So far we have consistently assumed — with only very few exceptions — that the photodissociation starts from the lowest vibrational state of the parent molecule. The corresponding bound-state wavefunction is typically a narrow multi-dimensional Gaussian-like function centered at the equilibrium configuration in the electronic ground state. This wavefunction defines the starting zone for the motion of the time-dependent wavepacket or the swarm of classical trajectories on the excited-state potential energy surface (PES). If the dissociation proceeds in a direct way, the forces near the Franck-Condon region determine to a large extent the fate of the wavepacket and ultimately the energy and state dependence of the dissociation cross sections. Since the initial wavefunction for a normal, chemically bound molecule such as H2O has a typical width of the order of 0.1−0.2 Å, the evolving wavepacket explores only a relatively small portion of the dissociative PES (see Figures 3.2, 9.8, and 9.9 for example).

By starting the photodissociation from an excited vibrational level one can access a significantly wider range of the upper-state PES (see Figure 13.1) and to some extent manipulate and steer the reaction path. One example has already been discussed in Section 10.1: dissociation of excited bending states of H2O through the à state probes a much wider angular region of the corresponding PES than dissociation of the lowest bending state. The influence of the increased anisotropy for smaller HOH angles clearly shows up in the final rotational state distribution of the OH product (see Figure 10.5).

Theoretically, the calculation of photodissociation cross sections for excited vibrational states proceeds in exactly the same way as for the dissociation of the lowest level.