The bending vibrational levels of the acetylene cation: A case study of the Renner-Teller effect in a molecule with two degenerate bending vibrations

Forty three vibronic levels of C2 H2+, X̃ Πu2, with 4 =0-6, 5 =0-3, and K=0-4, lying at energies of 0-3520 cm-1 above the zero-point level, have been recorded at rotational resolution. These levels were observed by double resonance, using 1+ 1′ two-color pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. The intermediate states were single rovibrational levels chosen from the à Au1, 4 3 (K=1-2), 5 3 (K=1), 2 +4 3 (K=0), and 47 206 cm-1 (K=1) levels of C2 H2. Seven of the trans-bending levels of C2 H2+ (4 =0-3, K=0-2) had been reported previously by Pratt [J. Chem. Phys. 99, 6233 (1993)]; our results for these levels agree well with theirs. A full analysis has been carried out, including the Renner-Teller effect and the vibrational anharmonicity for both the trans- and cis-bending vibrations. The rotational structure of the lowest 16 vibronic levels (consisting of the complete set of levels with 4 + 5 ≤2, except for the unobserved upper Πu2 component of the 2 4 overtone) could be fitted by least squares using 16 parameters to give an rms deviation of 0.21 cm-1. The vibronic coupling parameter ε5 (about whose magnitude there has been controversy) was determined to be - 0.02737. For the higher vibronic levels, an additional parameter, r45, was needed to allow for the Darling-Dennison resonance between the two bending manifolds. Almost all the observed levels of the 4 + 5 =3 and 4 polyads (about half of the predicted number) could then be assigned. In a final fit to 39 vibronic levels with 4 + 5 ≤5, an rms deviation of 0.34 cm-1 was obtained using 20 parameters. An interesting finding is that Hund's spin-coupling cases (a) and (b) both occur in the u components of the 4 +2 5 combination level. The ionization potential of C2 H2 (from the lowest rotational level of the ground state to the lowest rotational level of the cation) is found to be 91 953.77±0.09 cm-1 (3).