First example CH3ONO

The Si PES, calculated by Nonella and Huber (1986), has a shallow minimum above the ground-state equilibrium, or expressed differently, a small potential barrier hinders the immediate dissociation of the excited S complex. Although the height of the barrier is less than a tenth of an eV, it drastically affects the dissociation dynamics, even at energies which significantly exceed the barrier. The excited complex lives for about 5-10 internal NO vibrational periods before it breaks apart. The photodissociation of CH3ONO through the Si state exemplifies indirect photodissociation or vibrational predissociation (Chapter 7). 

In contrast, the PES of the S2 state calculated by Suter, Briihlmann, and Huber (1990) is repulsive along the CH3O-NO dissociation coordi- 

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In indirect photofragmentation, on the other hand, a potential barrier or some other dynamical force hinders direct fragmentation of the excited complex and the lifetime amounts to at least several internal vibrational periods. The photodissociation of CH3ONO via the 51 state is a representative example. The middle part of Figure 1.11 shows the corresponding PES. Before CH30N0(5i) breaks apart it first performs several vibrations within the shallow well before a sufficient amount of energy is transferred from the N-0 vibrational bond to the O-N dissociation mode, which is necessary to surpass the small barrier. 

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