Study Of The Characteristics Of Roaming Trajectories In Triatomic Systems With Quasi-classical Trajectory Method

"Roaming",since it was firstly discovered in the experiment of HCOH,has been widely regarded as an universal phenomenon in unimolecular dissociation and bimolecular reactions.It is also an important reaction mechanism that has been ignored for a long time.A special manifestation of roaming is that the roaming path can bypass the minimum energy path on the potential energy surface(PES)which is associated with the traditional transition state,and roams in a high and flat region on PES(the origin of the word "roaming").In the course the roaming path competes with the path that produces radicals,and they intertwine with each other.Usually the roaming path leads to unexpected typical vibrationally excited molecular products.Roaming has an impact on the transition state theory(TST).In a series of studies the newly discovered reactive path can explain the discrepancy between the tradition TST and the forecast in experiments for the rate constant and the state distribution of products.It is of great significance to the branching ratio of the products in unimolecular dissociation reactions.It also makes our understanding for the reaction dynamics especially in unimolecular dissociation reactions develop further.It has practical significance in atmospheric chemistry,combustion chemistry,dissociation of energetic malarial,planetary chemistry,and many other fields[1].Furthermore,because roaming radical moves slowly,this course may have general importance to ultracold reaction systems[2].As a traditional theoretical method in molecular reaction dynamics,quasi-classical trajectory(QCT)method is also a powerful tool in the study of roaming problem.In this thesis,the roaming process in triatomic reaction systems is studied with QCT method based on previous work expecting to find some laws in common.For LiH2 and MgH2 systems the following questions are discussed.Firstly,systematic research is made with QCT method for H’+LiH system based on the newest PES,which completes the understanding about the dynamics of this system.Secondly,for the roaming phenomenon found in this system,an universal theoretical method(FPM)is proposed to explain and reproduce the quasi-periodic bound movement in roaming trajectories.FPM is applied to H’+ LiH and H’+MgH systems.At last,for two isotopic reactions of H+MgD and D+MgH the effects of different stages that the roaming process appears in a trajectory are discussed.The following main conclusions are drawn:1.Based on the newest PES,the effects of the reactants’ rovibrational excitation for H’+LiH system are studied in the low collision energy range.It is found that for the exchange path rotational excitation of reactants will promote the reaction,which provides a theoretical basis for the experiments.2.Roaming trajectories are found in H’+LiH reaction.And a new roaming phenomenon is found in H’+LiH and H’+MgH systems.That is,when an atom roams,the other two atoms do a quasi-periodic bound movement around their center of mass approximately in the planes of themselves.In order to explore the physical mechanism of this phenomenon,FPM is proposed to interpret the reason of the quasi-periodic bound movement.Then the theory is applied to H’+LiH and H’+MgH systems.A method is applied to find roaming trajectories,our hypothesises about roaming process are verified,and the trajectories in quasi-periodic bound movement are reproduced.Therefore we can conclude that FPM can be applied to other triatomic reaction systems to analyze the dynamic mechanism in roaming process and provide some useful dynamic information.There is some hope to generalize this method to polyatomic reaction systems.3.The roaming effects in noreaction and abstract paths for H+MgD and D+MgH reactions are studied.According to the time that a roaming process occurs in a trajectory,roaming processes are classified into two types(roaming just before a reaction ends and roaming during a reaction).It is found that the time that a roaming process occurs in a trajectory can affect ICS,differential cross section,and the rovibration state distribution of the products.