Jet-cooled ethylene cavity ring-down spectroscopy between 5880 and 6200 cm−1.

• An innovative slit nozzle with adjustable opening is used to cool C2H4 down to 6 K. • Cavity ring-down spectroscopy provides high sensitivity in the 1.6–1.7 µm range. • Rotational cooling allows observation of 14 new vibrational bands. • 320 rovibrational lines are assigned to 20 bands using the TheoReTS linelist. Absorption spectra of jet-cooled ethylene (ethene, C 2 H 4) are recorded at three different rotational temperatures (6/8 K, 12 K, 38 K) using cavity ring-down spectroscopy (CRDS) in the 5880–6200 cm−1 spectral region. Rotational cooling is used to determine the various vibrational band centers by simplifying drastically the rotational band structure. A line-by-line assignment, based on a direct comparison with the TheoReTS variational line list and a systematic use of lower state combination difference (LSCD), is performed. Experimental line lists including line position and line integrated absorption cross sections are drawn up. The 6/8 K, 12 K and 38 K spectra contain 668, 1553 and 1679 absorption lines respectively. Overall, 320 rovibrational lines are assigned across 20 interacting vibrational bands. Among the 20 vibrational cold bands identified in this work, 14 had never been observed before. Line intensities are in the range of 10−24 - 10−20 cm/molecule. A direct comparison between our work and the TheoReTS and ExoMol theoretical line lists, as well as with the recent experimental work of Ben Fathallah et al. (2024) is provided. [ABSTRACT FROM AUTHOR]