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

Abstract

Forty three vibronic levels of C2H2+, X (2)Pi(u), with upsilon(4)=0-6, upsilon(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 A (1)A(u), 4 nu(3) (K=1-2), 5 nu(3) (K=1), nu(2)+4 nu(3) (K=0), and 47 206 cm(-1) (K=1) levels of C2H2. Seven of the trans-bending levels of C2H2+ (upsilon(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 upsilon(4)+upsilon(5)<= 2, except for the unobserved upper (2)Pi(u) component of the 2 nu(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 epsilon(5) (about whose magnitude there has been controversy) was determined to be -0.0273(7). For the higher vibronic levels, an additional parameter, r(45), was needed to allow for the Darling-Dennison resonance between the two bending manifolds. Almost all the observed levels of the upsilon(4)+upsilon(5)=3 and 4 polyads (about half of the predicted number) could then be assigned. In a final fit to 39 vibronic levels with upsilon(4)+upsilon(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 Sigma(u) components of the nu(4)+2 nu(5) combination level. The ionization potential of C2H2 (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 sigma). (c) 2006 American Institute of Physics.