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Sub-promille measurements and calculations of CO (3--0) overtone line intensities

Intensities of lines in the near-infrared second overtone band (3--0) of $^{12}$C$^{16}$O are measured and calculated to an unprecedented degree of precision and accuracy. Agreement between theory and experiment to better than 1 $\permil$ is demonstrated by results from two laboratories involving two independent absorption- and dispersion-based cavity-enhanced techniques. Similarly, independent Fourier transform spectroscopy measurements of stronger lines in this band yield mutual agreement and consistency with theory at the 1 $\permil$ level. This set of highly accurate intensities can provide an intrinsic reference for reducing biases in future measurements of spectroscopic peak areas.