An unsuspected Fermi perturbation in the acetylene à 1A u 3ν3 level

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Abstract
A previously undetected perturbation in the HCCH à 1Au 3ν3 (trans‐bend) level has been found and analyzed. The perturbing level is identified as the à 1Au ν2+2ν4 level. Locally deperturbed molecular constants for the 3ν3 and perturbing level are obtained, along with the perturbation matrix element and the J‐dependent mixing coefficients. The Ã‐state torsional fundamental, ν4≈850 cm−1, is estimated from the energy of perturbing level. The presence of the perturbation explains large differences between the rotational line intensity patterns observed in the absorption and fluorescence excitation spectra. The perturbed 3ν3 and extra lines are especially prominent in a fluorescence excitation spectrum of the à 1Au–X̃ 1Σ+g 33 0 K10 subband of acetylene rotationally cooled in a supersonic jet.