16 - Real-time dynamics of photodissociation

Summary

In the preceding fifteen chapters of this monograph we have described how one can infer information about the dissociation process and ultimately about the multi-dimensional potential energy surface(s) (PES) in the excited electronic state(s) from the observables which one measures in “conventional” experiments, namely the absorption spectrum, the emission spectrum of the transient molecule, and the various final state distributions of the fragments. By “conventional” we mean those experiments in which the molecule is irradiated by a long, more or less monochromatic laser pulse. The last open question, which we will address here, concerns the true time dependence of the molecular system as it evolves from the Franck-Condon region, through the transition state, into the possible fragment channels and how this motion can be made transparent in the laboratory.

The lifetime of the complex in the excited electronic state is either in the range of 10⁻¹⁵−10⁻¹³ seconds for direct dissociation respectively 10⁻¹² seconds or longer for indirect fragmentation. In contrast, conventional experiments are performed with pulse lengths of the photolysis laser of the order of 10⁻⁹ seconds. Thus, no matter how refined such experiments are — full preparation of the initial state and complete resolution of the fragment states — they are inherently unable to resolve the real time dependence of the breakup process. They need accompanying theoretical studies (classical trajectories or quantum mechanical wavepackets) in order to disentangle the interaction among the various degrees of freedom and to disclose the evolution of the system.^†