Theoretical Study on the Spectroscopic Properties and Line Intensity of Silicon Monosulfide Cation

The SiS⁺ cation is considered a potential astromolecule, yet there is limited documentation regarding its spectroscopic properties and spectral line intensities. In this paper, the potential energy curves, dipole moments, and transition dipole moments of the SiS⁺ cation are calculated using the icMRCI + Q method. By solving the one-dimensional Schrödinger equation for the nucleus, we have obtained spectroscopic constants for both the ground state and 11 low-lying excited states. Utilizing the vibrational transition energy levels of these 12 bound states, we calculated the partition function for the SiS⁺ cation over the temperature range of 300-10,000 K. Upon deriving the partition function and Einstein coefficients, we have computed the spectral line intensities for the X²Π state, the A²Σ⁺ state, and the X²Π ↔ A²Σ⁺ transition at 300 and 3000 K for Δν = 0,1,2. At T = 300 K, the intensity of the X²Π state 1-0 band reaches its maximum, while at T = 3000 K, the intensity of the A²Σ⁺ state 0-0 band reaches its maximum. The spectral line intensities of the X²Π ↔ A²Σ⁺ transition, on the other hand, are relatively small, with the overall intensity being smaller compared to the spectral line intensities of the X²Π and A²Σ⁺ states by about 10^-6.