Saturation cavity ring-down spectrometry using a dynamical relaxation model

We propose a new simple approximate solution to the two-state rate equation model for analyzing decay signals of saturation cavity ring-down spectrometry in the adiabatic and low-saturation regime. It helps obtain baseline-immune Doppler-free spectra for hyperfine transitions and linear absorption coefficients of a gas in the saturation regime. To demonstrate it, a baseline-immune Lamb dip spectrum of the R1A2 transitions in the 2v₂ + v₃ band of methane was recorded. The line position was determined to be 6 076.108 457 7(11) cm^-1, the relative uncertainty being 1.8 × 10^-10.