Progress towards an accurate determination of the Boltzmann constant by Doppler spectroscopy

TL;DR

This paper reports significant progress in measuring the Boltzmann constant using Doppler spectroscopy of ammonia, achieving a statistical uncertainty of 6.4 ppm, and discusses future improvements for even higher accuracy.

Contribution

The study demonstrates laser spectroscopy as a viable alternative to acoustical methods for determining the Boltzmann constant with high precision.

Findings

Achieved a statistical uncertainty of 6.4 ppm in measuring k.

Recorded hyperfine structure and linewidth broadening of ammonia line.

Showed negligible impact of saturation effects on linewidth measurements.

Abstract

In this paper, we present significant progress performed on an experiment dedicated to the determination of the Boltzmann constant, k, by accurately measuring the Doppler absorption profile of a line in a gas of ammonia at thermal equilibrium. This optical method based on the first principles of statistical mechanics is an alternative to the acoustical method which has led to the unique determination of k published by the CODATA with a relative accuracy of 1.7 ppm. We report on the first measurement of the Boltzmann constant by laser spectroscopy with a statistical uncertainty below 10 ppm, more specifically 6.4 ppm. This progress results from improvements in the detection method and in the statistical treatment of the data. In addition, we have recorded the hyperfine structure of the probed saQ(6,3) rovibrational line of ammonia by saturation spectroscopy and thus determine very precisely the induced 4.36 (2) ppm broadening of the absorption linewidth. We also show that, in our well chosen experimental conditions, saturation effects have a negligible impact on the linewidth. Finally, we draw the route to future developments for an absolute determination of with an accuracy of a few ppm.