Absolute frequency measurements of CHF3 Doppler-free ro-vibrational transitions at 8.6 um

We report on absolute measurements of saturated-absorption line-center frequencies of room-temperature trifluoromethane using a quantum cascade laser at 8.6 ?m and the frequency modulation spectroscopy method. Absolute calibration of the laser frequency is obtained by direct comparison with a mid-infrared optical frequency comb synthesizer referenced to a radio-frequency Rb standard. Several sub-Doppler transitions falling in the v vibrational band are investigated at around 1158.9 cm with a fractional frequency precision of 8.6·10 at 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6·10 is mainly limited by the reproducibility of the frequency measurements.