Theoretical Study on the Spectroscopic Properties and Line Intensities of the O₂⁺ Cation

O₂⁺ cation, as one of the major gas components in the near space environment, has attracted significant attention due to its spectroscopic properties. In this study, we systematically investigate the spectroscopic properties of the O₂⁺ cation using ab initio methods. The potential energy curves and transition dipole moments of O₂⁺ were obtained using the icMRCI + Q method combined with the ACV5Z-DK basis set. Subsequently, the vibrational and rotational energy levels, as well as the corresponding spectroscopic constants for both ground and excited states, were determined by solving the one-dimensional radial Schrödinger equation. Based on the vibrational and rotational energy levels of bound electronic states, the internal partition function of O₂⁺ was computed over the temperature range of 100-10,000 K. Utilizing the precise potential energy functions, transition dipole moment functions, and internal partition functions, the line intensities for the First Negative Band System (a⁴Π_u-b⁴Σ_g^-) and the Second Negative Band System (X²Π_g-A²Π_u) were calculated. For the first negative band system, the spectral line intensity of Δν = 1 is maximized at temperatures ranging from 100 to 7000 K. In the case of the second negative band system, the strongest vibrational band shifts with increasing temperature. We also discuss the impact of temperature on spectral lines; at higher temperatures, a greater number of energy levels are populated, allowing for the observation of more spectral lines. These findings are significant for understanding the spectral behavior of high-temperature nonequilibrium plasmas and their role during spacecraft reentry, providing a theoretical basis for experimental research.