Rovibrational assignments of the ethylene absorption spectrum near 3.3 µm based on variational calculations

• The strong and congested ethylene absorption spectrum near 3.3 µm is revisited by Fourier transform spectroscopy • Rovibrational assignments rely on position and intensity agreement with a variational line list and are validated by Lower State Combination Difference (LSCD) relations • A total of 5807 transitions of the 2912 - 3270 cm -1 region are assigned to eight cold bands and four hot bands • A very good agreement with the HITRAN list is noted, although HITRAN assignments are limited to four cold bands and one hot band • The recent ECaSDa calculated list limited to the four strongest bands shows significant deviations The congested absorption spectrum of ethylene (C 2 H 4 ) is analyzed in the 2912-3270 cm -1 region dominated by the strong ν 9 and ν 11 CH stretching bands. A list of about 6870 lines has been retrieved from a room temperature Fourier transform spectrum ( P = 2.568 hPa, L = 19.7 cm). Relying on the position and intensity agreements with a line list of 12 C 2 H 4 transitions calculated by the variational method, a total of 4287 lines are assigned to 5797 transitions of eight cold bands, four of them being newly reported. In addition, about 200 transitions are assigned to four hot bands. Most of the reported assignments are confirmed by Lower State Combination Difference (LSCD) relations i.e. that 1824 of the 1897 determined upper states energies have coinciding determinations through several transitions (up to 8). Compared to their variational counterpart, the obtained empirical energy values show a good agreement with a small overestimation with an amplitude increasing with the rotational energy but limited to 0.6 cm -1 at most. Overall, the assigned lines represent 94.0 % of the total experimental intensity in the region. The band-by-band intensity comparison with variational calculations shows an excellent agreement, mostly within the experimental uncertainties of about 3 %. Systematic line-by-line comparisons with previous literature studies are discussed in detail. In the current HITRAN line list, rovibrational assignments are limited to the four strongest cold bands and one hot band. Excellent agreement is observed for line positions and line intensities. Suggestions for improvements are provided. Comparisons with the recent ECaSDa line list (Ben Fathallah et al., J Quant Spectrosc Radiat Transf 2024;323:108995 https://doi.org/10.1016/j.jqsrt.2024.108995 ), calculated using an effective operator approach developed within the tensorial formalism and limited to the four strongest cold bands, show a number of significant disagreements.