The ethylene absorption spectrum between 6075 and 8050 cm-1: Empirical line list and rovibrational assignments

The highly congested absorption spectrum of ethylene (C₂H₄) is analyzed between 6075 and 8050 cm^-1. In the 6200–8050 cm^-1 range, a list of about 32,650 lines is retrieved from a room temperature spectrum recorded by Fourier transform spectroscopy (P = 15.72 mbar, L = 45 m). This dataset was merged with a set of about 3460 lines available in the literature in the region of the strong ν₅+ν₉ band near 6150 cm^-1. In addition, two FTS spectra at 130 K provide complementary information in the 6020–6320 cm^-1 range (about 7570 lines).

Relying on the position and intensity agreements with a line list of ¹²C₂H₄ transitions calculated by the variational method, a total of 4090 transitions is assigned to eighteen bands, ten of them being newly reported. All the reported assignments are confirmed by Lower State Combination Difference (LSCD) relations i.e. all the upper states (1749 in total) have coinciding determinations of their energies through several transitions (up to 6). The obtained empirical energy values are given and compared to their variational counterpart. As an additional validation test of the lower state assignments, the room temperature intensities are extrapolated at 130 K and compared to their experimental values. Overall, a large fraction of the strong lines of the region is assigned and the total intensity of the assigned transitions represents 46 and 55% of the total variational and experimental intensities at 296 K, respectively. In the considered range, variational positions deviate from measurements by a few cm^-1 and the total variational absorption is underestimated by 40 %.