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 less sophisticated of my forbears avoided foreigners at all costs, for the very good reason that, in their circles, speaking in tongues was commonly a prelude to snake handling. The more tolerant among us regarded foreign languages as a kind of speech impediment that could be overcome by willpower.”
—Barbara Ehrenreich (b. 1941)
“Humanity has passed through a long history of one-sidedness and of a social condition that has always contained the potential of destruction, despite its creative achievements in technology. The great project of our time must be to open the other eye: to see all-sidedly and wholly, to heal and transcend the cleavage between humanity and nature that came with early wisdom.”
—Murray Bookchin (b. 1941)
“I have witnessed the tremendous energy of the masses. On this foundation it is possible to accomplish any task whatsoever.”
—Mao Zedong (1893–1976)
“Footnotes are the finer-suckered surfaces that allow tentacular paragraphs to hold fast to the wider reality of the library.”
—Nicholson Baker (b. 1957)