【作者】 吴春晓；

【导师】 田善喜；

【作者基本信息】 中国科学技术大学 ， 物理化学， 2020， 博士

【摘要】 离子-分子反应在星际空间、星球大气、燃烧火焰和等离子体刻蚀中广泛存在,并且在其物质演化中发挥着重要作用,反应动力学研究可帮助我们更准确的建立这些物质演化的模型,因此在相关研究或实际应用中具有重要意义。一个世纪以来,随着技术的发展实验方法在不断更新。得益于超声分子束技术和粒子探测技术的发展,目前进行离子-分子反应研究的主要方法为信号较弱但包含动力学信息最多的交叉束方法。交叉束研究在反应动力学领域取得了众多令人欣喜的成果,其相关理论也得到了相应的发展,我们实验组也在离子-分子反应领域做了大量工作。首先,为了实现全三维牛顿球的直接探测,我们对实验室现有的低能离子-分子反应速度成像装置的探测系统进行了更新。我们重新设计了适用于三维速度成像的透镜,并将探测器从荧光屏+CCD相机升级为延迟线阳极探测器,从而使整个探测系统具备了直接全三维速度成像的能力。在国际上,我们首次将直接全三维离子速度成像技术引入低能离子-分子反应实验中,这将给其立体反应动力学研究带来革新。由于探测器的升级避免了高压时间切片脉冲的使用,提高了实验重复频率,这些极大的提高了实验效率。使用自主开发的数据提取程序包我们成功复原出了离子产物的全三维牛顿球,并且针对不同的反应体系编写了后续的数据处理分析程序。在仪器升级后,我们进行了下述离子-分子反应实验。(1)在质心碰撞能7.23 eV～15.96 eV范围内,我们研究了 Ar+与CO2的电荷转移反应。经分析发现CO2+产物主要分布在基态X2Πg态和单电子激发态A2Πu、B2Σu+和C2Σg+态;在较高的碰撞能下,也存在CO2+的多电子激发态。CO2+产物效率曲线与CO2的光电离电子谱明显的不同,说明从Ar+到CO2的电荷转移速度不是预期的快速电荷转移过程。短寿命中间产物(Ar-CO2)+中的强电子关联应是多电子激发态CO2+产生的原因。(2)在3.40 eV～9.24 eV的质心碰撞能量范围内研究了 Ar+(2P)和O2(X3Σ-g)之间的电荷转移反应。随着碰撞能的增加,产物O2+的内能逐渐增强,前向散射的O2+离子在较高碰撞能下分布在较宽的散射角范围内。在3.40 eV的低碰撞能下,类似于光子电离过程的共振电荷转移导致了O2+离子a4Πu态呈现类似Franck-Condon跃迁的振动态布居。在较高的碰撞能下,除了在光电离过程中可见的a4Πu态和高能态之外,O2+产物可以布居在非Franck-Condon区的一些电子束缚态上。目前的观察结果再次表明电荷交换反应强烈的碰撞能依赖性,但与我们以前对Ar++NO→Ar+NO+研究的发现明显不同。(3)几十年来,由反应物之间相互准直或取向引起的分子碰撞反应的立体动力学一直是化学界的一个热门话题。然而从未在实验上观察到共线和非共线碰撞共存的现象,而且,根据我们的常识,对于随机取向的反应物,通常缺乏立体动力学方面的内容。我们研究了 Ar+与O2的解离电荷转移过程,在随机取向的O2与低能Ar+离子的解离电荷交换反应中,利用三维离子速度图成像技术,清楚地观察到了 O+沿碰撞轴的线性排列和近似各向同性的分布。我们用两个多普勒动力学模型解释了上述观测结果及其相应的能量转移效率,并用O2+的自旋态耦合进一步解释了这些观测结果,表明在共线碰撞和一定角度的碰撞中存在显著的电荷和能量转移偏好。更多还原

