Molecular oxygen fine structure with sub-kHz accuracy

Highlights: • Oxygen fine-structure lines up to N = 43 are investigated by means of two spectrometers. • Measured frequencies have several times higher accuracy than previous data. • The most accurate to date effective molecular constants are obtained. Two spectrometers with complementary capabilities, in particular, a spectrometer with radioacoustic detection of absorption and a conventional video spectrometer, were used for systematic measurement of the frequencies of all magnetic-dipole transitions of the fine structure of the ¹⁶O¹⁶O molecule in the ground state X³${\Sigma }_g^{-}$ up to rotational states with N = 43. The obtained data set made it possible to refine significantly (the uncertainty was reduced from 4 to 6 times) the molecular constants responsible for the fine structure of the rotational oxygen levels, and to reduce the uncertainty in predicting the frequencies of the corresponding transitions by about an order of magnitude. For all fine-structure lines essential for atmospheric applications, the uncertainty is less than 1 kHz.