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.