In indirect photodissociation a potential barrier or some other dynamical constraint hinders immediate dissociation of the complex that the light pulse has created in the excited electronic state. Figure 7.1 shows a typical one-dimensional example. The barrier may be due to an avoided crossing with another electronic state. The potential energy surface (PES) of the S1 state of CH3ONO (Figure 1.11) is a typical two-dimensional example. Depending on the efficiency of internal energy redistribution between the various degrees of freedom the lifetime of the complex may range from a few to several thousand internal vibrational periods, in contrast to direct processes where the fragmentation finishes in less than one internal period. Because of the long lifetime, the final state distributions of the photofragments no longer reflect the initial coordinate distribution of the parent molecule in the ground electronic state like they do in direct dissociation. The coupling between the various modes in the complex gradually erases the memory of the initial state before the molecule finally breaks apart.

The main characteristics of indirect dissociation are resonances in the time-independent picture and recurrences in the time-dependent approach. Resonances and recurrences are the two sides of one coin; they reveal the same dynamical information but provide different explanations and points of view. To begin this chapter we discuss in Section 7.1, on a qualitative level, indirect photodissociation of a one-dimensional system. A more quantitative analysis follows in Section 7.2. The time dependent and the time-independent views of indirect photodissociation are outlined and illustrated in Sections 7.3 and 7.4, respectively, with emphasis on vibrational excitation of the NO moiety in the photodissociation of CH3ONO(S1).