Ab Initio Calculation On Potential Energy Curves And Vibration-rotation Spectrum Of Cl₂⁺

Molecular spectroscopy is the disciplines to study the interaction betweenmolecules and radiation, which is helpful in understanding the electronic structuresand the transitions properties in molecules. Most of the information aboutmolecules is based on molecular spectroscopy. Molecular spectroscopy is one ofthe most important means in studying molecular structure. Nowadays，it can beconcluded that most knowledge on the molecular structure comes from molecularspectroscopy. Spectroscopy is a widely used technology, which provides us with alot of reliable data in determining the chemical composition and the structure ofmatter.Reports and documents of Cl2molecular structure and spectra at this stage isubiquitous, in contrast, reports of Cl₂⁺molecular ion are very rare. Cl₂⁺molecularion plays an important role in nature world; it is not only the intermediates of thechemical, physical processes of many molecules with chlorine, but also theionization product of Cl2molecules. The study of Cl₂⁺electronic structure andenergy levels will be useful to understand the physical and chemical properties ofthe Cl2molecules and Cl₂⁺molecular ions.We use the quantum chemistry package MOLPRO2010calculate someaspects of Cl₂⁺molecular ion theoretically: 1. The complete active space self-consistent field （the CASSCF） andmulti-reference configuration interaction （MRCI） methods were used tocalculate the potential energy curve of the ground state X2Π+gof Cl2molecularion. Firstly we employ different basis sets cc-PVXZ （X=T, Q,5,6） in ab initiocalculations, the calculated results were extrapolated to complete basis set（CBS）. At the same time, we also take the scalar relativistic, spin-orbit coupling,and core-valence （CV） shell electron correlation energy corrections intoaccount. The results show that the CV correction has a greater impact on thepotential energy curve, and the modified potential energy curve match with theexperimental results better.2. According to the calculation result of ground state X2Πgof Cl₂⁺molecular ion,we calculate the vibration-rotational levels of ground state X2Πgof Cl₂⁺molecular ion, and they are comparable with available experimental values.The results show that the calculation error falls into the experimental error bar,and the lower vibrational levels of calculated results in line with theexperimental ones, at the same time, the calculation results complement thelack of vibration-rotational energy levels on the experiment.3. In this article, we discuss the isotope effect （35Cl and37Cl） on vibrational-rotational constants. The results show that the isotope effect of Cl₂⁺molecularion is not palpable; there was a downward trend of vibrational-rotationalenergy with increasing of mass. In addition, we adopt the coupled-clustertheory to calculate dissociation energy of X2Π+gstate of Cl2molecular ion. 4. At MRCI/aug-cc-pV5Z theoretical level, we calculated potential energy curvesof excited states of Cl₂⁺molecular ion that correspond to the first threedissociation limits. And we also compare these excited state with the oneswhich already been observed. As the same time, we give some excited stateswhich have not been observed. We also calculate spectroscopy constants ofbound states of Cl₂⁺molecular ion.