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 ${P}_{\mathrm{e}}(12)$ line of the $(30012)\ensuremath{\leftarrow}(00001)$ band of $\mathrm{C}{\mathrm{O}}_{2}$ at 1.578 \textmu{}m 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 $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ (type A) and $11\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ (type B), which ranks the system at the first place among optical methods. Thanks to a measurement time of only $\ensuremath{\approx}5\phantom{\rule{0.16em}{0ex}}\mathrm{h}$, the technique represents a promising pathway toward the optical determination of the thermodynamic temperature with a global uncertainty at the ${10}^{\ensuremath{-}6}$ level.