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We report on our ongoing effort to measure the Boltzmann constant, $k_B,$using the Doppler broadening technique on ammonia. This paper presents some ofthe improvements made to the mid-infrared spectrometer including the use of aphase-stabilized quantum cascade laser, a lineshape analysis based on a refinedphysical model and an improved fitting program 2 increasing the confidence inour estimates of the relevant molecular parameters, and a first evaluation ofthe saturation parameter and its impact on the measurement of k B. A summary ofthe systematic effects contributing to the measurement is given and the optimalexperimental conditions for mitigating those effects in order to reach acompetitive measurement of $k_B$ at a part per million accuracy level areoutlined.

Measuring the Boltzmann constant by mid-infrared laser spectroscopy of ammonia