Cavity-ring-down Doppler-broadening primary thermometry

A step forward in Doppler-broadening thermometry is demonstrated using a comb-assisted cavity-ring-down spectroscopic approach applied to an isolated near-infrared line of carbon dioxide at thermodynamic equilibrium. Specifically, the line-shape of the Pe(12) line of the (30012) ← (00001) band of CO2 at 1.578 μm is accurately measured and its Doppler width extracted from a refined multispectrum fitting procedure accounting for the speed dependence of the relaxation rates, which were found to play a role even at the very low pressures explored, from 1 to 7 Pa. The thermodynamic gas temperature is retrieved with relative uncertainties of 8 × 10−6 (type A) and 11 × 10−6 (type B), which ranks the system at the first place among optical methods. Thanks to a measurement time of only ≈5 h, the technique represents a promising pathway toward the optical determination of the thermodynamic temperature with a global uncertainty at the 10−6 level.