Abstract
Accurate molecular spectroscopy in the mid-infrared region allows precision measurements of fundamental constants. For instance, measuring the linewidth of an isolated Doppler-broadened absorption line of ammonia around 10 $\mu$m enables a determination of the Boltzmann constant k B. We report on our latest measurements. By fitting this lineshape to several models which include Dicke narrowing or speed-dependent collisional effects, we find that a determination of k B with an uncertainty of a few ppm is reachable. This is comparable to the best current uncertainty obtained using acoustic methods and would make a significant contribution to any new value of k B determined by the CODATA. Furthermore, having multiple independent measurements at these accuracies opens the possibility of defining the kelvin by fixing k B, an exciting prospect considering the upcoming redefinition of the International System of Units.
Highlights
Since 1889 the unit of mass has been defined by K, the international prototype of the kilogram, a platinum cylinder located at Bureau International des Poids et Mesures (BIPM) in Sèvres (France)
We report on our latest progress on the determination of the Boltzmann constant kB by accurate molecular spectroscopy in the mid-infrared region, using the Doppler Broadening Technique (DBT)
The Rautian profile (RP) assumes strong hard sphere collisions against infinitely massive particles, after each of which the memory of the initial velocity is lost [37, 38]. Another subtle effect is the fact that the collisional broadening and shifting slightly depend on the molecular speed. Taking this into account leads to the speed-dependent Voigt profile (SDVP) with an overall narrowing compared to the usual Voigt profile
Summary
Since 1889 the unit of mass has been defined by K, the international prototype of the kilogram, a platinum cylinder located at Bureau International des Poids et Mesures (BIPM) in Sèvres (France) In more than a hundred years, comparisons of K with other prototypes showed drifts on the order of 50 g (5 × 10−8 in relative value) This directly impacts the definition of the mole, the ampere and the candela which depend on it. One objective is to avoid using artifacts or macroscopic properties of matter Another objective is to redefine units in terms of fundamental constants expected to not vary in time. We are aiming to provide a competitive measurement using an alternative method in order to contribute to the CODATA value and to the new definition of the kelvin
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