Kohärente Steuerung von Wellenpaketdynamik an konischen Durchschneidungen

In a general perspective, chemistry is concerned with the composition, the behaviour and the conversion of matter. On a molecular scale, this means that chemistry deals with the formation, the dissociation, or more generally the development of molecular bonds. Observing such events in real time, leading towards a deeper understanding of chemical reactions and their dynamics is the aim of Femtochemistry. The prefix “Femto” (10-15) results from the natural time scale of such phenomena. The vibrational period of a C=C stretching vibration in a polyene backbone, which will turn out to be of great importance in this work, is in the order of 30 femtoseconds. Spectroscopy with light pulses of similar duration promises to reveal the inner workings of molecular dynamics far away from thermal equilibrium. Phenomena like internal vibrational redistribution amongst molecular normal modes or the coupling between these degrees of freedom to the surrounding are now part of physical chemistry due to rapid advances in the field of ultrashort laser pulse technology. Femtochemistry deals with questions fundamental to chemistry: How are chemical bonds formed? What is the nature of a transition state? The reason why spectroscopic information is not lost or washed out after interaction with an ultrashort and hence spectrally broad pulse lies within the coherence between the excited quantum states. The phase relation between the energy states associated with this notion is not only preserved, it can be controlled and directed towards a desired target state via modulating the excitation pulse. This is the basic idea behind coherent control, whose general aim it is to influence a photochemical or photophysical process by shaping the temporal structure of the exciting light wave. What is the contribution of this work to the field of coherent control outlined above? From a spectroscopist’s point of view, a relevant and conclusive section is promoted, namely state selective quantum control spectroscopy. It is not the aim of this branch to enhance the yield of a photochemical species while suppressing others. The pursued objective is rather to prepare certain quantum states, in order to contrast their development in time to the one after unmodulated excitation. This comparison already led to the elucidation of the energetic deactivation network in complex biological systems. It is the aim of this work to investigate and control the molecular dynamics around a decisive point of a potential energy surface, which is hard if not impossible to analyze by conventional spectroscopic techniques. This work deals with the coherent control of vibrational dynamics near a conical intersection. This important concept in modern quantum chemistry challenges the conceptual fundamentals of the description of molecular dynamics. The notion of adiabatic electronic potential energy surfaces is based upon the Born-Oppenheimer approximation. It is this assumption of the separate solvability of electronic and nuclear wavefunctions that allows one to describe potential electronic energy surfaces in the space of internal nuclear coordinates. Ultrafast processes presuming an intersection between two such surfaces do not allow for a separate treatment of electronic and structural dynamics. Only after abandoning this fundamental assumption, a variety of quantumdynamical phenomena becomes treatable. Considering benzene as a prototypical system, the geometrical properties of a conical intersection determine the product ratio of a photochemical reaction. Control over such a non-adiabatical passage is therefore the key step in a universal method for influencing wavepacket dynamics and hence photochemistry. The aim of this work is to coherently control such ultrafast phenomena on electronically excited molecular potential energy surfaces.