6 - Direct photodissociation: The reflection principle

Summary

Photodissociation can be roughly classified as either direct or indirect dissociation. In a direct process the parent molecule dissociates immediately after the photon has promoted it to the upper electronic state. No barrier or other dynamical constraint hinders the fragmentation and the “lifetime” of the excited complex is very short, less than a vibrational period within the complex. For comparison, the period of an internal vibration typically ranges from 30 to 50 fs. The photodissociation of CH₃ONO via the S₂ state is a typical example; the corresponding potential energy surface (PES) is depicted in the upper part of Figure 1.11. A trajectory or a quantum mechanical wavepacket launched on the S₂-state PES immediately leads to dissociation into products CH₃O and NO.

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 S₁ state is a representative example. The middle part of Figure 1.11 shows the corresponding PES. Before CH₃ONO(S₁) 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.

Direct dissociation is the topic of this chapter while indirect photofragmentation will be discussed in the following chapter. Both categories are investigated with the same computational tools, namely the exact solution of the time-independent or the time-dependent Schrodinger equation. The underlying physics, however, differs drastically and requires different interpretation models.