On the 12C2H2 near-infrared spectrum: absolute transition frequencies and an improved spectroscopic network at the kHz accuracy level

Abstract

Lamb dips of twenty lines in the P, Q, and R branches of the ν₁ + ν₃ + ν₄¹ vibrational band of ¹²C₂H₂, in the spectral window of 7125–7230 cm⁻¹, have been measured using an upgraded comb-calibrated frequency-stabilized cavity ring-down spectrometer, designed for extensive sub-Doppler measurements. Due to the large number of carefully executed Lamb-dip experiments, and to the extrapolation of absolute frequencies to zero pressure in each case, the combined average uncertainty of the measured line-center positions is 15 kHz (5 × 10⁻⁷ cm⁻¹) with a 2-σ confidence level. Selection of the twenty lines was based on the theory of spectroscopic networks (SN), ensuring that a large number of transitions, measured previously by precision-spectroscopy investigations, could be connected to the para and ortho principal components of the SN of ¹²C₂H₂. The assembled SN contains 331 highly precise transitions, 119 and 121 of which are in the ortho and para principal components, respectively, while the rest remain in floating components. The para- and ortho-¹²C₂H₂ energy-level lists, determined during the present study, contain 82 and 80 entries, respectively, with an accuracy similar to that of the lines. Based on the newly assembled lists of para- and ortho-¹²C₂H₂ empirical energy levels, a line list, called TenkHz, has been generated. The TenkHz line list contains 282 entries in the spectral range of 5898.97–7258.87 cm⁻¹; thus far, only 149 of them have been measured directly via precision spectroscopy. The TenkHz line list includes 35 intense lines that are missing in the HITRAN2020 database.