Abstract:We report on our on-going effort to measure the Boltzmann constant, k B , using the Doppler broadening technique on ammonia. This paper presents some of the improvements made to the mid-infrared spectrometer including the use of a phase-stabilized quantum cascade laser, a lineshape analysis based on a refined physical model and an improved fitting program * Present address:, LNE-SYRTE, Observatoire de Paris, CNRS, UPMC, 61 Avenue de l'Observatoire, 75014 Paris, France † Author, to whom any correspondence should be addressed.2 increasing the confidence in our estimates of the relevant molecular parameters, and a first evaluation of the saturation parameter and its impact on the measurement of k B . A summary of the systematic effects contributing to the measurement is given and the optimal experimental conditions for mitigating those effects in order to reach a competitive measurement of k B at a part per million accuracy level are outlined.

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The semiclassical formalism of Robert and Bonamy is used to calculate the linewidths of rovibrational transitions of ammonia in collision with argon. Two accurate ab initio potential energy surfaces (PES) have been applied to model the NH 3 -Ar interactions. In this work and contrary to our previous calculations, the transitions with DK = ±3n (n integer) have been introduced into the expressions of the differential collision cross section S(b). Comparisons with previous theoretical and experimental studies are reported. A detailed analysis of the contribution of the various anisotropic components of the PES shows that the induced DK = ±3 transitions play a crucial role. Calculations performed in the m 4 and m 1 vibrational bands of NH 3 are in good agreement with the experimental data and correct dependences of the broadening coefficients with the rotational quantum numbers J and K are obtained.