【作者】 王晓林；

【导师】 郑雨军； 杨欢；

【作者基本信息】 山东大学 ， 原子与分子物理， 2022， 博士

【摘要】 “Roaming”现象,从2004年第一次在甲醛的单分子解离实验中被发现,到现在已经被公认为是在单分子解离、双分子反应中可能发生的一种非常普遍的现象,也是一种重要的、却被长期忽略了的反应机制。它的特殊性表现在:(1)Roaming路径会绕过势能面上与传统的过渡态相关联的最低能量路径,而在一个高能、平坦的区域“游历”(这也是roaming一词的由来);(2)该路径与形成原子基团的其他通道相互竞争,纠缠在一起;(3)通常生成典型的振动激发的分子产物。这种现象给过渡态理论带来了冲击。一系列研究证明,这种新发现的反应路径能用来解释传统的过渡态理论(Transition state theory,TST)和实验上对于速率常数和产物态分布的预测之间的不符情况,它对单分子解离中产物的分支比有重要的意义。这种新的反应机制使我们对反应动力学,特别是对单分子解离过程有了更深入的理解。它在大气化学、燃烧化学、含能材料的分解和行星化学等多个专业领域具有现实意义[1]。此外,由于roaming原子基团运动非常缓慢,故这一过程对于超冷反应体系可能具有普遍的重要意义[2]。准经典轨线法(QCT)作为一种传统的分子动力学理论方法,在研究roaming问题方面也是一个有力的工具。本论文在前人工作的基础上,应用QCT方法对三原子反应体系的roaming过程进行研究,期望找到一些具有共性的规律。针对LiH2和MgH2体系本论文主要讨论了以下问题:首先,对H’+LiH反应体系,基于一个新的势能面采用准经典轨线法进行系统的动力学研究,完善对该反应动力学的认识;其次,对于在该反应中发现的roaming现象,提出一种对三原子反应体系普适的理论方法(有效势法),用于解释和重现roaming轨线中出现的准周期束缚运动,并应用到H’+LiH和H’+MgH体系;最后,对H+MgD和D+MgH两种同位素反应,讨论在反应不同阶段出现的roaming过程对产物特性的影响。研究发现:1.采用最新势能面,在低碰撞能区对H’+LiH体系反应物的振转激发效应进行了系统研究,发现对于交换反应转动激发会促进反应的进行,为实验提供了理论依据。2.在H’+LiH反应体系中发现roaming轨线,并且在该体系以及H’+MgH体系中发现,一个原子作roaming运动时,另外两个原子围绕它们的质心近似在各自平面内作一种准周期性的束缚运动。为了探索这一现象的物理机制,我们用有效势法解释这种轨线的成因,并将该理论成功地应用到H’+LiH和H’+MgH体系,提出了一种寻找roaming轨线的方法,验证了我们关于roaming过程特征的一些假设,并将这种准周期性轨迹复现出来。由此我们认为,这一方法可以推广到其他三原子反应体系,分析roaming过程轨线的动力学机制,给出roaming过程的一些有用的动力学信息,并有希望被推广到多原子反应体系。3.对H+MgD和D+MgH反应的不反应通道和提取通道的roaming机制进行了研究,根据roaming过程在反应中出现的时间节点的不同,将反应中的roaming过程分为两类(一类恰好在反应完成前,另一类发生在反应中间),发现roaming过程在反应中出现的时间节点会影响产物的积分截面、微分截面及转振态分布。更多还原

【Abstract】 "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.更多还原