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Accurate molecular spectroscopy in the mid-infrared region allows precisionmeasurements of fundamental constants. For instance, measuring the linewidth ofan isolated Doppler-broadened absorption line of ammonia around 10 $\mu$menables a determination of the Boltzmann constant k B. We report on our latestmeasurements. By fitting this lineshape to several models which include Dickenarrowing or speed-dependent collisional effects, we find that a determinationof k B with an uncertainty of a few ppm is reachable. This is comparable to thebest current uncertainty obtained using acoustic methods and would make asignificant contribution to any new value of k B determined by the CODATA.Furthermore, having multiple independent measurements at these accuracies opensthe possibility of defining the kelvin by fixing k B, an exciting prospectconsidering the upcoming redefinition of the International System of Units.

Accurate determination of the Boltzmann constant by Doppler spectroscopy: Towards a new definition of the kelvin