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

Abstract

A previously undetected perturbation in the HCCH à ¹A_u 3ν₃ (trans‐bend) level has been found and analyzed. The perturbing level is identified as the à ¹A_u 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 ¹A_u–X̃ ¹Σ⁺_g 3³ ₀ K¹₀ subband of acetylene rotationally cooled in a supersonic jet.