The Boltzmann constant from the shape of a molecular spectral line

Abstract We report on our recent determination of the Boltzmann constant, k B , by means of Doppler broadening thermometry. This relatively new method of primary gas thermometry was implemented by using a pair of offset-frequency locked extended-cavity diode lasers at 1.39 μm, to probe a particular vibration–rotation transition of the H 2 18 O molecule. Adopting a rather sophisticated and extremely refined line shape model in the spectral analysis procedure, we were able to determine the Doppler width from high-quality absorption spectra with unprecedented accuracy. Our spectroscopic determination of k B exhibits a combined (type A plus type B) uncertainty of 24 parts over 10 6 . The complete uncertainty budget is presented and discussed.