New vibrational assignments in the Ā ¹ A u -[Xtilde] ¹ Σ ⁺ g electronic transition of acetylene, C ₂ H ₂ : the v′ ₁ frequency

New laser-induced fluorescence spectra of supersonic jet cooled acetylene (C2H2) in the wavelength region 230–205 nm have led to an improved understanding of the vibrational structure of the A 1Au state. Among the new bands observed are two weak perturbed bands at 46008 cm−1 and 46116 cm−1. Rotational analyses of these bands, together with the corresponding ‘hot’ bands arising from the ground state v4 fundamental, have shown that the upper states have asymmetric top K structure that is unaffected by a axis Coriolis coupling; this means that they do not involve overtones of the low frequency bending vibrations and therefore must be combinations of ag vibrational normal modes. From their positions in the manifold, their vibrational assignments can only be 22 031 0; and 11 0;31 0. These assignments lead to values of x 22, x13, and a revised value for the symmetric CH stretching frequency, ν1 = 2880.5cm−1; this revised value is 160cm−1 lower than the previously accepted value, but consistent with new ab initio calculations that we performed at the EOM-CCSD level using a TZ2P (triple-zeta plus double polarization) basis set.