Theoretical Studies On Superatomic States Under High Pressure

Superatoms,which are derived from mass spectrometry experiments of clusters,have attracted more and more attention in basic science and related applied research.As artificial unit,superatoms can exhibit electron configuration characteristics similar to natural atoms’,which is expected to create special physical and chemical properties,and therefore it is believed that they can be used to simulate or even replace natural atoms in forming a rich and diverse material world.At present,the research and exploration of superatoms largely rely on stable structures under conventional conditions.Obviously,whether comparing with atoms or in the sight of the application requirements,extending the study of superatoms to extreme conditions such as the pressure could help us form regular understanding and general grasp of the physical effects toward superatoms.As we all know,by affecting microstructure and interatomic interactions,extreme conditions such as high pressure can create strange physical properties that are generally difficult to obtain under conventional conditions,so as to effectively extend traditional structure research into state research,which is still a direction need to be urgently developed for superatoms.In this paper,the structural evolution processes of superatoms under different compression conditions are systematically studied based on the theoretical calculation of first-principles to reveal the movement law of the energy level structure of superatomic molecular orbitals with high pressure as a regulation dimension and furthermore,to develop the regulation approach of superatomic electron energy level arrangement based on group theory.Firstly,we introduced pressure as regulation dimension and extended the study of superatoms from structure to the state under high pressure.Specifically,isotropic compression of CH₄@C₆₀ system formed by embedding methane in fullerene C₆₀molecules was simulated theoretically.Interestingly,when the distance between CH₄and C₆₀ was compressed until within the van der Waals boundary,the local molecular orbitals contributed by CH₄ gradually transformed into superatom molecular orbitals（SAMOs）.And new occupied molecular orbitals—3P SAMOs formed by the linear combination of methane’s 1P SAMOs and C₆₀’s 2P SAMOs.These results indicate that environmental conditions such as pressure can be a key factor in maintaining and even acquiring superatomic properties,which is consistent with the basic understanding that high pressure is a powerful tool for acquiring new physical properties.In addition,we calculated the potential energy surface and analyzed the tunneling effect for the rotational behavior of methane molecules,and found that the orders of magnitude of methane molecules’tunneling probability is as high as 10^-2 at the low temperature（20K）where the thermodynamic effect is small,which indicates that tunneling effect can make methane molecules rotate relatively freely in the C₆₀ confined space.Our work helps to break through the long-standing habit of thinking that the study of superatoms is limited to stable structures,and opens up a new direction from structure to state for the physical exploration of superatoms.Then,considering that high pressure has been used as an important tool forunderstanding structural evolution,this provides an opportunity to reveal the connection between superatoms of different dimensions.Therefore,by axial compressions of the typical carbon-based superatom U@C₂₈（T_d symmetry）,it is found that ground state of U@C₂₈ remained as triplet when axial compression was carried out in the direction that makes the symmetry of the system decrease to D_2d.And the ground state of U@C₂₈ changes from triplet to singlet when symmetry of system is axially compressed towards C_2v or C_s.The spin population analysis shows that the change of electron state is due to the change of electron occupation mode caused by the different response of electron spin to different axial compressions.Moreover,we confirmed the evolution process of superatom from stereoscopic to near-planer structure and the relationship between their electronic structures.In the axial compression that reduces the symmetry of the system to D_2d,the electron density distribution of SAMOs（D _z 2 and F _z 3）which extends along the compression direction gradually shrank,and some special delocalization properties of SAMOs related to the compression direction were destroyed.In the way,the superatom exhibits the characteristic that tends to be planar in its electronic structure.In addition,we found the splitting phenomenon in Raman spectra and the hyperchromic effect as well as red shift of characteristic peak in ultraviolet?visible（UV-Vis）absorption spectra in this process,which are expected to be used in fingerprinting of superatomic planarization.The above results reveal the effect of axial compression on the electronic structure of superatoms,which can provide reference for understanding the planarization process of three-dimensional superatoms.Finally,based on above understanding of superatomic states and their electron configuration under the influence of pressure,we hope to develop an effective method in principle to regulate the energy level distribution of superatomic molecular orbitals by pressure.To this end,we confirmed by first-principles calculation that the SAMOs’splitting caused by reducing the point group symmetry of fullerene superatom C₂₀^2-under compression is consistent with the basic theory of point group symmetry evolution.Based on this result,we theoretically further realized the SAMOs’splitting adjusted by axial compression or its the reverse process—expansion.Correspondingly,in the process of compression or expansion,with the change of structural parameters,cross of energy levels happens between the highest occupied molecular orbital and the lowest unoccupied molecular orbital with different angular momentum,which reduces the symmetry of the system and further led to the decrease of system energy.This phenomenon is consistent with conclusion of the well-known Jahn-Teller effect,which shows that the system develops towards a low energy state with low symmetry.Moreover,the above results were extended to C₆₀ and U@C₂₈ superatomic systems to confirm the general significance of regulating SAMOs’splitting by directionally decreasing its point group symmetry.These results provide new insights into the electronic structure of superatoms and are expected to lay a solid foundation for the continuous development of functional regulation of superatoms.In summary,through the first-principles systematic theoretical research on isotropic compression,axial compression and its inverse process,axial expansion of superatoms,this paper extends the research scope from the superatomic structure under conventional condition to its state under high pressure,reveals that through the axial compression of superatoms we can obtain the transformation approach of superatomic electronic state and the evolution law of the geometric and electronic structures between superatoms of different dimensions,and develops a modulation idea to regulate splitting of SAMOs through axial compression and expansion.The research carried out in this paper will not only inspire the important perspective of understanding and regulating the properties of superatomic electronic structures,but also provide a valuable direction for realizing the regulation of the functional properties of superatoms,and contribute to the continuous progress of basic molecular physics research.