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In this paper, we present the progress performed in an experiment dedicated to the determination of the Boltzmann constant by an accurate measurement of the Doppler absorption profile of a line of ammonia in the gas phase in thermal equilibrium. We have recorded the hyperfine structure of the v2 saQ(6,3) rovibrational line by saturation spectroscopy and determined very precisely the induced broadening of the absorption line in linear absorption. In addition, we present an accurate analysis of the shape of the probed self-broadened line. Experimental spectra are confronted to various models that take into account Dicke narrowing or speed-dependent effects. Our results show a deviation from the Voigt profile dominated by speed-dependent broadening and shift. We also theoretically demonstrate that there is strictly no additional broadening from finite transit time across the laser beam. Finally, from the reported analysis, we anticipate that a first optical determination of the Boltzmann constant with a competitive accuracy of 1.8 parts per million is now reachable.

High precision line shape studies in low pressure ammonia for an accurate determination of the Boltzmann constant