 eV and from 0.4 to 0.6 eV, respectively. The red line is a quadratic function of the form a+b(cos(θ))² fitted to the data as a guide for the eye. The molecular orientation is fixed as shown in the middle of the picture.

Contrary to these results, ionization and dissociation in two independent steps would lead to symmetric angular distributions. That means that the escaping electron and the fragmentation of the molecule cannot be treated separately and the process can no longer be classified as a Franck-Condon transition. While we have observed this effect in H₂, the simplest system where it can occur, we speculate that the effect is general for all symmetric molecules and for all processes ejecting an electron.

In the corresponding publication, our results are discussed in much more detail and are compared to the theoretical prediction by Serov and Kheifets:

Electron Localization in Dissociating H₂+ by Retroaction of a Photoelectron onto Its Source
M. Waitz, D. Aslitürk, N. Wechselberger, H. K. Gill, J. Rist, F. Wiegandt, C. Goihl, G. Kastirke, M. Weller, T. Bauer, D. Metz, F. P. Sturm,
J. Voigtsberger, S. Zeller, F. Trinter, G. Schiwietz, T. Weber, J. B. Williams, M. S. Schöffler, L. Ph. H. Schmidt, T. Jahnke, and R. Dörner
Physical Review Letter 116, 043001 (2016)

In a former work, we examined a process where different ionization pathways involving doubly excited states mix and lead to asymmetric photo electron angular distributions. Thus the two protons in the dissociating H₂⁺ molecule can be distinguished. The experimental data is compared to quantum mechanical ab initio calculations, which also allows to observe interference effects between different channels.

More details can be found here:

Single photon induced symmetry breaking of H₂ dissociation
F. Martín, J. Fernández, T. Havermeier, L. Foucar, Th. Weber, K. Kreidi, M. Schöffler, L. Schmidt, T. Jahnke, O. Jagutzki, A. Czasch, E. P. Benis, T. Osipov, A. L. Landers, A. Belkacem, M. H. Prior, H. Schmidt-Böcking, C. L. Cocke, R. Dörner
Science 315, 629 (2007) 

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