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:
“But look what we have built ... low-income projects that become worse centers of delinquency, vandalism and general social hopelessness than the slums they were supposed to replace.... Cultural centers that are unable to support a good bookstore. Civic centers that are avoided by everyone but bums.... Promenades that go from no place to nowhere and have no promenaders. Expressways that eviscerate great cities. This is not the rebuilding of cities. This is the sacking of cities.”
—Jane Jacobs (b. 1916)
“Children are potentially free and their life directly embodies nothing save potential freedom. Consequently they are not things and cannot be the property either of their parents or others.”
—Georg Wilhelm Friedrich Hegel (1770–1831)
“I have witnessed the tremendous energy of the masses. On this foundation it is possible to accomplish any task whatsoever.”
—Mao Zedong (1893–1976)
“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)