Determination of the Boltzmann Constant by Laser Spectroscopy as a Basis for Future Measurements of the Thermodynamic Temperature

In this paper, the latest results on the measurement of the Boltzmann constant kB, by laser spectroscopy of ammonia at 10μm are presented. The Doppler absorption profile of a rovibrational line of an NH3 gas sample at thermal and pressure equilibrium is measured as accurately as possible. The absorption cell is placed inside a large 1 m3 thermostat filled with an ice–water mixture, which sets the temperature very close to 273.15 K. Analyzing this profile, which is related to the Maxwell–Boltzmann molecular speed distribution, leads to a determination of the Boltzmann constant via a measurement of the Doppler width (proportional to \({\sqrt{k_{\rm B} T})}\). A spectroscopic determination of the Boltzmann constant with an uncertainty as low as 37 ppm is obtained. Recent improvements with a new passive thermostat lead to a temperature accuracy, stability, and homogeneity of the absorption cell of 1 ppm over a day.