Non-adiabatic effects in molecular systems

Agnes Vibok

By separating the motion of the rapidly moving electrons and the slowly mov-
ing nuclei in a molecular system the Born-Oppenheimer approximation usually
can treat the dynamical process in a molecule after absorbing a photon. In
this scheme the nuclei move over a potential energy surface (PES) provided by
the electronic (adiabatic) eigenstates and therefore electrons and nuclei do not
easily exchange energy. Although several chemical and physical processes can
be rationalized on a single Born-Oppenheimer PES, there is no doubt that in
many important cases like radiationless relaxation of excited electronic states,
dissociation, proton transfer or isomerization processes of polyatomic molecules
etc. this approximation breaks down. The nuclear and electronic motion then
couple and so called conical intersection (CI) arises. In this situation the energy
exchange between the electrons and nuclei can become signicant. The CIs play
an important role in the radiationless deactivation processes of the excited state
systems as they can provide pathway for ultrafast interstate crossing on the
femtosecond timescale. CIs can be evolved between different electronic states
starting from triatomic systems to a truly large polyatomic molecules.
It was also shown that molecules interacting with standing [1] or running [2]
laser waves can produce CIs. The presence of such CIs offers new possibilities
to control the photo-induced molecular dynamics.

[1] N. Moiseyev, M. Sindelka and L.S. Cederbaum, J. Phys. B: At. Mol. Opt.
Phys. 41 (2008) 221001.

[2] M. Sindelka, N. Moiseyev and L.S. Cederbaum, http://arxiv.org/abs/1008.0741.



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