New assignments and a rare peculiarity in the high sensitivity CRDS spectrum of acetylene near 8000 cm−1.

The absorption spectrum of acetylene has been recorded at room temperature (296 K) using high sensitivity Cavity Ring Down Spectroscopy in the 7914 and 8252 cm −1 interval. The noise equivalent absorption of the spectra is α min ∼ 5×10 −11 cm −1 . A list of about 5600 absorption features was constructed. The smallest intensities are on the order of 10 −29 cm/molecule. A total of 1325 rovibrational lines of 12 C 2 H 2 were assigned by comparison with accurate predictions provided by a global effective operator model. In addition, 132 rovibrational lines of 12 C 13 CH 2 present in natural isotopic abundance were assigned on the basis of their published positions. The assigned 12 C 2 H 2 lines belong to 12 new and 6 already known bands, for which additional J -lines were assigned. The line intensities of the three cold bands of 12 C 13 CH 2 are reported for the first time. The new data will be valuable to refine the parameters of the global effective Hamiltonian and dipole moments of 12 C 2 H 2 in the region. Spectroscopic parameters of the 12 C 2 H 2 and 12 C 13 CH 2 upper vibrational levels were derived from a band-by-band fit of the line positions (typical rms values are on the order of 0.001 cm −1 ). A few of the analyzed bands were found to be affected by rovibrational perturbations, which are discussed. In particular, the rotational structure of the 2 ν 1 + ( ν 4 + ν 5 ) 0 Σ u + –Σ g + band near 7994 cm −1 exhibits a particularly surprising intensity distribution: while the P (19) and R (17) transitions share the same J = 18 upper level, the R (17) line has an intensity about 4 orders of magnitude smaller than the P (19) line. This unusual situation is quantitatively interpreted as resulting from a Coriolis interaction between the ν 1 + 2 ν 2 + ν 5 1 and 2 ν 1 + ( ν 4 + ν 5 ) 0 bands with a energy crossing at J = 18. The accidental nearly perfect cancelation of the two terms contributing to the line strength of the R (17) line leads to the near disappearance of this line. [ABSTRACT FROM AUTHOR]