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Quasiclassical Studies of Chemical Reaction Dynamics with Inclusion of Tunneling and Nonadiabatic Transition.
Quasiclassical Studies of Chemical Reaction Dynamics with Inclusion of Tunneling and Nonadiabatic Transition.
OLOYEDE, OLUWAPONMILE
オロヤード, オルウポンマイル
OLOYEDE, Oluwaponmile
The subject of chemical dynamics typically consists of two steps. The first is the study of the behavior of the potential energy between interacting groups, and the second is the use of such potential energy behavior to study the chemical reactions between these chemical species. The latter is the subject matter of this thesis. <br /><br /> The availability and development of increasingly fast computers with bigger storage capabilities continues to influence the course of chemical dynamics in no small measure; this is considered a bit of good fortune and an invitation to study chemical dynamics in systems which are more complicated than those that were previously being studied. In spite of the eapacity of these computational resources, full quantum-mechanical scheme are still far from routine when the system of interest is large and complicated. In such a case, the quantum mechanical equations become so complex that finding the solutions becomes impractical.<br /><br />Classical mechanics-based approaches continue to be useful in the study of chemical dynamics in systems of various size and complexity. However, the absence of quantum effects like interference and oscillations in purely classical descriptions imply that classical mechanics, when used as a stand-alone technique cannot be entirely correct. In chemical reactions, the important quantum effects include the zero-point energy problem, tunneling, resonance, nonadiabatic transitions, amongst others. <br /><br />Advances in semi-classical theory have ensured that classical trajectory methods, with nonadiabatic transitions and tunneling taken into account, would be very much feasible as well as useful for chemical dynamics. A method that has become popular for treating nonadiabatic transition within the classical framework is the Trajectory Surface Hopping Method where the nuclear motion is treated classically on a single adiabatic potential surface with analytical expressions. In the case of tunneling there have been approximate methods to treat tunneling just as it has been done for Nonadiabatic transition. A classical trajectory is made to evolve in a classically-allowed region of space and on encountering an intervening barrier separating two separated classically-allowed regions, the trajectory makes an instantaneous transition in real time. <br /><br />He intends to show in this thesis that, there is a whole lot more that can be achieved with classical simulations if one can find good semiclassical analytical theory to deal with the important quantum effects such as nonadiabatic transitions and tunneling. In Chapter 2, he reports on his study of caustics, an important feature in semiclassical analysis. He presents numerical demonstrations of how to locate caustics of trajectories in the Henon-Heiles potential and also extended the same to the caustics in a potential of a triatomic system undergoing collision. His final published method capably treats the case of multiple caustics and looks to be a good candidate for use in studying multi-dimensional systems. The light masses of the constituent atoms in the H<SUB>3</SUB> system makes it a good testing ground for many tunneling theories. With this realization, he seizes the advantage offered by the caustics-locating method, merging it with a simple tunneling recipe to calculate the thermal rate constant of the H<SUB>3</SUB> system. <br /><br />Chapter 3 concerns the semiclassical study of Nonadiabatic transition. He reports in this chapter, an improvement and generalization of the Surface Hopping Method of Tully and Preston which was briefly mentioned above. In the original method, the hopping points were dictated by the location of the seam line and the probability of hop was determined by integrating the coupled quantum equations or by using the Landau-Zener equation. <br /><br />For large and complicated systems, this geometrical construct is not easily defined and hence, cannot be easily obtained for use in calculation. The method presented in this chapter, avoids these problems by seeking a generalized method which does not require the knowledge of the seam line. The use of the Zhu-Nakamura theory is also introduced, with the approximation that the nonadiabatic transition can be reduced to a one-dimensional problem. The Zhu-Nakamura theory adds a crucial advantage to classical treatments of nonadiabatic transitions because the theory is valid for classically-forbidden hops. <br /><br />The goal of chapter 4 is the extension of the aforementioned quasiclassical methodologiesto the subject of the chemical reactions in the OHCl system which is an important molecule in atmospheric chemistry. The possible mediating effects of the excited surfaces have never been considered in quasiclassical calculations and the results of this chapter should help in answering the question of what contributions nonadiabatic processes make to important atmospheric events. <br /><br />In the final chapter, he presents the conclusion and summarizes what the future holds for quasiclassical methods. It is the claim of his thesis that classical approaches remain relevant to the practice of chemical dynamics as long as it is carefully coupled to semiclassical theories.
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総研大甲第891号
eng
thesis
https://ir.soken.ac.jp/records/338
博士（理学）
2005-09-30
総合研究大学院大学
https://ir.soken.ac.jp/record/338/files/甲891_要旨.pdf
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333.4 kB
2016-02-17
https://ir.soken.ac.jp/record/338/files/甲891_本文.pdf
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4.0 MB
2016-02-17