Theory of saturated-absorption cavity ring-down: radiocarbon dioxide detection, a case study

The theoretical bases of saturated-absorption cavity ring-down (SCAR) spectroscopy are treated and described in detail in different transition broadening regimes. All the approximations that are introduced to achieve a model as simple and accurate as possible are discussed. In the Voigt broadening regime, a calibration factor that relates the measured spectral areas in the saturated and linear absorption regimes is retrieved. A relevant application of this technique to the measurement of radiocarbon dioxide concentration at both enriched and natural abundance (∼10−12) is presented. The population relaxation rate of one ro-vibrational state of the ν3 band is measured for this highly diluted CO2 isotopologue.