Determination of the Boltzmann constant by laser spectroscopy as a basis for future measurements of the thermodynamic temperature

TL;DR

This paper reports a highly precise laser spectroscopic measurement of the Boltzmann constant using ammonia gas, achieving an uncertainty of 37 ppm, which supports future thermodynamic temperature measurements.

Contribution

It introduces a novel laser spectroscopy method for determining the Boltzmann constant with unprecedented precision using ammonia at 10 µm.

Findings

Achieved a Boltzmann constant measurement with 37 ppm uncertainty.

Demonstrated temperature stability and homogeneity better than 1 ppm over a day.

Validated the method as a basis for future thermodynamic temperature standards.

Abstract

In this paper, we present the latest results on the measurement of the Boltzmann constant kB, by laser spectroscopy of ammonia at 10 ?m. The Doppler absorption profile of a ro-vibrational 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 1m3 thermostat filled with an ice-water mixture, which sets the temperature very close to 273.15 K. Analysing 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 tosqrt(kBT)). 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 better than 1 ppm over a day.