Chapter 15: Strong Field Excitation

Until now we have dealt only with the interaction of molecules with weak fields, where perturbation theory is valid. In this Chapter, we turn t0 the interaction with strong fields. The molecular dynamics becomes much more complicated in the presence of strong field, and it is generally very difficult to have detailed intuition for what will happen. Nevertheless, for certain situations we are on solid ground. For structureless two-level systems in a single frequency field there is an (almost exact) analytic solution, known as the Rabi solution. The concept of Rabi cycling, and its dependence on intensity and detuning, provides the foundation for all other strong field behavior. Next we describe the Feynman-Vernon-Hellwarth (FVH) geometrical approach to the two-level system in the presence of a strong field. This approach gives intuition into a wide variety of processes involving two-level systems in the presence of fields ,with time-varying envelopes. One of these processes is the process of adiabatic passage, which provides the basis for understanding the STIRAP (Stimulated Raman Adiabatic Passage) in three- level systems.