Continuum intermediate states and bond stretching

When a molecule is excited to a continuum state, at least one of the bonds in the molecule will start to stretch. If the molecule is then further excited, before dissociation can occur, the effect is to widen the Franck-Condon window, compared with the ground state, in at least one coordinate. A good example of how this principle can be used to explore the higher excited states of molecules is provided by work on CH3I. 

The UV spectrum of CH3I exhibits a broad continuum with a maximum 3.1X 250 nm. Excitation in this region leads to photodissociation, and this topic 

The simplest form of unimolecular rate theory, RRK theory, gives the following expression for the dissociation rate coefficient k E)  

High-resolution zero kinetic energy photoelectron spectroscopy 

The most widely used laser photoionization technique is undoubtedly REMPI. This technique yields valuable spectroscopic information on the resonant (neutral) intermediate states involved, but generally yields little or no information on the ionization step itself. The practical details and virtues of the REMPI technique have been described earlier, and the reader is referred to Chapter 9 for further discussion. 

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