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

Abstract

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 ${}^{16}$O${}_2$ molecule. The model covers seven states, namely the X ${}^3{\Sigma }_g^{-}$, a ${}^1{\Delta }_g$, b ${}^1{\Sigma }_g^{+}$, I ${}^1{\Pi }_g$, II ${}^1{\Pi }_g$, I ${}^3{\Pi }_g$ and II ${}^3{\Pi }_g$ 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 $\Pi$ states, and the resultant properties are used to produce a line list for higher-order transitions of astrophysical interest.

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