The Role of the Lone Pairs in Hydrogen Bonding

Tuesday, February 6 10:45-11:45
Density-Matrix Study of the Hydrogen-Antihydrogen Molecule
Alejandro Saenz
AG Moderne Optik, Institut für Physik, Humboldt-Universität zu Berlin Hausvogteiplatz 5-7, D-10 117
Berlin, Germany
There is a long history of the interest in antimatter and its properties. Starting from the theoretical
prediction of its existence by Dirac and its first experimental detection antimatter stimulated scientists,
but even science fiction authors. The scientific interest is closely linked to its role with respect to fundamental
symmetries of nature. According to the standard model of physics, equal amounts of matter and
antimatter were formed in the Big Bang creating our universe. However, we are evidently (and in view
of annihilation processes fortunately!) practically only surrounded by matter. There is no evidence for
large matterantimatter annihilation taking place in our (visible) part of the universe. This leads directly
to a fundamental open question: where is the antimatter gone? A good candidate for antimatter research
are antihydrogen atoms that consist of an antiproton and a positron. They are polarisable and can thus
be trapped. On the other hand, they are electrically neutral which avoids that they are sensitive to stray
fields. In addition, the matter analogy (atomic hydrogen) has a long history of being the workhorse for
validating quantum mechanics as well as quantum electrodynamics, because it is accessible to very precise
theoretical and experimental studies. This motivated on-going experiments at CERN and planned
future ones at GSI (Darmstadt). One interesting question in the context of antihydrogen research is
its interaction with ordinary atoms. The simplest example is evidently atomic hydrogen. As our studies
have shown, the hydrogen-antihydrogen system is an interesting object by itself. Although it is related to
molecular hydrogen, the change of the sign of the charges and the absence of exchange interaction leads
to a number of effects that are specific to antimatter-matter systems, but absent for ordinary molecules.
Nevertheless, hydrogen-antihydrogen can form an in the chemical sense bound molecular system. On
the first glance, the bonding should be of electrostatic nature and thus ionic. Based on a study of the
leptonic densities and density matrices the character of the bond is, however, investigated in more detail
in this work. Such investigations are non-trivial, since the leptons (electron and positron) are highly
correlated. The theoretical description of the hydrogen-antihydrogen molecule requires therefore very
accurate quantum-chemical approaches.
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