CO₂laser stabilized on narrow saturated absorption resonances of CO₂, improved absolute frequency measurements

We have stabilized a CO 2 laser on narrow saturated absorption resonances of CO 2 detected on the transmitted beam of an external Fabry-Perot cavity in the 30 THz spectral region. By using another CO 2 laser stabilized on a close OsO 4 line whose absolute frequency is precisely known, and measuring the beat frequency 4 with a precision of 100 Hz, we determined the absolute frequency of twenty CO 2 laser lines, with uncertainties between 100 Hz and 1 kHz mainly limited by the accuracy of the OsO 4 grid. The set of measurements reveal some discrepancies with data based on measurements performed with the widely used saturated fluorescence technique; we attribute these to pressure shifts of several kilohertz under the conditions of this last method.