Crossings and Avoided Crossings of Potential Energy Surfaces
Vibronic coupling is large in the case of two adiabatic potential energy surfaces coming close to each other (that is, when the energy gap between them is of the order of magnitude of one oscillation quantum). This happens in the neighbourhood of an avoided crossing of potential energy surfaces corresponding to distinct electronic states of the same spin symmetry. At the vicinity of conical intersections, where the potential energy surfaces of the same spin symmetry cross, the magnitude of vibronic coupling approaches infinity. In either case the adiabatic or Born–Oppenheimer approximation fails and vibronic couplings have to be taken into account.
The large magnitude of vibronic coupling near avoided crossings and conical intersections allows wavefunctions to propagate from one adiabatic potential energy surface to another, giving rise to nonadiabatic phenomena such as radiationless decay. The singularity of vibronic coupling at conical intersections is responsible for the existence of Berry phase, which has been discovered by Longuet-Higgins in this context.
Read more about this topic: Vibronic Coupling
Famous quotes containing the words avoided, potential, energy and/or surfaces:
“The approval of the public is to be avoided like the plague. It is absolutely essential to keep the public from entering if one wishes to avoid confusion. I must add that the public must be kept panting in expectation at the gate by a system of challenges and provocations.”
—André Breton (1896–1966)
“The planet on which we live is poorly organized, many areas are overpopulated, others are reserved for a few, technology’s potential is only in part realized, and most people are starving.”
—Friedrich Dürrenmatt (1921–1990)
“But often the presence of mind and energy of a person remote from the spotlight decide the course of history for centuries to come.”
—Stefan Zweig (18811942)
“But ice-crunching and loud gum-chewing, together with drumming on tables, and whistling the same tune seventy times in succession, because they indicate an indifference on the part of the perpetrator to the rest of the world in general, are not only registered on the delicate surfaces of the brain but eat little holes in it until it finally collapses or blows up.”
—Robert Benchley (1889–1945)