Hot reaction ground state

Fig. 1. Idealized photoisomerization mechanisms. These can lead to different final products as shown. However, one should realize that hot ground state reactions could also occur, obscuring a simple relationship between products and excited state dynamics.

Hot ground state reaction A hot state reaction of the ground electronic state. 

Hot" Ground State Reaction reaction of a molecule in an excited vibrational level of the ground state, reached indirectly as a consequence of photoexcitation. 

Figure 5.12. Qualitative state diagram for the fluorescence quenching of benzene by radiationless transition into one of the higher vibrational levels of the isomeric benzvalene. The back reaction is a hot ground-state reaction.

If the initial excitation was not into the lowest excited state of given multiplicity, a fast crossing to that state will occur via internal conversion, that is, typically to the S, or T, state (Kasha s rule, cf. Section 5.2.1). In some cases a funnel in S is accessible and internal conversion from S, to Sg can be so fast that the first thermal equilibration of the vibrational motion in these molecules is achieved in a minimum in the Sg state. Such a process is referred to as a direct reaction. Here as well, the excess kinetic energy of the nuclei may take the molecule over barriers in the S state into valleys other than the one originally reached analogous to the above-mentioned reactions, such processes are referred to as hot ground-state reactions. 

In comparing these results with experimental data it has to be remembered that in contrast to ketones, azo compounds can also undergo photochemical trans-cis isomerizations. (Cf. Section 7.1.7.) In the gas phase n-> r excitation results in photodissociation with nearly unit quantum efficiency. At higher pressures, however, and especially in solution, this reaction almost completely disappears and photoisomerization dominates. The latter is observed even at liquid nitrogen temperatures. This is understandable if it is accepted that photodissociation proceeds in the gas phase as a hot ground-state reaction. According to Figure 7.18, it has to overcome a barrier in the excited state and is therefore not observed in solution. For the trans-cis isomerization, on the other hand, no excited-state barrier is to be expected from the results in Section 7.1.7. 

The failure in the earlier study to observe the production of large quantities of C2F4 was due to the quenching out of the hot ground-state reaction because of... 

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