Buffer-gas cooling of hydrogen cyanide quantified by cavity-ringdown spectroscopy

[Display omitted] •Laboratory measurement of rotationally resolved spectra of the first overtone of the C-H stretch in the near-infrared.•System benchmarked using HCN, applicable to a range of astrophysically relevant compounds.•Buffer-gas cooling is shown to be effective in thermalizing translational and rotational energies. We describe an instrument that uses continuous-wave (CW) cavity-ringdown spectroscopy to measure the translational and rotational temperature of buffer-gas cooled molecules and demonstrate its use on hydrogen cyanide. This instrument can access the near-infrared region around 1.5 μm—a rich spectral region that features the rotationally resolved first overtone of the C-H stretch for many astrophysically relevant molecules. Molecules are probed directly inside the buffer-gas cell, further enabling quantitative measurements of the effectiveness of this cooling technique.