RENNER-TELLER COUPLING IN THE 3D-BETA 1-PHI(U) RYDBERG STATE OF ACETYLENE

The ungerade Rydberg states of acetylene in the energy range from 79 4 00 to 85 000 cm-1 have been investigated by (3 + 1) ionization-detecte d absorption spectroscopy. Bending vibrational transitions induced by Renner-Teller coupling have been discovered and analyzed. The structur e observed corresponds closely with analogous bands in C2H2+ recently obtained and assigned to trans-bending (nu4) by Pratt et al, using ele ctron spectroscopy following resonant two-photon photoionization via t he A1A(u) state [J. Chem. Phys. 95, 6238 (1991)]. Based on intensities observed in our higher-resolution Rydberg spectrum, we offer a reassi gnment of this bending level structure that suggests an acetylene cati on, both isolated and at the core of nonpenetrating Rydberg states, in which trans-bend is of relatively high frequency and strongly coupled by Renner interaction to the cation 2PI electronic degeneracy.