【作者】 刘政；

【导师】 王志刚；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2022， 硕士

【摘要】 超原子（Superatom）作为由多个原子形成的复杂高对称性分子系统,它的分子轨道展现出相近于原子中依壳层排布的电子轨道对称性,所以超原子具有丰富的物理内涵,并在模拟乃至替代原子方面有重要前景。尤其注意到,在一般分子的里德堡态激发（Rydberg state excitation,RSE）研究中,受分子轨道的对称性影响,控制价电子实现激发到更高或特定的里德堡态是较为复杂乃至困难的。相比之下,具有高点群对称性的超原子可带来重要前景。不仅如此,超原子作为分子,它的性质也必然受到原子核运动的影响。考虑到近平面超原子（Nearly planar superatom,NPS）这一特殊类型的存在,其内部原子核的运动可引起构象发生自异构化,从而出现不同于传统稳定结构的动态特性。应该说,正是由于超原子兼具在电子激发和原子核运动两方面的特性,所以引起了广泛的研究兴趣。而对其相应物理规律的把握,都需要有针对性的开展研究工作,以进一步促进对超原子的基础乃至应用研究进展。正是在这种背景下,本论文于理论上研究了金属超原子的电子激发特性和近平面超原子的自异构化过程。首先,我们对一系列金超原子开展了电子激发特性研究。结果表明,在这些金超原子Au_n（n=4、6、7和13）的高激发态和低激发态中,它们的超原子分子轨道（Superatomic molecular orbitals,SAMOs）对电子跃迁都有重要贡献,体现出了类里德堡态激发的特征。电子密度的径向分布函数显示,最低未占据分子轨道和其他更高未占据分子轨道中电子的主要分布均远离几何中心,并且这些轨道具有较高的对称性,因此可以明确地将它们标识为里德堡轨道。进一步,考虑到里德堡态激发的一个极端例子,即电子被电离到离核无限远,我们在中性结构基础上研究了相应一价阳离子结构的性质,以探讨超原子在里德堡态激发下的稳定性。虽然Au_n⁺（n=4、6、7和13）发生了轨道重排,但它们仍然是稳定的超原子结构。此外,由于平面超原子的分子轨道具有二维延展性,超原子在里德堡态激发调控方面可能具有一定优势。基于以上,我们提出了超原子的里德堡态激发（Superatomic Rydberg state excitation,SRSE）这一新概念。这些发现为基于超原子的里德堡态激发提供了新来源,并且,也可能为高效快速制备里德堡态提供一个全新的视角。基于以上对超原子中电子行为的认识,我们进一步考虑到分子的异构化对分子的电子结构和光谱有重要的影响。而超原子作为特殊的分子,它的异构化行为尚不清楚,特别是考虑到在近平面超原子中沿对称平面的异构化是不可避免的。我们通过第一性原理计算探索了由近平面超原子Th@Au₆与外围金环Au₁₂形成近平面超原子Th@Au₁₈的自异构化过程,并构建了以Au₁₂环为对称平面的势能面（Potential energy surfaces,PESs）。其中,Th@Au₁₈的自异构化过程是从全局稳定结构出发,首先经历一个Th原子向几何中心方向的运动过程;第二步,Th原子先后穿过内圈金环和外圈金环;第三步,Th原子朝着远离几何中心方向运动,由此实现了Th原子运动幅度为3.4（?）的完整自异构化过程。结果清楚地表明在周期性的自异构化过程中,单重态S₀和三重态T₁势能面存在交叉行为。S₀态和T₁态之间的跃迁可以通过系间窜越实现,而不是辐射跃迁。在T₁态下,不同异构体的最大能量差在0.1 e V以下,说明在不需要打破化学键的前提下自异构化过程可以发生。电子结构分析表明,在这个过程中各个极值点都保持了超原子特性,其超原子组态可表示为1²1_<sup>21_<sup>21²1²_2-22²1_<sup>21_<sup>22_<sup>22_<sup>22¹2¹_2-2。此外,紫外-可见吸收光谱和振动光谱都反映了自异构化过程的周期性特征。这些发现将为超原子自异构化过程的理论研究和实验观测提供有价值的参考。通过上述研究,我们发现超原子可展现出类里德堡态激发特性,并由此提出了超原子的里德堡态激发这一新概念。通过对锕系内嵌近平面超原子的自异构化过程分析,发现这一个过程中存在不同电子态之间的势能面交叉行为,表明在不需要打破化学键的情况下就可以实现自异构化过程。通过本论文的工作,我们对超原子的电子激发和核运动行为形成了一定程度上的规律性把握,希望为以电子结构为基础的超原子激发态相关研究提供重要参考,并促进实验上观测超原子异构化的实验与理论研究发展。更多还原

