FT-spectroscopy of the ¹²C¹⁸O rare isotopologue and deperturbation analysis of the A¹Π(v = 3) level

Research on ¹²C¹⁸O was carried out using two complementary Fourier-transform methods: (1) vacuum-ultraviolet absorption spectroscopy, with an accuracy ca. 0.03 cm⁻¹ on the DESIRS beamline (SOLEIL synchrotron) and (2) visible emission spectroscopy with an accuracy of about 0.005–0.007 cm⁻¹ by means of the Bruker IFS 125HR spectrometer (University of Rzeszów). The maximum rotational quantum number of the energy levels involved in the observed spectral lines was J_max = 54. An effective Hamiltonian and the term-value fitting approach were implemented for the precise analysis of the A¹Π(v = 3) level in ¹²C¹⁸O. It was performed by means of the PGOPHER code. The data set consisted of 571 spectral lines belonging to the A¹Π - X¹Σ⁺(3, 0), B¹Σ⁺ - A¹Π(0, 3), C¹Σ⁺ - A¹Π(0, 3) bands and several lines involving states that perturb the A¹Π(v = 3) level as well as to the previously analysed B¹Σ⁺ - X¹Σ⁺(0, 0) and C¹Σ⁺ - X¹Σ⁺(0, 0) transitions. A significantly extended quantum–mechanical description of the A¹Π(v = 3) level in ¹²C¹⁸O was provided. It consists of the 5 new unimolecular interactions of the spin–orbit and rotation-electronic nature, which had not been taken into account previously in the literature. The ro-vibronic term values of the A¹Π(v = 3, J_max = 55), aʹ³Σ⁺(v = 13), D¹Δ(v = 4) and I¹Σ^–(v = 5) levels were determined with precision improved by a factor of 10 relative to the previously known values.