Scattering theory, resonances threshold structures

CO, H2CO, and CO9 Molecules. Studies of photolonlzatlon of CO, H2CO, and CO2 molecules provide an opportunity to observe possible similarities of shape resonances In chromophorlc C-0 bonds (lA). The o- ka polarization channels of CO photolonlzatlon cross sections are known to Include strong features 10 eV above thresholds that are associated with shape resonances In scattering-theory parlance (31). Similar structures are found In ax H ax channels In H2CO (32), and In ag- kay, Ou-+kag channels In CO2 (30). By contrast, ir->kir channels In CO, and related channels In H2CO and CO2, exhibit broad structureless Ionization cross sections, but are known to Include strong dis-... 

One of the nice aspects of classical variational TST is that the theory provides upper and lower bounds to the exact classical rate of reaction. To date, there is no such quantal theory. Quantally, transition state theory is an approximation - one of the objectives of the theorist is to optimize the approximation. The concept of a transition state remains though as an important guide towards understanding the quantal structure of the reaction probability. In this section we shall see that quantal barriers and wells and their associated properties determine both threshold behaviour and resonance phenomena in quantal reactive scattering. 

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