A classification of photochemical reaction pathways

The observation of emission from a product B (or triplet triplet absorption by 3B in flash spectroscopy) upon excitation of a reactant A would seem to be clear evidence for an adiabatic reaction, class (b). However, this is true only when excitation of A is possible at a wavelength where B does not absorb. Otherwise, diabatic reactions can easily masquerade as adiabatic ones due to re-excitation of the photoproduct B by the light source used for excitation of A.149 

Class Diabatic reactionss proceed directly from the excited state to the ground state of the photoproduct via geometries, at which the two surfaces cross (conical intersections, Special Topic 2.5) or nearly cross (avoided crossings, funnels). In such reactions, no intermediates other than the excited state reached by absorption can usually be detected, because the molecule arrives on or near to a cusp of the ground-state surface and immediately proceeds to a stable product. 

Equation 2.53 Landau Zener model for diabatic reactions 

8 Also called nonadiabatic reactions, a double negation. 

V 2 is the electronic coupling term between the zero-order states at the crossing point, h is Planck s constant, v (not id) is the nuclear velocity along the reaction coordinate as the molecule approaches the crossing point and si s21 is the difference between the slopes of the two intersecting PESs at this point. Even in systems for which an accurate calculation of p 12 is possible nowadays, the Landau Zener model provides a useful first estimate. 

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