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Figure 3.7 contains the FM spectra collected with the in situ instrument discussed above for (a) ¹²C¹⁶O₂ and (b) ¹³C¹⁶O₂. All of the FMS instruments built at LANL were designed to be research instruments that are capable of probing multiple absorption features as depicted in Figure 3.7.

Figure 3.7 The derivative shaped FMS spectrum recorded for several (a) ¹²C¹⁶O₂ and (b) ¹³C¹⁶O₂ transitions. The blue trace in both plots represents the baseline where the laser did not probe CO₂.

The White cell contained ~100 ppm ¹²C¹⁶O₂ and ¹³C¹⁶O₂, and the laser was scanned across multiple absorption features. Since the CO₂ concentration remained constant, the intensity of the FM trace changed with the intensity of the population in each of the rovibrational states. In both experiments, the laser was scanned over the 1,605–1,607 nm range where the population in the ¹³CO₂ states remained relatively constant while the ¹²CO₂ populations were declining. For both experiments, the baseline is depicted in the blue trace at a spectral wavelength that is not absorbed by CO₂ and demonstrates the dynamic range available by this method. In principal, only one rovibrational state in ¹²CO₂ and ¹³CO₂ is necessary to calculate an isotope ratio. While these spectra were collected with the in situ instrument with a constant CO₂ concentration, one records similar spectra with the remote instrument.