Cavity ring-down spectroscopy measurements of sub-Doppler hyperfine structure

Frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) was used to measure magnetic dipole transitions in the b^1Σ_g^+ ← X^3Σ_g^-(0,0) band of O_2. The ^(17)O-containing isotopologues show unresolved hyperfine structure due to magnetic hyperfine splitting in the ground state. The sensitivity and stability of FS-CRDS allow for quantitative sub-Doppler measurements of the hyperfine constants, even when the hyperfine splittings are much smaller than the Doppler width. Unlike saturation spectroscopy, this linear absorption technique can be applied to weak transitions and employed to quantitatively measure intensities and line shapes. This method may be an attractive approach for measuring unresolved hyperfine structure in excited electronic states.