Accurate Computational Study On Spectroscopy Of Small Molecules In Astrophysics

Molecular spectroscopy is one of the most important methods to study molecular structure.The study of molecular spectra provides information about the electronic structure,transition properties,electronic state coupling and other aspects of molecular information,which gains knowledge about molecular structure.The spectroscopic properties and structures of electronic states of small molecules in astronomy play a key role in physical and chemical reactions and interactions between molecules.Therefore,the study of accurate spectroscopic calculation of small molecules has important theoretical significance and application value.In this work,accurate spectroscopic of AlCl and NS molecules have been carried out using the explicit correlation multi-reference configuration interaction(MRCI-F12)and multi-reference configuration interaction(MRCI)methods,respectively.The detailed research contents are as follows:(1)The potential energy curves,spectroscopic constants and dipole moments of AlCl molecules are calculated and studied by the MRCI-F12 method.The electronic structure of AlCl molecule is accurately calculated by MRCI-F12 method.The potential energy curves(PECs)of 13?-S electronic states corresponding to the two dissociation limits of the AlCl molecule are obtained with the SOC effects,consideration of SR effects and Davidson correction(+Q)in the calculation.Based on PECs,the spectroscopic constants of the three lowest bound electronic states are computed.The calculated spectroscopic constants are in good agreement with the experimental values.We computed permanent dipole moments of the 13(43)-S states.All the calculated PDMs at the large internuclear distance are almost zero a.u.with the exception of the 3¹?⁺state,indicating that the 3¹?⁺state is generated at the ion pair dissociation limit,and other electronic states are generated at the neutral atom dissociation limit.(2)The effect of the SOC effect on the transitions and electronic states of AlCl molecules has been studied.Taking the SOC effect into account,the two dissociation limits of AlCl molecule split into five dissociation limits,and the 13(43)-S states split into 24?states.The potential energy curves of 24?states are calculated and the spectroscopic constants of the X¹(50)⁺,A¹(47)and a³(47)states after splitting are fitted.With the help of the calculated SOC matrix elements of the electronic states,the dissociation mechanism of A⁽¹⁶⁾(47)state is discussed and analyzed.We also calculated the transition dipole moments from the ground state X0⁺to 0⁺(2),1(1)and 1(2)states,discussed and analyzed the influence of the spin-orbit coupling effect on the transition properties.The radiative lifetime of the 1(2)-X0⁺state of AlCl molecule is predicted to be 5.48 ns,which is within the range of reported experiment 6.4�2.5 ns.(3)The spectroscopic constants and transition properties of NS molecules are calculated.In this work,the spectroscopic constants of the 12?-S state of NS molecules are computed.We computed results are relatively close to the theoretical and experimental results.The results of the spectroscopic constants are closer to the experimental values than the previous theoretical ones in this paper.At the same time,the transition properties and electronic structure of NS molecule are also computed,including the electronic dipole moment,the PECs,the FCFs and the TDM.And the corresponding radiative lifetime is also predicted.The radiative lifetimes of2²(47)-X²(47)(11)1²(?)-X²(47)(11)2²(50)⁽¹⁰⁾-X²(47)and 1²(50)^--X²(47)transitions are reported for the first time.The radiative lifetime of 2²(47)-X²(47)transition of the NS molecule are in good agreement with the experimental results.