Comparison of CRDS to ICL-PAS and phase-shift CRDS spectroscopies for the absolute intensities of C–H (ΔvCH=6) overtone absorptions

Cavity ring-down spectroscopy (CRDS) has been used to obtain the visible overtone spectra (ΔvCH=6) of neo-pentane, C(CH3)4, propane, C3H8, and n-butane, C4H10, yielding absolute f-values for the transitions to better than 3%. For the neo-pentane overtone intensity, comparison with a recent measurement using intra-cavity laser photoacoustic spectroscopy (ICL-PAS) provides favourable agreement, with improved precision. Being absolute this value may be used as a standard for relative intensity measurements obtained by ICL-PAS. The measured propane and n-butane overtone intensities, when compared to recent work using phase-shift CRDS, indicate a lack of agreement to quoted uncertainties.