【Abstract】 As complex and highly symmetric molecular systems formed by multiple atoms,superatoms exhibit molecular orbital symmetry similar to that of the electrons arranged in shells of atoms.Therefore,superatoms have abundant physical connotations and important prospect of simulating or even replacing atoms.Especially,it is noticed that controlling the excitation of valence electrons to higher or specific Rydberg states is difficult due to the symmetry of molecular orbitals in the research of general molecular Rydberg state excitation（RSE）.However,superatoms with high point group symmetry hold important promise.Not only that,because superatoms are polyatomic molecules,their properties are inevitably affected by the motion of atomic nuclei.Considering the existence of a special type of nearly planar superatom（NPS）,the nucleus motion can cause seif-isomerization,resulting in dynamic properties different from traditional stable structures.Due to the properties of superatoms in both electron excitation and nuclear motion,they have attracted extensive research interest.To grasp the corresponding laws of physics,it is necessary to carry out relevant work in a targeted manner to further promote the progress of the basic and even applied research on superatoms.It is under this background that the electron excitation properties of metal superatoms and the self-isomerization process of NPS were studied theoretically in this thesis.First,we studied the electron excitation properties of a series of gold superatoms.The results show that their superatomic molecular orbitals（SAMOs）have important contributions in electronic transitions in the high and low excited states of Au_n（n=4,6,7,and 13）.The radial distribution function of electronic density shows that the main distribution of electrons in the lowest unoccupied molecular orbitals and other higher unoccupied SAMOs is extremely far from the geometric center,thus,they can be unambiguously identified as Rydberg orbitals.As an extreme case of RSE,an electron is ionized to an infinite distance.Therefore,we investigate the structural properties of the corresponding monovalent cation species based on neutral structures to investigate the stability of superatoms under RSE.Although the orbital rearrangement of Au_n⁺（n=4,6,7,and 13）appears,they are still stable superatomic structures.In addition,due to the two-dimensional ductility of planar SAMOs,superatoms are superior in the RSE regulation.Thus,we propose a new concept of superatomic Rydberg state excitation（SRSE）.These findings may provide a new source of superatom-based RSE and a new perspective on the regulation and efficient preparation of Rydberg states.The isomerization of molecules has important implications on their electronic structure and spectra.However,this behavior in superatoms（as special molecule）is unclear,especially considering that isomerization along the symmetry plane is inevitable in NPS.We explored the self-isomerization process of NPS Th@Au₁₈formed by NPS Th@Au₆with an outer gold ring Au₁₂.By constructing the potential energy surfaces（PESs）with the Au₁₂ring as the symmetry plane,it was found that the process can be divided into three steps.Firstly,the Th atom moves toward the geometric center;secondly,it passes through the inner ring and the outer ring;lastly,the Th atom moves away from the geometric center.Thus,a complete seif-isomerization process with Th atom motion amplitude of 3.4（?）is realized.The results clearly show that in the periodic process of self-isomerization,there are PES crossing behaviors between singlet S₀and triplet T₁states.The transition between two electron states can be achieved by intersystem crossing（such as between E1-E2 or E4-E5）,rather than radiative transition.Moreover,the maximum energy difference of different isomers is below 0.1 e V in T₁state,indicating that the self-isomerization process occurs relatively easily without breaking the chemical bond.The analysis of electronic structure shows that each structure on extreme point maintains the superatomic properties during the whole process,and its superatomic configuration can be expressed as 1²1_<sup>21_<sup>21²1²_2-22²1_<sup>21_<sup>22_<sup>22_<sup>22¹2¹_2-2.In addition,both vibration and UV-Vis absorption spectra reflect the periodic properties matching with the self-isomerization process.These findings will provide a valuable prospect for theoretical investigation and experimental observation of superatomic seif-isomerization.In summary,we found superatoms can exhibit Rydberg-like excitation properties,and proposed the concept of SRSE.Moreover,by analysing the seif-isomerization process of actinide embedded NPS,different electronic states exhibit the SPESs crossing behavior,indicating that the seif-isomerization process is easy to occur without breaking bonds.Based on the above work,we have formed a certain degree of regular understanding in the electronic excitation and nuclear motion behavior of superatoms.These results can provide an important reference for the related research of superatom excited states based on electronic structure,and promote the experimental observation of superatomic isomerization.更多还原