The Schumann-Runge absorption bands of 16O18O in the wavelength region 175-205 nm and spectroscopic constants of isotopic oxygen molecules

Abstract High-resolution absorption spectra of 16 O 18 O have been photographed, at 300 and 78 K, throughout the region 175–205 nm, in the first order of a 2400 lines/mm grating in a 6.65-m vacuum spectrograph at a dispersion of 0.06 nm/mm. Precise wavelength measurements and rotational analyses of the Schumann-Runge bands (2,0)-16,0) have been completed. Spectroscopic constants of the B 3 Σ u − state of 16 O 18 O for 2 ≤ v ≤ 16 have been determined. Rotational perturbations are observed in the (16,0) band. The concept of mass-reduced vibrational quantum numbers, μ − 1 2 (v + 1 2 ) , has been used to combined isotopic molecular constants from 16 O 2 , 16 O 18 O, and 18 O 2 . It has been shown that the functions of the vibrational spacings, μ 1 2 Δ G v+ 1 2 , rotational constants μB v , and μ 2 D v , spin-spin constants, λ v , and spin-rotation constants, μγ v , are isotopically invariant functions of μ − 1 2 (v + 1 2 ) . The isotopic dependence of the spin-spin constants λ v is discussed in terms of the unique perturber approximation. Values of γ v and λ v have been obtained by interpolation for the levels v = 2–8, which correspond to bands of 16 O 18 O with unresolved triplet structure. In a theoretical investigation, the calculations of level shifts and perturbations have been reexamined. Excellent agreement between calculated and experimental level shifts has been obtained for all three isotopic molecules.