Fourier transform spectroscopy of CO intercombination bands

Several bands of CO intercombination systems have been recorded for the first time under high-resolution Fourier transform spectroscopy: B¹Σ⁺-e³Σ^-(0-1), B¹Σ⁺-d³Δ_i(0-5), and B¹Σ⁺-d³Δ_i(0-12), B¹Σ⁺-a′³Σ⁺(0-14), and B¹Σ⁺-a′³Σ⁺(0-17). The emission spectrum was produced from an electric discharge through a mixture of formic acid and helium. Observed wavenumbers of rotational lines (0 < J < 19) are presented. The line position uncertainties range from 0.001 to 0.02 cm^-1, depending on the transition intensity. These bands are forbidden by electric-dipole selection rules and borrow intensity from the main Angstrom B¹Σ⁺-A¹Π system, via spin-orbit interactions. It is shown that these data are consistent with recent studies of perturbations arising in the A¹Π levels. In addition, the mixing coefficients, which represent the fractional A¹Π character of each line, are also reported.