Towards an optical measurement of the Boltzmann constant at the 10-5level

We present a new method for direct determination of the Boltzmann constant by laser spectroscopy. The principle consists in recording the linear absorption in a vapour of an atomic or molecular line at a controlled temperature around 273.15 K. By eliminating the pressure broadening, we deduced the pure Doppler width which gives a first optical measurement of the Boltzmann constant kB. The present determination should be significantly improved in the near future and contribute to a new definition of kelvin.