【Abstract】 The ion-molecule reaction is frequently observed in interstellar space,planetary atmosphere,combustion flames,and plasma etching,and play an important role in their material evolution.Its dynamics research helps us to establish these material evolution models more accurate,which is of great significance in relevant research or practical application.For a century,experimental methods have been continuously improved with the development of technology.Due to the development of supersonic molecular beam technology and particle detection technology,at present,the main method for ion-molecular reaction research is the cross beam method with weak signal but most dynamic information.Many gratifying results have been obtained in the field of reaction dynamics with cross beam research,and the related theories have been developed accordingly.Our group has also done a lot of work in the field of ion-molecular reaction.Firstly,to realize the direct detection of the full three-dimensional Newton sphere,the detection system of our low-energy ion-molecule reaction velocity map imaging device was updated.We have redesigned the lens suitable for three-dimensional velocity map imaging and upgraded the detector from a phosphor screen+CCD camera to a delay-line anode detector so that the entire detection system has the capability of full three-dimensional direct imaging.Internationally,for the first time,we have introduced direct full-three-dimensional ion velocity imaging technology into low-energy ion-molecule reaction experiments,which will bring innovations to the study of stereodynamics.owing to the upgrade of the detector,the use of high pulse voltage in time slices is avoided and improves the repetition rate of the experiment,which greatly improves the efficiency of the experiment.We successfully recovered the full three-dimensional Newton sphere of ion products using a self-developed data extraction package and wrote the subsequent data processing and analysis program for different reaction systems.With the upgraded instrument,we have carried out the following experiments.(1)In a center-of-mass collision energy range of 7.23 eV～15.96 eV,we studied the charge transfer reaction between Ar+and CJ2.After analysis,we found that CO2+products are primarily populated at the ground state X2Πg and the single-electron excited states A2Πu,B2Σu+ and C2Σg+;the multielectrou excited states of CO2+ are also found at the higher collision energies.The production efficiency profiles of CO2+ are distinctly different from the photoionization electron spectrum of CO2,implying that the charge transfer from Ar+would be not as fast as expected.The strong electron correlations in the short-lived intermediate(Ar-CO2)+should be responsible for the CO2+ yields at the multielectron excited states.(2)The charge transfer reactions between Ar+(2P)and O2(X3Σg-)are investigated in the collision energy range of 3.40 eV-9.24 eV within the center-of-mass coordinate,by using the ion momentum imaging technique.The internal energy of the product O2+is enhanced gradually with the increase of collision energy,and the forward-scattered O2+ ions are distributed in the broader range of scattering angles at higher collision energies.At the low collision energy of 3.40 eV,the resonant charge transfer,similar to a photon ionization process,leads to the Franck-Condon-like vibrational state population of O2+ at the a4Πu state.At the higher collision energies,besides a4Πu and the high-lying states that are visible in the photoionization process,the O2 products could be populated at some electronically bound states in the non-Franck-Condon region.The present observations indicate again the strong collision-energy dependences of the charge exchange reactions,but distinctly different from our previous finding for Ar++NO→Ar+NO+.(3)Stereodynamics of molecular collisional reaction arising from the mutual alignment or orientation between the reactants has been an attractive topic in chemistry for decades.However,co-existence of the collinear and no-collinear collisions was never observed experimentally,furthermore,on our common sense,the stereodynamics aspects are usually absent for the randomly orientated reactants.We studied the dissociation charge transfer process of Ar+and O2,here between randomly orientated O2 and low-energy ion Ar+,we,using three-dimensional ion velocity-map imaging technique,clearly observed a linear alignment and a nearly isotropic distribution of the O+yields along the collision axis.Above observations,as well as their corresponding energy-transfer efficiencies,are interpreted with two Doppler dynamics models and further elucidated with the spin-state couplings of O2+,indicating the remarkable charge-and energy-transfer preferences in the collinear and no-collinear collisions.更多还原