Theoretical Study On The Electronic Structure And Spectral Properties Of Homonuclear Diatomic Molecules And Ions

Homonuclear diatomic molecules and ions accurately describe the electronic structure of the ground state and excited states by means of the D_2h subgroup under the high symmetry Abel point group D_∞h.The wavefunctions of the electronic states of such molecules and ions have clear parity,which makes the transitions between electronic states and the spin-orbit coupling（SOC）effects with a stricter selection rule.The excited states of diatomic molecules and ions of the carbon and oxygen groups have a high electronic state density.The SOC effects between the densely distributed electronic states can cause the predissociation of some bound states,resulting in bands fracture or dispersion and increasing the difficulty of spectral analysis and identification.In addition,the higher electronic density of states in the calculation needs to consider more solutions for energy eigenvalues,which is more difficult to solve involving multiple roots at the same time and poses a great challenge in terms of computational amount and computational accuracy.S and Se are important elements in astrophysics,which are widely present in the cosmic atmosphere and other celestial environments.S also has important contributions to the research and technical application of laser materials.Germanium and tin are important semiconductors,and their infrared spectrum can fully reflect many properties of semiconductors such as lattice absorption and free carrier absorption.Therefore,the spectral data of these molecules can provide important reference information for microelectronics and other related fields.The in-depth study of the spectra and transition characteristics of the electronic states of S₂,Ge₂,Sn₂molecules and Se₂⁺ions has laid the foundation for understanding and mastering the complex interactions and dynamics between the electronic states of these molecules and ions.（1）The ground state and excited states of S₂ are calculated for the dynamic correlation energy using the multi-reference configuration interaction method considering Davidson’s correction（MRCI+Q）.In the calculation,12 electrons of the3s3p orbitals of the S atom are distributed into 10 orbitals（4-5σ_g,4-5σ_u,2π_u,2π_g and3π_u）.The influence of the SOC effect on the electronic structure is also considered by the state interaction method of Breit-Pauli Hamiltonian.The potential energy curves（PECs）and spectroscopic constants of 19Λ-S states and 52Ωstates are calculated.The curve of the line width of the B³Σ_u^--X³Σ_g^-transition with the vibrational quantum number v?of the B³Σ_u^-state is calculated using the Fermi golden rule.Through analysis,we believe that the predissociation of B³Σ_u^-state at v′=11 is the result of coupling with the d¹Π_u state.The SOC of B³Σ_u^-state with both the 3⁵Π_ustate and C³Σ_u⁺state can cause the B³Σ_u^-state to dissociate at high vibrational levels,resulting in a diffuse structure when v’≥18.（2）The electronic structure and spectral properties of Ge₂ molecule are studied using the MRCI+Q method.Based on the calculated correlation energies,the PECs and spectroscopic constants of theΛ-S states andΩstates of the Ge₂ are obtained.The oscillator intensities of F³Σ⁺_u1-X³Σ^-_g1 and H³Σ^-_u1-X³Σ^-_g1 transitions are also calculated.According to the calculated PECs and SOC matrix elements ofΩstates,the interactions between F³Σ⁺_u1 and H³Σ^-_u1 states and other excited states are studied.Our calculations provide sufficient evidence that the previously observed bands of Ge₂ in the range of 20500-22000 cm^-1 should be redistributed into the transition between theΩ=1_g component of the X³Σ^-_g1 state and theΩ=1_u component of the F³Σ⁺_u1 state.Moreover,due to the strong SOC of the 2⁵Π_u repulsive state,the H³Σ^-_u1 state dissociates into the Ge（³P₂）+Ge（³P₁）channel at the vibrational level higher than v′=6.Our theoretical research will provide comprehensive information for understanding the spectroscopy and dynamics of the electronic excited states of Ge₂.（3）The electronic structure of Sn₂ molecule is studied by MRCI+Q method.The SOC effect and the core-valence correlation effect of 4d¹⁰ electrons for Sn atom are also taken into account in the present calculation.The PECs and spectroscopic constants of 19Λ-S states and 52Ωstates of Sn₂ are obtained.We also discuss the perturbation of the c¹Π_u state and d¹Σ_g⁺state by the adjacent electronic state and explain the interaction between F³Σ_u⁺、H³Σ_u^-and 2⁵Π_u states through the crossing and the avoided crossing of the PECs,the SOC matrix elements and the change of theΛ-S states component of theΩstates.According to the calculated predissociation line width of H³Σ_u^-state,the predissociation of H³Σ_u^-state is analyzed.From our calculations,it can be seen that the strong coupling between the bound state H³Σ_u^-and the repulsive state 2⁵Πcauses the H³Σ_u^-state to dissociate into the Sn（³P₂）+Sn（³P₁）channel at the vibrational level higher than v’=8.（4）Using the MRCI+Q method,the 15Λ-S states and 46Ωstates of Se₂⁺ions are studied.Based on the obtained PECs and SOC matrix elements,the perturbations of c⁴Σ_g^-and e⁴Δ_g states are analyzed.The vibrational linewidths of c⁴Σ_g^-and e⁴Δ_g states are calculated using the Fermi Golden Rule.The results show that the predissociation of the c⁴Σ_g^-state at v?=33 and v?=66 is caused with 2⁶Π_g and 4⁶Σ_g⁺states by SOC,respectively.The coupling of e⁴Δ_g and 2⁶Π_g states induces the predissociation of e⁴Δ_g at the vibrational levels of v?≥5.