Microwave and terahertz frequencies of O2 determined with saturated absorption spectroscopy near 763 nm

We report kilohertz-level precision measurements of magnetic dipole transitions in the 16O2A-band using optical frequency comb-referenced cavity ring-down saturation absorption spectroscopy. Under a zero magnetic field, 30 transitions were recorded with center frequency uncertainties within several kilohertz, representing an improvement of two orders of magnitude over previous studies. From these measurements, 19 ΔN = 0 (microwave) and 12 ΔN = 2 (terahertz) ground-state rotational frequencies were derived using the combination difference method. A global fit of the data produced a new set of spectroscopic parameters for the X3Σg-(0) and b1Σg+(0) states, achieving a root-mean-square deviation of 18 kHz and surpassing earlier benchmarks. Our results reveal systematic deviations in existing terahertz frequency data and demonstrate that saturation spectroscopy provides superior accuracy for determining rotational energies compared to direct microwave or terahertz measurements, particularly at high rotational quantum numbers. This work establishes the most precise frequency reference to date for the oxygen A-band and terahertz region, supporting future advances in high-resolution spectroscopic databases, atmospheric remote sensing, and interstellar O2 searching.