Theoretical Study Of The Analytical Potential Function,Spectroscopy And Radiative Transition Properties Of LiS

Multi-reference configuration interaction method is an effective and practical quantum chemistry tool, and the electron correlation effect has been almost fully considered. It is highleveled accurate quantum chemistry methods for predicting the total energy of molecular system, its prediction of energy spectrum of small molecule system can be conformed to that from experimental data. 6Li32 S is a simple sulfide diatomic molecule; it is helpful to make clear the mechanism of the process of binding between the lithium atom and Sulphur atom in the lithium battery for the study of the potential energy function and electronic state about the molecule. The potential energy functions and their spectrum constants about the nine low lying electronic states（X2Π, a4Δ, B2Δ, b4Δ, A2Σ+, C2Π, F2Σ –, E2Σ+ and D2Π） of 6Li32 S diatomic molecule have been studied by the method of multi-reference configuration quantum chemistry method（MRCI/ CASSCF/ aug-cc-p V5Z） provided by Molpro 2010 software package. The results show that the ground electronic state of6Li32 S is X2Π, its equilibrium inter-nuclear distance is 2.141 ?, and its infrared spectrum frequency is 638.6 cm-1. There is only one minimum energy point on the curve of the potential energy function. Its dissociation limit of the ground state is ⁶Li³²S→6Li（2Pg） + 32S（3Pg）, in which the atom 6Li and 32 S are all in their ground state 6Li（2Pg） and 32S（3Pg）. Electronic states B2Δ, a4Δ and b4Δ have different potential energy functions and various stabilities, but they have the same dissociation limit as the ground state, which the latter two states have the spin multiplicities of four differ from that of ground state. The electronic states of A2Σ+, C2Π, F2Σ – and E2Σ+ have the same dissociation limits of 6Li（2Pg） + 32S（1Pg） and the same spin multiplicities of two as the ground state. They are all in low lying excited states with low stabilities, which the lithium atom is also in the ground state in the dissociation limit, but the sulfur atom is in the singlet excited state in the dissociation limit. The electronic state D2Π is a low lying excited state of the neutral diatomic molecule 6Li32 S, where the dissociation limit is cation 6Li+ and anion 32S-,suggesting the electronic state is ion-pair state.The total electronic energies of the 9 low lying states of 6Li32 S as well as the various nuclear distances have been calculated by multi-reference configuration interaction method with the Davison correction and without Davison correction. The electronic potential energiesof the 9 electronic states were treated as the part of the potential of the nuclear movements.The transition from the first low lying excited state A2Σ+to ground state X2Π has been studied with the method of software LEVEL8.0 which solves the one dimension Schr?dinger equation provided by Le Roy. The electron transition dipole moment functions, Einstein coefficients, Frank- Condon factors and the radiative lifetimes of transition from excited state A2Σ+to ground state X2Π have been studied. The lifetimes of the six vibrational energy levels from the lowest excited state A2Σ+ transited to the ground state X2Π have been calculated. The results show that the lifetimes become shorter with the quantum numbers increasing. But the lifetimes of the six vibrational energy levels have the same order of magnitudes and in the range between 1.5 to 3.5 ns. The lifetime with longest of the vibrational energy is about 3.48 ns. That is to say, the spectrum of the six vibrational energy levels can be observed experimentally.The electron transition dipole moment functions, Einstein coefficients, Frank- Condon factors and the radiative lifetimes of transition from excited state C2Π, A2Σ+and B2Δ to ground state X2Π have been studied. The results show that the electron transition dipole moment functions from lowest excited electronic state A2Σ+to ground electronic state X2Π is rapid decreasing from a small positive value（less than 1 Debye） to a small negative value（larger than-1 Debye） with the distance between two nucleus increasing from short distance to longer distance, then increase slowly to 0 Debye as the distance between two nucleus increasing.Electronic transition moment, Einstein coefficients, Frank-Condon factors and radiative lifetime for the A2Σ+—X2Π, B2Δ—X2Π, C2Π—X2Π system have been calculated. The balanced distance between two nuclei, harmonic frequencies and moment of inertias of ground state X2Π predicted in this paper are in accordance with that of experimental datum. The balance distances between the two nucleuses in the electronic states of b4Π, C2Π, D2Π are all longer than 4 ?, so they are very unstable. The D2Π electronic state will dissociate to Li+ion and Sion far away from each other. The electronic transition dipole moments, Einstein coefficients,Franck-Condon factor and radiative lifetimes in the transition from lowest excited A2Σ+state to ground state X2Π are predicted in this paper. The electronic transition dipole moments fromthree low lying electronic state A2Σ+, B2Δ and C2Π to the ground state X2Π has been calculated under aug-cc-p V5Z/MRCI level. The results show that the electronic transition dipole moment of A2Σ+→X2Π has a small positive value while the nuclei distance is short, and rapidly decrease down to a small negative value during the nuclei distance increasing to around balance distance. Then it is stable at zero value while the nuclei distance continually increase. The electronic transition dipole moment of B2Δ→X2Π has a small negative value（which is larger than that of A2Σ+→X2Π） at the short nuclei distance, and it soars up to a small positive value during the nuclei distance close to about balance distance. Then it slow down to zero while the nuclei distance increasing to about 4 ?. Finally it keeps stable about zero value while the nuclei distance continually increase. The electronic transition dipole moment of C2Π→X2Π is more sophisticated, but it has more large values than that of other two transitions.So the low-lying electronic state A2Σ+is more stable than B2Δ, and B2Δ is more stable than that of C2Π. The results also show that the ground state X2Π and the lowest excited state A2Σ+have similar IR frequencies with their difference within 8 cm-1, they cannot be distinguished by IR spectrum. The A2Σ+has shorter balanced distance than ground X2Π about 0.076 ?. It means A2Σ+has stronger chemical bond than that of ground X2Π.This paper first describes the basic theory of molecular structure and molecular spectra,the born-Oppenheimer approximation of the molecular potential energy function, diatomic molecular potential energy function properties, atomic orbital and molecular orbital symmetry and molecular orbital theory basic point, construction method and bisection electronic state simple introduction and comparison. Secondly,it describes the introduction of the quantum chemistry calculation based, including many electron wave functions of the system and the energy expression, processing related to the electronic post HF methods. In the post HF methods, it mainly expounds the concept of the electronic correlation energy, configuration interaction, completely space self consistent field and multi-parameter test configuration interaction and the wave function of the reference single state and multi reference configuration method of constructing.