On the determination of the Boltzmann constant by means of precision molecular spectroscopy in the near-infrared

Intensity-stabilized laser absorption spectroscopy, in the near-infrared, represents an extremely powerful tool for primary gas thermometry, provided that an accurate procedure of frequency calibration is used. The method consists in retrieving the Doppler width from highly accurate observations of the shape of a spectral line in any atomic or molecular system in the gas phase, at the thermodynamic equilibrium. Applied to CO2 samples in the 2-μm wavelength region, it has recently allowed to perform a spectroscopic determination of the Boltzmann constant with a relative accuracy of 1.6 × 10−4. We here discuss the main factors, of both fundamental and technical nature, limiting the accuracy of this method and report on the possibility of approaching the target accuracy of 10−6 by changing the molecular sample, the operation wavelength and the frequency calibration technique.