Renner–Teller coupling in the 3dδ 1Φu Rydberg state of acetylene

The ungerade Rydberg states of acetylene in the energy range from 79 400 to 85 000 cm−1 have been investigated by (3+1) ionization-detected absorption spectroscopy. Bending vibrational transitions induced by Renner–Teller coupling have been discovered and analyzed. The structure observed corresponds closely with analogous bands in C2H2+ recently obtained and assigned to trans-bending (ν4) by Pratt et al., using electron spectroscopy following resonant two-photon photoionization via the A 1Au state [J. Chem. Phys. 95, 6238 (1991)]. Based on intensities observed in our higher-resolution Rydberg spectrum, we offer a reassignment of this bending level structure that suggests an acetylene cation, both isolated and at the core of nonpenetrating Rydberg states, in which trans-bend is of relatively high ffrequency and strongly coupled by Renner interaction to the cation 2Π electronic degeneracy.