Photoselective chemistry access to the transition state region

Photoselective chemistry access to the transition state region 

As the wavelength of lasers gets shorter and shorter we can also envisage approaching the limit where the light will be used in a diffraction mode, simitar to the current uses of X-rays. Combined with ultrashort pulses, Chapter 8, this will allowtcacldng the structure of molecules during chemical, biochemical, and material (e.g., fracture) processes. 

The quantum analog of the classical Franck-Condon principle is that the probability of the nuclei making a transition from one potential surface to another is 

for identical potential curves in the ground and excited states, the vibrational selection rule Av = 0 is strictly obeyed for whatever vibrational level v in which the nuclei start. We refer to such a limiting situation as having diagonal Franck-Condon factors. 

Suppose that the two potential surfaces are dissimilar. Then the Franck-Condon factors are less than imity and you get different probabilities for making transitions to final v vibrational levels depending on the vibrational overlap. We shall make repeated use of the Franck-Condon principle in imderstanding which vibrational levels are populated in various dynamical processes. In Section 9.2 we will generalize the principle so that it also applies to excitation as a result of a collision (where we need not be in the sudden limit). 

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