Conical Intersection

In quantum chemistry, a conical intersection of two potential energy surfaces is the set of molecular geometry points where the two potential energy surfaces are degenerate (intersect) and the non-adiabatic couplings between these two states are non-vanishing. In the vicinity of conical intersections, the Born-Oppenheimer approximation breaks down, allowing non-adiabatic processes to take place. The location and characterization of conical intersections are therefore essential to the understanding to a wide range of reactions, such as photo-reactions, explosion and combustion reactions, etc.

Conical intersections are ubiquitous in both trivial and non-trivial chemical systems.

In a system with n coordinates, degenerate points lie in what is called the intersection space, or seam. The dimensionality of the seam is n-2. For a conical intersection, the remaining two dimensions that lift the energetic degeneracy of the system are known as the branching space.

The conical intersections are also called molecular funnels or diabolic points. This comes from the very important role they play in non-radiative de-excitation transitions from excited electronic states to the ground electronic state of molecules. For example, the stability of DNA with respect to the UV irradiation is due to such conical intersection. The molecular wave packet excited to some electronic excited state by the UV photon follows the slope of the potential energy surface and reaches the conical intersection from above. At this point the very large vibronic coupling induces a non-radiative transition (surface-hopping) which leads the molecule back to its electronic ground state.

A well-written introduction to the topic of conical intersections in chemistry is Diabolical Conical Intersections, David Yarkony, Rev. Mod. Phys. 68, 985-1013 (1996).

A clear authoritative book on the intricate mathematical aspects of conical intersections can be found in Beyond Born-Oppenheimer: Electronic Nonadiabatic Coupling Terms and Conical Intersections by Michael Baer (Wiley-Interscience, 2006).

Read more about Conical Intersection:  Local Characterization of Conical Intersections, Categorization By Symmetry of Intersecting Electronic States

Famous quotes containing the word intersection:

    If we are a metaphor of the universe, the human couple is the metaphor par excellence, the point of intersection of all forces and the seed of all forms. The couple is time recaptured, the return to the time before time.
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