Absolute frequency measurements of CO2 transitions at 4.3 μm with a comb-referenced quantum cascade laser

The infrared spectrum of the (0111–0110) ro-vibrational band of 12C16O2 in the range 2306–2312 cm−1 is investigated with saturated-absorption sub-Doppler spectroscopy. The absolute frequencies of six transitions belonging to the P-branch of this band are measured by recording their Lamb-dip features in a pump-probe detection scheme employing a room-temperature quantum cascade laser. The laser is phase-locked to a subkilohertz-linewidth difference-frequency-generated radiation source, which is referenced to an optical frequency comb synthesiser. The achieved relative uncertainties range from 1×10−11 to 5×10−11, improving by three to four orders of magnitude the previous tabulated values for such frequencies. Moreover, thanks to this precision level, self-pressure-shift coefficients due to collisional processes of CO2 molecules are reported for the first time.