The simplest molecule in the
universe, H_{2}, 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 H_{2}:
The electric field of the ejected photoelectron is sufficient to
preferentially localize the bound electron at one side of the remaining
H_{2}^{+} 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 H_{2}, 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 H**_{2}**+ 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 H_{2}^{+} 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 H**_{2} 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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**
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