Cavity ring-down spectroscopy of the fifth overtone of CO

•First observation of the 6–0 band of CO.•Upper-level energies determined with an accuracy of 3×10−4 cm−1.•Line parameters of the 6–0 band of 12C16O are presented.

The extremely weak 6–0 absorption band of the 12C16O molecule was recorded for the first time using a very sensitive cavity ring-down spectrometer. Frequencies of a few atomic transitions in the 0.8 μm region, transferred to the passive modes of a thermally stabilized Fabry-Pérot interferometer, are used to calibrate the observed spectra with an absolute accuracy of 3×10−4 cm−1. Line parameters, including line positions, intensities, self-broadening and -shifting coefficients were derived by fitting the observed spectra with the Voigt profile.