Deperturbation analysis of the A1Π(v = 2) level in the 12C18O isotopologue

© 2021 The Author(s)The rotational structure of the A1Π(v = 2) level of 12C18O is re-examined using high-accuracy experimental data comprised of 541 molecular lines obtained by two complementary Fourier-transform techniques. The absorption spectrum of the A1Π – X1Σ+(2, 0) band, in the range 66,500 - 67,650 cm–1, was recorded by the vacuum-ultraviolet FT spectrometer at the DESIRS beamline of the SOLEIL synchrotron. Visible emission spectra of the B1Σ+ – A1Π(0, 2) and C1Σ+ – A1Π(0, 2) bands in the range 19,200 - 20,000 and 24,300 - 24,800 cm–1 were obtained with a Bruker IFS-125HR spectrometer at the University of Rzeszów. The absolute accuracy of line frequencies are 0.01 and 0.005 - 0.01 cm−1, respectively. Results from the B1Σ+ – X1Σ+(0, 0) and C1Σ+ – X1Σ+(0, 0) absorption bands of 12C18O were added to the experimental data set. A deperturbation analysis of A1Π(v = 2) is performed with an effective Hamiltonian and a term-value fitting approach. Accurate molecular constants for A1Π(v = 2) and the e3Σ–(v = 4), d3Δ(v = 7), aʹ3Σ+(v = 12) and I1Σ–(v = 3) perturbing levels were determined. Perturbation parameters of the spin-orbit A1Π(v = 2) ~ [e3Σ–(v = 4), d3Δ(v = 7), aʹ3Σ+(v = 12)] and rotation-electronic (L-uncoupling) A1Π(v = 2) ~ [I1Σ–(v = 3, 4), D1Δ(v = 3)] interactions, were obtained. A significant, indirect influence of the a3Π state on the A1Π state was detected in 12C18O and has therefore been included in the final fit by taking into account the simultaneous a3Π(v = 13) ~ [e3Σ–(v = 4), d3Δ(v = 7), aʹ3Σ+(v = 12)] ~ A1Π(v = 2) spin-orbit/spin-electronic/L-uncoupling and spin-orbit interactions as well as the a3Π(v = 13) ~ [D1Δ(v = 3), I1Σ–(v = 3)] ~ A1Π(v = 2) spin-orbit and L-uncoupling interactions. This work results in determination of 110 rotational term-values for the A1Π(v = 2) state and its perturbers.