SPECTRAL ANALYSIS OF THE BENDING LEVELS OF ACETYLENE CATION

Abstract

Twenty-seven vibronic levels $(v_{4}=0-6, v_{5}=0-2, K=0-3)$ of the ground electronic state of acetylene cation have been previously studied by 1+1' two-color pulsed-electric field induced zero-kinetic energy (PFI-ZEKE) photoelectron $spectroscopy^{a}$. These levels, rotationally resolved, are about $0-3520 cm^{-1}$ above the zero-point energy level. Previously, only the Renner-Teller effects of the trans- and cis-bending vibration and the vibrational anharmonicity were included in the effective $Hamiltonian^{b}$; one standard deviation of the fit was $4.1 cm^{-1}$, about six times of the measurement uncertainties. Large discrepancies between the observed and calculated frequencies were identified at combination levels. In this fit, effects such as vibrational $\ell$---type doubling and second-order anharmonic interaction were also considered due to the near degeneracy of the trans- and cis-bending vibration. The results of the fit are satisfactory; most of the vibronic bands can be fitted within one standard deviation of $1.0 cm^{-1}$, except the lower component of $(v_{4}=0, v_{5}=1, K=0)$. The results of the fit will be presented and few observed vibronic levels, yet unassigned, will be discussed.