Passive and active control of photochemical processes

Many elementary chemical and physical processes such as the vibrational mot ion of atoms within a molecule or the breaking and formation of chemical bo nds take place on a femtosecond time scale. By means of ultrafast laser pul ses it became possible to monitor these events in real time with a temporal resolution in the sub-20 fs regime [1]. Moving beyond the mere observation of a reaction to the control of its outcome is the next logical step. We i llustrate with the aid of selected model systems the concepts and possibili ties of 'passive' and 'active' control and their differences,