Chemical dynamics is concerned with all transient aspects of chemistry at the molecular level. In general terms, these include, e.g., the energy flow between and within molecules, the transformation of reactants to products, or the motion induced by the interaction of the species with electro-magnetic fields. For most chemically-relevant considerations, these molecules are well described as being composed of two separate entities: electrons and nuclei. Due to their large mass difference, chemical dynamics is usually explained in most textbooks in terms of the Born–Oppenheimer approximation¹ : the slow-moving, heavy nuclei evolve on an energy landscape that is due to the fast-moving, light electrons. The very concept of a potential energy surface, introduced by Eyring and Polanyi in 1931, is built around this mass mismatch, effectively separating the timescales of the electron and nuclei dynamics.^2,3  Although the idea of having electrons reacting instantaneously to the motion of the nuclei was originally met with skepticism, it is now ubiquitous in chemical dynamics and it has lead to the definition of many concepts familiar to the chemists. Static molecular structures define the shape and the orientation of nuclei, and stable configurations in the vicinity of local minima of the potential energy surface define the reactants and the products for a given reaction, as well as various metastable intermediates. Further, following the minimal energy path connecting these molecular structures, it is possible to define a so-called reaction coordinate that forms the basis of transition state theory,^4–6  in which the local maxima between stable/metastable structures are short-lived species that yield information about the rate of change between two chemical species. These ideas have been immensely successful at explaining the reactivity of molecules in the gas and in the condensed phase⁷  and they will form the basis of the dynamical investigations discussed in this chapter.