Calculation of electric quadrupole linestrengths for diatomic molecules: Application to the H₂, CO, HF, and O₂...

We present a unified variational treatment of the electric quadrupole (E2) matrix elements, Einstein coefficients, and linestrengths for general open-shell diatomic molecules in the general purpose diatomic code Duo. Transformation relations between the Cartesian representation (typically used in electronic structure calculations) to the tensorial representation (required for spectroscopic applications) of the electric quadrupole moment components are derived. The implementation has been validated against accurate theoretical calculations and experimental measurements of quadrupole intensities of ¹H₂ available in the literature. We also present accurate electronic structure calculations of the electric quadrupole moment functions for the X¹Σ⁺ electronic states of CO and HF, as well as for the a¹Δ_g-b¹Σ_g ⁺ quadrupole transition moment of O₂ with the MRCI level of theory. Accurate infrared E2 line lists for ¹²C¹⁶O and ¹H¹⁹F are provided. A demonstration of spectroscopic applications is presented by simulating E2 spectra for ¹²C¹⁶O, H¹⁹F, and ¹⁶O₂ (Noxon a¹Δ_g-b¹Σ_g ⁺ band).