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Kowalewski, Markus (2012): Quantendynamik isolierter molekularer Systeme. Dissertation, LMU München: Fakultät für Chemie und Pharmazie



The thesis involves three different aspects of quantum dynamics in cold and isolated molecular systems that are investigated theoretically with methods ranging from wavepacket dynamics to density matrix propagation. In the first part the efficiency of cavity sideband cooling of trapped molecules is theoretically investigated for the case in which the infrared transition between two ro-vibrational states is used as a cycling transition. The molecules are assumed to be trapped either by a radio frequency or optical trapping potential, depending on whether they are charged or neutral, and confined inside a high-finesse optical resonator that enhances radiative emission into the cavity mode. Using realistic experimental parameters and Carbonyl sulfide as a molecular representative, we show that in this setup, cooling to the trap ground state is feasible. The second part investigates femtosecond pump-probe spectroscopy of single MgH+ ions confined in an ion trap. The molecular ions are embedded in a coulomb crystal of atomic magnesium ions which are laser cooled to a few millikelvin. Single molecules are addressed by femtosecond ultraviolet laserpulses and the induced molecular processes are investigated. The simulations of the wave packet motion and dissociation behavior were used to predict the experimental parameter regime and are compared directly to the laboratory results. For this purpose a multiscale model is developed which involves the wave packet motion as well as the problem of vibrational heating occurring on a millisecond time scale under laboratory conditions. The third part investigates the gas phase SN2 collision reaction of chloride and methyl iodine. Motivated by the experimental results of Roland Wester and co-workers [Mikosch et al., Science 319, 183 (2008)], remaining questions were addressed. The collision reaction is simulated by solving the time dependent Schrödinger equation on ab initio potential energy surfaces. With the chosen reactive coordinates it is possible to reproduce the basic features of the immediate collision reaction. The internal molecular coordinates are transformed into reaction path based model which allows for an efficient numerical description of the nuclear dynamics. The energy transfer in the system is investigated and compared to the experimental results. From the new insight into the process, an intuitive concept of a dynamical barrier can be derived. Moreover the role of the spectator mode can be clarified.