VUV-VIS FT spectroscopy of the rare 13C18O isotopologue of carbon monoxide: Analysis of the A1Π(v = 1) multiply-perturbed level

Ro-vibronic spectra of the ¹³C¹⁸O carbon monoxide isotopologue were obtained with (i) emission spectroscopy in the visible region using a Bruker IFS 125HR spectrometer (University of Rzeszów) and (ii) vacuum-ultraviolet absorption spectroscopy using the wave-front-division spectrometer on the DESIRS beamline of the SOLEIL synchrotron. A deperturbation analysis of the ¹³C¹⁸O A¹Π(v = 1) level was conducted from 598 observed transitions from the B¹Σ⁺ - A¹Π(0, 1), C¹Σ⁺ - A¹Π(0, 1), A¹Π - X¹Σ⁺(1, 0), B¹Σ⁺ - X¹Σ⁺(0, 0), C¹Σ⁺ - X¹Σ⁺(0, 0), I¹Σ^- - X¹Σ⁺(2, 0) bands and five further nominally forbidden bands. An effective Hamiltonian and term-value fitting analysis was implemented. Consequently, 135 parameters were floated: 23 molecular parameters, including molecular constants for A¹Π(v = 1), I¹Σ^-(v = 2), d³Δ(v = 6), e³Σ^-(v = 3) and D¹Δ(v = 1); rotation-electronic (L-uncoupling) mixing of A¹Π(v = 1) ∼ [D¹Δ(v = 1), I¹Σ^-(v = 1), I¹Σ^-(v = 2)] and spin-orbit interaction parameters for A¹Π(v = 1) ∼ [d³Δ(v = 6), e³Σ^-(v = 3), a'³Σ⁺(v = 11)]; the spin-orbit/spin-electronic/L-uncoupling a³Π(v = 12) ∼ d³Δ(v = 5) and spin-orbit a³Π(v = 12) ∼ [D¹Δ(v = 1), I¹Σ^-(v = 2)] perturbation parameters; as well as 112 ro-vibronic term values of B¹Σ⁺(v = 0) up to J = 50 and C¹Σ⁺(v = 0) up to J = 60. The significant, indirect a³Π(v = 12) ∼ [e³Σ^-(v = 2, 3), d³Δ(v = 5, 6)] ∼ A¹Π(v = 1) spin-orbit/spin-electronic/L-uncoupling interaction and a³Π(v = 12) ∼ [I¹Σ^-(v = 2), D¹Δ(v = 1)] ∼ A¹Π(v = 1) spin-orbit/L-uncoupling interaction were detected and analysed. Thus, this study, using modern experimental methods and deperturbation analysis, leads to a much improved description in terms of molecular constants and interaction parameters, compared to previous studies of the A¹Π(v = 1) energy region in the ¹³C¹⁸O isotopologue. This research is a continuation of the studies on the A¹Π state and its numerous perturbers in the CO isotopologues made by our team.