Chapman, Emma Louise
(2009).
Theoretical Studies on the ultrafast photodissociation of molecules.
University of Birmingham.
Ph.D.
Abstract
Ultrafast photodissociation is a fundamental process in nature. In this thesis we present a study of three very different systems which undergo photodissociation on the femtosecond timescale. A major feature of these processes is the often complex topology of the potential energy surfaces due to coupling between the nuclear and electron motion: termed vibronic coupling. A known feature in potential energy surfaces due to vibronic coupling is the conical intersection and these play a role in all three systems studied. In the first study we investigate NH\(_3\), which exhibits an intersection between the ground and first excited state. As a consequence the dissociation occurs on both states. We use two models to study this system, a 2D and a 6D, contrasting greatly in their complexity and ability to describe the whole molecule. Wavepacket dynamics are used to probe the reaction mechanism and to calculate the branching ratio. A detailed investigation of electronic structure theory methods forms a large part of this research. We apply it to the FNO molecule, a system in which there is coupling amongst the states giving rise to certain topological features on the first excited state. These features are both subtle and difficult to reproduce with ab initio methods. We also present a potential fit of this data and implement wavepacket dynamics simulations on the surfaces. A study of Cr(CO)\(_6\) using electronic structure theory is the final system investigated in this work. In contrast to the other systems, Cr(CO)\(_6\) has many low lying excited electronic states and we investigate this system using Complete Active Space Self- Consistent Field (CASSCF) methods. Using a large active space allows us to include all of the states of interest within one calculation.
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