Wave packet dynamics in molecules

With ultrashort pulses one can initiate molecular processes at well-defined moments in time, and take snapshots of their progress. In this situation the molecular dynamics can be described with quantum mechanical wave packets, which are superpositions of molecular vibrational states, in the same way as pulses of light are superpositions of plane waves in classical optics. Wave packet dynamics forms a complementary description to the traditional spectroscopic description of molecular vibrations, based on Franck - Condon factors and individual vibrational states. In many cases the motion of wave packets can be thought of in terms of classical Newtonian dynamics, which is often helpful both computationally and conceptually. They are, nevertheless, quantum objects and thus show interesting matter wave characteristics such as decoherence and interference. The basics of wave packet dynamics and their use in molecular processes which are initiated or probed with ultrashort laser pulses are outlined. This includes also a description of recent work on the application of quantum tomography measurements on molecules.