Doppler broadening thermometry based on cavity ring-down spectroscopy

A Doppler broadening thermometry (DBT) instrument is built based on cavity ring-down spectroscopy (CRDS) for the precise determination of the Boltzmann constant. Compared with conventional direct absorption methods, the high-sensitivity of CRDS allows one to reach a satisfactory precision at lower sample pressures, which reduces the influence due to collisions. By recording the spectrum of C2H2 at 787 nm, we demonstrate a statistical uncertainty of 6 ppm (part per million) in the determined linewidth values by several hours' measurement at a sample pressure of 1.5 Pa. As for the spectroscopy-determined temperatures, although with a reproducibility better than 10 ppm, we found a systematic deviation of about 800 ppm, which is attributed to 'hidden' weak lines overlapped with the selected transition at 787 nm. Our analysis indicates that it is feasible to pursue a DBT measurement toward the 1 ppm precision using cavity ring-down spectroscopy of a CO line at 1.57 m.