Femtosecond spectroscopy

The development of ferntosecond laser systems has opened the door to studyi ng ultrafast dynamical processes, which range from nuclear motion in molecu les to relaxation mechanisms of charge carriers in solids. A characteristic of the approach is that the system investigated is no longer in thermodyna mic equilibrium. It is rather in an excited state whose decay into other de grees of freedom is being probed. We will, in this article, outline the gen eral experimental set-up needed for such studies and discuss typical experi ments involving excited electrons in solids.