The Quantum Dynamics Calculation Of Atom-Polyatomic Reaction Systems

In the 1960s, the branch of gas reaction dynamics developed gradually in the field of physical chemistry, and chemical reactions can be studied from the point of molecular collision mechanics, that is a new stage of micro-reaction dynamics. In recent years, the remarkable development of quantum scattering theory and the increasing abilities in relevant calculations have drawn more and more attention to the study of quantum dynamics of polyatomic reactions. The study of state-to-state chemical reaction dynamics from first principles is to be a major goal in quantum scattering calculations. During the last two decades, the quantum scattering theory has been substantially developed and rigorous quantum reactive scattering calculations have already been done to reactions involving four atoms. The chemical or biological reactions, however, involve more than four atoms; therefore, considering the present calculation ability, it is necessary to develop some practical computational methods in order to carry out quantitatively accurate calculations in studying the quantum dynamics of polyatomic molecule reactions. In this paper a recently offered theory model—semirigid vibrating rotor target (SVRT) model, which is a dimension-reduced model handling polyatomic system reaction, is adopted to study polyatomic molecule reaction. In SVRT model polyatomic molecule whose spacial locomotion can be accurately treated as a regular non-symmetry rotor is dealt with as two different rigid segments which both can vibrate one-dimensionly through the line of their centroid. Since SVRT model can relatively correctly deal with the spacial locomotion, it can exactly demonstrate reaction system's steric dynamics effects, as is a very crucial factor in the research for polyatomic molecule reaction. This model is adaptive to polyatomic molecule one of whose bond is relatively weaker and which can be divided into two segments at the end of the reaction. For atom-polyatomic molecule reaction system, only 4 degrees of freedom are enough to describe it. According to this theory,the reactive polyatomic molecule NH3 is regarded as a diatomic molecule H-NH2, therefore the reaction system can be regarded as an atom-diatom reaction system, the reaction polyatomic molecule H-NH2 is regarded as a semirigid vibrating rotor which is made up of one H atom and one NH2 whose geometry structure is fixed. Since NH2 is dealt as rigid and maintain C2v symmetry in the reaction process, four degrees of freedom are enough to describe the reaction system. In the process of calculation,the time-dependent wave packet method is used to obtain the Hamiltonian of reaction system; the split-operator method is employed to propagate the wave packet. To avoid boundary reflection of wave function, an optical absorbing potential is used in the calculation process. Garcia provided potential energy surface is adapted to calculate separately the reaction systems'reaction probability, the total cross-section and the rate constant. After comparing and analyzing the calculated results, we get the following conclusions: First, the reaction systems has observable reaction probability when it approaches the barrier, which indicates quantum tunnel effects exist obviously; the graph of the calculated reaction probability changing with the energy dependence shows a steplike behavior, as is similar in characteristics to H+H2, H+CH4 abstract reaction. Second, The fact that H-NH2 molecules'vibrating exciting increase the reaction probability enormously while they decreases the threshold evidently illustrates that the molecule's vibrating energy makes great contribution to collision reaction. Third, the different vibrating states for the molecule have on the reaction probability influences, including that the increase of molecules'vibrating energy makes great contribution to abstract reaction while it has little effects on the reaction threshold and that the initial geometry orientation for the reaction molecule has important influence on the reaction probability. Fourth, the total cross-section of each of the three reaction systems increases with the enlargement of the translational energy while the rate constant enhances with the rising of the temperature. For H+NH3 reaction, the vibrating exciting increases enormously the total cross-section while the reaction threshold lowers, as is consistent with the regulations of the reaction probability changing. Moreover, the fact that the rate constant is far higher in the vibrating excited state than in the ground state indicates that vibrating exciting benefits the reaction process. Fifth, we compared the rate constant with the experimental result, our rate constant is lower than...