By separating the motion of the rapidly moving electrons and the slowly moving 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 significant. 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.