The à ¹A_u state of acetylene: Ungerade vibrational levels in the region 45,800-46,550 cm^-1

The ungerade vibrational levels of the ùA_u (S₁-trans) state of C₂H₂ lying in the region 45,800-46,550 cm^-1 have been assigned from IR-UV double resonance spectra. The aim has been to classify the complete manifold of S ₁-trans levels in this region, so as to facilitate the assignment of the bands of S₁-cis C₂H₂. The rotational structure is complicated because of the overlapping of vibrational polyads with different Coriolis and Darling-Dennison parameters, but assignments have been possible with the help of predictions based on the properties of polyads at lower energy. An important result is that the analysis of the (1 ¹4¹, 1¹6¹) polyad determines the anharmonicity constants x₁₄ and x₁₆, which will be needed to proceed to higher energies. Some regions of impressive complexity occur. Among these is the band given by the 3³6¹, K=1 state at 45,945 cm^-1, where a three-level interaction within the S₁ state is confused by triplet perturbations. Several probable S₁-cis states have been observed, including cis-6², K=1; this vibrational level appears to show a K-staggering, of the type that arises when quantum mechanical tunnelling through the barrier to cis-trans isomerization is possible. The total number of identified cis vibrational states is now 6 out of an expected 10 up to the energies discussed in this paper.