An ab initio spectroscopic model of the molecular oxygen atmospheric and infrared bands

We present a unified variational treatment of the magnetic dipole matrix elements, Einstein coefficients and line strength for general open-shell diatomic molecules in the general purpose diatomic code Duo. Building on previous work in which similar expressions for the electric quadrupole transitions were developed, we also present a complete ab initio spectroscopic model for the infrared, electric dipole-forbidden, spectrum of the ¹⁶O₂ molecule. The model covers seven states, namely the X ³Σ⁻_g, a ¹Δ_g, b ¹Σ⁺_g, I ¹Π_g, II ¹Π_g, I ³Π_g and II ³Π_g states, for which 7 potential energy, 6 electronic angular momentum, 7 spin-orbit, and 14 quadrupole moment curves are calculated using ic-MRCI theory and an aug-cc-pV5Z basis set. These curves are diabatised to remove avoided crossings between the excited Π states, and the resultant properties are used to produce a line list for higher-order transitions of astrophysical interest.