Theoretical Study On The Electronic Excitation And Self-isomerization Process Of Superatoms

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.