The first observation of the 12C16O2 absorption bands near 660 nm

The absorption spectra of carbon dioxide were recorded in the region from 15,000 to 15,300 cm⁻¹, using a Bruker IFS 125 HR Fourier transform spectrometer and a 30 m multipass cell with the White type optical system. The recording was performed at a spectral resolution of 0.044 – 0.050 cm⁻¹, room temperature, a path length of 1057.95 m and pressures of 185 and 362 mbar. Utilization of a LED as a light source provided a sensitivity (noise equivalent absorption) at the level of k_ν = 1.23 × 10^-10 cm⁻¹ and allowed detection of a number of lines of two 3005i − 00001 (i = 2,3) bands and several lines of the 60031 – 00001 band of ¹²C¹⁶O₂ with the intensity values down to 10^-30 cm⁻¹/(molecule cm⁻²) at 296 K. These bands were observed for the first time. The uncertainty of the line position measurements was estimated to be about 0.005 cm⁻¹ for the unblended lines with a high signal-to-noise ratio. The uncertainties of the retrieved line intensities for the strongest unblended lines are at the level of 15 %. The spectroscopic constants for observed bands were fitted to the observed line positions. The vibrational transition dipole moments squared of these bands were fitted to the observed line intensities. The measured line positions were compared to those from the HITRAN2020 database and to those predicted with the global effective Hamiltonian. The measured line intensities were compared to the values from the HITRAN2020 database and from the Ames2021 line list.