Viewing the Reaction Path with the Help of Time-Resolved Femtosecond Spectroscopy

Abstract

The spectroscopy of the transition state, which separates products from educts in a chemical reaction has been the subject of numerous studies [1]. A beautiful idea, which by now has been applied to a great variety of molecular systems is to use electron detachment spectroscopy to directly resolve resonance features which are the fingerprints of quasi-bound states of the molecule [2]. The latter are localized in the transition-state region. Nevertheless this kind of spectroscopy does not tell us about the history of the system when it evolves from this intermediate region towards the product channels. In this chapter we want to line out, from a theoretical point of view, how time-resolved spectroscopy can be used to monitor the evolution of a system which is prepared in the transition state, until it ends up in several product channels. This idea has been promoted by Zewail and coworkers over the last years and exciting experiments were performed. It is not the purpose of this contribution to review those experiments (see [3]). Rather, we will describe the principles of timeresolved spectroscopy using quantum mechanical and classical theory. In particular it will be seen that the motion along the reaction path can be directly taken from an experimental signal.