Fig. 1. Sketch of the experimental setup.

Fig. 2. Comparison between Lamb-dip profiles of the C2H2 (ν1+ν3+ν4←ν4) R(14)e rovibrational transition, obtained by the CRD (upper frame) and SCAR (lower frame) fit of the cavity decay events, respectively, as a function of the gas pressure. In the SCAR case, an SNR as high as 110 is reached (for p=0.59  Pa).

Fig. 3. Linear fit (with intercept set to zero) to the y0(p) and γg(p) trends. While the CRD measurements obviously cannot give meaningful information on the linear absorption regime, the SCAR slope, although extracted in the sub-Pa range (even in the strong saturation regime), aligns with that determined at much higher pressures (between 1500 and 5000 Pa) by a simple (not cavity-enhanced) laser absorption spectrometer.

Fig. 4. Lamb-dip FWHMs provided by the SCAR and CRD analysis, respectively.

Fig. 5. SCAR line-center frequency determinations as a function of p. As mentioned in the text, the self-collisional shift coefficient could not be significantly estimated (by the slope of a linear fit): δself=(−3±6)  kHz/Pa.