Adiabatic And Non-Adiabatic Quantum Dynamics Calculation Of Atom-Diatom Reactions

In recent years great advances have been achieved in understanding of microscop-ic mechanism of chemical reaction in molecular and atomic level both from theoret-ical and experimental aspect. It is important because not only it can provide basic knowledge for interpreting the process of the chemical reaction but also deepen the insight of the molecular collision in microscopic level. Here we used Chcbyshcv real wave packet method to calculate the gas phase atom-diatom reaction dynamics of O+D2(v=0,j=0)â†'OD(v’,j’)+D and C+H2(v=0,j=0)â†'CH(v’,j’)+H and obtained reaction dynamics information such as reaction probabilities, cross sections.etc.Reactions between excited atomic carbon and molecular hydrogen play an im-portant role in both combustion and astrochemistry. Significant experimental and theoretical efforts have been devoted in the past to the understanding of both kinet-ic and dynamic aspects of these reactions. In addition to its practical importance, the C+H2reaction serves as a prototype for studying the insertion mechanism due to the potential energy surface (PES) has a deep well, in which the carbon atom usually attacks the hydrogen molecule in the perpendicular approach. However, this reaction is considered to be a "clean" insertion reaction because of its near thermoneutrality and a relatively large barrier in the collinear abstraction pathway. Here, this reaction was studied by the Chcbyshev quantum wave packet method using a new potential energy surface. Fully Coriolis-coupled channel calculations have been carried out to get initial state-specified integral cross section and rate constant accurately. All projection quantum numbers Ω were considered for each total angular momentum J. At room temperature, the rate constant is reasonably in good agreement with the experimental value.The reactions of O(1D)+H2is an important benchmark for the understanding of elementary reaction dynamics. The electronic structure of this reaction system is somewhat complicated since the electronic state of the asymptotic reagent is five- fold degenerate. When the reactant atom and molecule approach each other, the degeneracy will be broken leading to five different potential energy surfaces. In the collinear geometry, there are a (?) state, a doubly degenerate â…¡ state, and a doubly degenerateâ–³state. Because theâ–³state is strongly repulsive, only three of them contribute to the total reaction. Surface-hopping quasi-classical trajectory (QCT) calculation and limited quantum mechanical studies have investigated the isotopic reaction O(1D)+D2. However, due to the limitation in treating the quantum effects the surface hopping model did not lead to a quantitative agreement with accurate quantum result. The exact quantum calculation should be carried out not only for J=0but also for higher partial waves, J>0. For these reactions, we obtained the initial state-specified (vi=0, ji=0) integral cross section and rate constant using the potential energy surfaces of Dobbyn and Knowlcs. Since deuterium is much heavier than hydrogen, more basis functions or grid points arc needed. It is still a challenge to obtain accurate dynamic information without dynamic approximation. A total of50partial wave contributions have been calculated using the Chcbyshcv wave packet method with full Coriolis coupling to achieve convergence up to the collision energy of0.28eV. The total integral cross section and rate constant are in excellent agreement with experimental as well as quasi-classical trajectory results. Contributions from the adiabatic pathway of the1A" state and the non-adiabatic pathway of the2A’/1A’ states, increase significantly with the collision energy. Com-pared to the O(1D)+H2system, the kinetic isotope effect (k(D)/k(H)) is found to be nearly temperature independent above100K and its value of0.77±0.01shows excellent agreement with the experimental result of0.81.This thesis consists of five chapters and our main work is described in the third and fourth chapter. We make a introduction in chapter1and giving a brief review about the development of molecular reaction dynamics in recent years. In chapter2, we give an detailed introduction of the time-independent wave packet method. In chapter3we theoretically calculated the reaction dynamics of C+H2and test the accuracy of new potential energy surface which deepen the insight of this inser- tion reaction. In chapter4, we present the adiabatic and non-adiabatic quantum dynamics calculations of O+D2and found that the non-adiabatic pathway gives important contribution both to the integral cross section and rate constant. Chapter5gives conclusion and outlook.